Chapter 3. Routing Dynamically

What is the purpose of dynamic routing protocols?

How does dynamic routing compare with static routing?

How do dynamic routing protocols share route information and achieve convergence?

What are the differences between the categories of dynamic routing protocols?

How does the algorithm used by distance vector routing protocols determine the best path?

What are the different types of distance vector routing protocols?

How do you configure the RIP routing protocol?

How do you configure the RIPng routing protocol?

How does the algorithm used by link-state routing protocols determine the best path?

How do link-state routing protocols use information sent in link-state updates?

What are the advantages and disadvantages of using link-state routing protocols?

How do you determine the source route, administrative distance, and metric for a given route?

How do you explain the concept of a parent/child relationship in a dynamically built routing table?

How do you describe the differences between the IPv4 route lookup process and the IPv6 route lookup process?

Can you determine which route will be used to forward a packet upon analyzing a routing table?

dynamic routing protocols

Routing Information Protocol (RIP)

Advanced Research Projects Agency Network (ARPANET)

Open Shortest Path First (OSPF)

Intermediate System-to-Intermediate System (IS-IS)

Interior Gateway Routing Protocol (IGRP)

Enhanced IGRP (EIGRP)

Border Gateway Protocol (BGP)

data structures

routing protocol messages

algorithm

convergence

classful routing protocols

classless routing protocols

autonomous system (AS)

Interior Gateway Protocols (IGP)

Exterior Gateway Protocols (EGP)

distance vector routing protocols

link-state routing protocols

discontiguous network

variable-length subnet mask (VLSM)

metrics

bounded triggered updates

Hello keepalive mechanism

automatic summarization

passive-interface

default static route

RIPng

Dijkstra’s algorithm

shortest path first (SPF)

link-state packet (LSP)

SPF tree

event-driven updates

OSPFv3

ultimate route

level 1 route

supernet route

level 1 parent route

level 2 child route

Routers forward packets by using information in the routing table. Routes to remote networks can be learned by the router in two ways: static routes and dynamic routes.

In a large network with numerous networks and subnets, configuring and maintaining static routes between these networks requires a great deal of administrative and operational overhead. This operational overhead is especially cumbersome when changes to the network occur, such as a down link or implementing a new subnet. Implementing dynamic routing protocols can ease the burden of configuration and maintenance tasks and give the network scalability.

This chapter introduces dynamic routing protocols. It explores the benefits of using dynamic routing protocols, how different routing protocols are classified, and the metrics routing protocols use to determine the best path for network traffic. Other topics covered in this chapter include the characteristics of dynamic routing protocols and how the various routing protocols differ. Network professionals must understand the different routing protocols available in order to make informed decisions about when to use static or dynamic routing. They also need to know which dynamic routing protocol is most appropriate in a particular network environment.

As networks evolved and became more complex, new routing protocols emerged. The RIP routing protocol was updated to accommodate growth in the network environment, into RIPv2. However, the newer version of RIP still does not scale to the larger network implementations of today. To address the needs of larger networks, two advanced routing protocols were developed: Open Shortest Path First (OSPF) and Intermediate System-to-Intermediate System (IS-IS). Cisco developed the Interior Gateway Routing Protocol (IGRP) and Enhanced IGRP (EIGRP), which also scales well in larger network implementations.

Additionally, there was the need to connect different internetworks and provide routing between them. The Border Gateway Protocol (BGP) is now used between Internet service providers (ISPs). BGP is also used between ISPs and their larger private clients to exchange routing information.

Table 3-1 classifies the protocols.

With the advent of numerous consumer devices using IP, the IPv4 addressing space is nearly exhausted; thus, IPv6 has emerged. To support the communication based on IPv6, newer versions of the IP routing protocols have been developed, as shown by the IPv6 row in Table 3-1.

RIP is the simplest of dynamic routing protocols and is used in this section to provide a basic level of routing protocol understanding.

Discovery of remote networks

Maintaining up-to-date routing information

Choosing the best path to destination networks

Ability to find a new best path if the current path is no longer available

The main components of dynamic routing protocols include:

Data structures: Routing protocols typically use tables or databases for their operations. This information is kept in RAM.

Routing protocol messages: Routing protocols use various types of messages to discover neighboring routers, exchange routing information, and perform other tasks to learn and maintain accurate information about the network.

Algorithm: An algorithm is a finite list of steps used to accomplish a task. Routing protocols use algorithms for facilitating routing information and for best path determination.

Figure 3-1 highlights the data structures, routing protocol messages, and routing algorithm used by EIGRP.

Routing protocols determine the best path, or route, to each network. That route is then added to the routing table. A primary benefit of dynamic routing protocols is that routers exchange routing information when there is a topology change. This exchange allows routers to automatically learn about new networks and also to find alternate paths when there is a link failure to a current network.

Compared to static routing, dynamic routing protocols require less administrative overhead. However, the expense of using dynamic routing protocols is dedicating part of a router’s resources for protocol operation, including CPU time and network link bandwidth. Despite the benefits of dynamic routing, static routing still has its place. There are times when static routing is more appropriate and other times when dynamic routing is the better choice. Networks with moderate levels of complexity may have both static and dynamic routing configured.

Static routing has several primary uses, including:

Providing ease of routing table maintenance in smaller networks that are not expected to grow significantly.

Routing to and from a stub network, which is a network with only one default route out and no knowledge of any remote networks.

Accessing a single default route (which is used to represent a path to any network that does not have a more specific match with another route in the routing table).

Figure 3-2 provides a sample static routing scenario.

The disadvantages of static routing include:

They are not easy to implement in a large network.

Managing the static configurations can become time consuming.

If a link fails, a static route cannot reroute traffic.

Table 3-2 highlights the advantages and disadvantages of static routing.

Imagine maintaining the static routing configurations for the seven routers in Figure 3-3.

What if the company grew and now has four regions and 28 routers to manage, as shown in Figure 3-4? What happens when a link goes down? How do you ensure that redundant paths are available?

Dynamic routing is the best choice for large networks like the one shown in Figure 3-4.

There are disadvantages to dynamic routing. Dynamic routing requires knowledge of additional commands. It is also less secure than static routing because the interfaces identified by the routing protocol send routing updates out. Routes taken may differ between packets. The routing algorithm uses additional CPU, RAM, and link bandwidth.

Table 3-3 highlights the advantages and disadvantages of dynamic routing.

Notice how dynamic routing addresses the disadvantages of static routing.

In general, the operations of a dynamic routing protocol can be described as follows:

1. The router sends and receives routing messages on its interfaces.

2. The router shares routing messages and routing information with other routers that are using the same routing protocol.

3. Routers exchange routing information to learn about remote networks.

4. When a router detects a topology change, the routing protocol can advertise this change to other routers.

After a router boots successfully, it applies the saved configuration. If the IP addressing is configured correctly, then the router initially discovers its own directly connected networks.

Notice how the routers proceed through the boot process and then discover any directly connected networks and subnet masks. This information is added to their routing tables as follows:

R1 adds the 10.1.0.0 network available through interface FastEthernet 0/0 and adds 10.2.0.0 available through interface Serial 0/0/0.

R2 adds the 10.2.0.0 network available through interface Serial 0/0/0 and adds 10.3.0.0 available through interface Serial 0/0/1.

R3 adds the 10.3.0.0 network available through interface Serial 0/0/1 and adds 10.4.0.0 available through interface FastEthernet 0/0.

With this initial information, the routers then proceed to find additional route sources for their routing tables.

If a routing protocol is configured, the next step is for the router to begin exchanging routing updates to learn about any remote routes.

The router sends an update packet out all interfaces that are enabled on the router. The update contains the information in the routing table, which currently comprises all directly connected networks.

At the same time, the router also receives and processes similar updates from other connected routers. Upon receiving an update, the router checks it for new network information. Any networks that are not currently listed in the routing table are added.

Figure 3-5 depicts an example topology setup between three routers, R1, R2, and R3. Notice that only the directly connected networks are listed in each router’s respective routing table.

Based on this topology, a listing of the different updates that R1, R2, and R3 send and receive during initial convergence is provided:

R1:

Sends an update about network 10.1.0.0 out the Serial 0/0/0 interface

Sends an update about network 10.2.0.0 out the FastEthernet 0/0 interface

Receives update from R2 about network 10.3.0.0 and increments the hop count by 1

Stores network 10.3.0.0 in the routing table via Serial 0/0/0 with a metric of 1

R2:

Sends an update about network 10.3.0.0 out the Serial 0/0/0 interface

Sends an update about network 10.2.0.0 out the Serial 0/0/1 interface

Receives an update from R1 about network 10.1.0.0 and increments the hop count by 1

Stores network 10.1.0.0 in the routing table via Serial 0/0/0 with a metric of 1

Receives an update from R3 about network 10.4.0.0 and increments the hop count by 1

Stores network 10.4.0.0 in the routing table via Serial 0/0/1 with a metric of 1

R3:

Sends an update about network 10.4.0.0 out the Serial 0/0/1 interface

Sends an update about network 10.3.0.0 out the FastEthernet 0/0 interface

Receives an update from R2 about network 10.2.0.0 and increments the hop count by 1

Stores network 10.2.0.0 in the routing table via Serial 0/0/1 with a metric of 1

Figure 3-6 displays the routing tables after the initial exchange.

After this first round of update exchanges, each router knows about the connected networks of its directly connected neighbors. However, did you notice that R1 does not yet know about 10.4.0.0 and that R3 does not yet know about 10.1.0.0? Full knowledge and a converged network do not take place until there is another exchange of routing information.

After initial discovery is complete, each router continues the convergence process by sending and receiving the following updates.

R1:

Sends an update about network 10.1.0.0 out the Serial 0/0/0 interface

Sends an update about networks 10.2.0.0 and 10.3.0.0 out the FastEthernet 0/0 interface

Receives an update from R2 about network 10.4.0.0 and increments the hop count by 1

Stores network 10.4.0.0 in the routing table via Serial 0/0/0 with a metric of 2

Same update from R2 contains information about network 10.3.0.0 with a metric of 1. There is no change; therefore, the routing information remains the same.

R2:

Sends an update about networks 10.3.0.0 and 10.4.0.0 out of Serial 0/0/0 interface

Sends an update about networks 10.1.0.0 and 10.2.0.0 out of Serial 0/0/1 interface

Receives an update from R1 about network 10.1.0.0. There is no change; therefore, the routing information remains the same.

Receives an update from R3 about network 10.4.0.0. There is no change; therefore, the routing information remains the same.

R3:

Sends an update about network 10.4.0.0 out the Serial 0/0/1 interface

Sends an update about networks 10.2.0.0 and 10.3.0.0 out the FastEthernet 0/0 interface

Receives an update from R2 about network 10.1.0.0 and increments the hop count by 1

Stores network 10.1.0.0 in the routing table via Serial 0/0/1 with a metric of 2

Same update from R2 contains information about network 10.2.0.0 with a metric of 1. There is no change; therefore, the routing information remains the same.

Figure 3-7 displays the routing tables after the routers have converged.

Distance vector routing protocols typically implement a routing loop prevention technique known as split horizon. Split horizon prevents information from being sent out the same interface from which it was received. For example, R2 does not send an update containing the network 10.1.0.0 out of Serial 0/0/0, because R2 learned about network 10.1.0.0 through Serial 0/0/0.

After routers within a network have converged, the router can then use the information within the route table to determine the best path to reach a destination. Different routing protocols have different ways of calculating the best path.

Convergence is both collaborative and independent. The routers share information with each other, but must independently calculate the impacts of the topology change on their own routes. Because they develop an agreement with the new topology independently, they are said to converge on this consensus.

Convergence properties include the speed of propagation of routing information and the calculation of optimal paths. The speed of propagation refers to the amount of time it takes for routers within the network to forward routing information.

As shown in Figure 3-8, routing protocols can be rated based on the speed to convergence; the faster the convergence, the better the routing protocol. Generally, older protocols, such as RIP, are slow to converge, whereas modern protocols, such as EIGRP and OSPF, converge more quickly.

Purpose: Interior Gateway Protocol (IGP) or Exterior Gateway Protocol (EGP)

Operation: Distance vector protocol, link-state protocol, or path-vector protocol

Behavior: Classful (legacy) or classless protocol

For example, IPv4 routing protocols are classified as follows:

RIPv1 (legacy): IGP, distance vector, classful protocol

IGRP (legacy): IGP, distance vector, classful protocol developed by Cisco (deprecated from 12.2 IOS and later)

RIPv2: IGP, distance vector, classless protocol

EIGRP: IGP, distance vector, classless protocol developed by Cisco

OSPF: IGP, link-state, classless protocol

IS-IS: IGP, link-state, classless protocol

BGP: EGP, path-vector, classless protocol

The classful routing protocols, RIPv1 and IGRP, are legacy protocols and are only used in older networks. These routing protocols have evolved into the classless routing protocols, RIPv2 and EIGRP, respectively. Link-state routing protocols are classless by nature.

Figure 3-9 displays a hierarchical view of dynamic routing protocol classification.

The Internet is based on the AS concept; therefore, two types of routing protocols are required:

Interior Gateway Protocols (IGP): Used for routing within an AS. It is also referred to as intra-AS routing. Companies, organizations, and even service providers use an IGP on their internal networks. IGPs include RIP, EIGRP, OSPF, and IS-IS.

Exterior Gateway Protocols (EGP): Used for routing between autonomous systems. It is also referred to as inter-AS routing. Service providers and large companies may interconnect using an EGP. The Border Gateway Protocol (BGP) is the only currently viable EGP and is the official routing protocol used by the Internet.

The example in Figure 3-10 provides simple scenarios highlighting the deployment of IGPs, BGP, and static routing.

There are five individual autonomous systems in the scenario:

ISP-1: This is an AS and it uses IS-IS as the IGP. It interconnects with other autonomous systems and service providers using BGP to explicitly control how traffic is routed.

ISP-2: This is an AS and it uses OSPF as the IGP. It interconnects with other autonomous systems and service providers using BGP to explicitly control how traffic is routed.

AS-1: This is a large organization and it uses EIGRP as the IGP. Because it is multihomed (i.e., connects to two different service providers), it uses BGP to explicitly control how traffic enters and leaves the AS.

AS-2: This is a medium-sized organization and it uses OSPF as the IGP. It is also multihomed; therefore, it uses BGP to explicitly control how traffic enters and leaves the AS.

AS-3: This is a small organization with older routers within the AS; it uses RIP as the IGP. BGP is not required because it is single-homed (i.e., connects to one service provider). Instead, static routing is implemented between the AS and the service provider.

Distance: Identifies how far it is to the destination network and is based on a metric such as the hop count, cost, bandwidth, delay, and more

Vector: Specifies the direction of the next-hop router or exit interface to reach the destination

For example, in Figure 3-11, R1 knows that the distance to reach network 172.16.3.0/24 is one hop and that the direction is out of the interface Serial 0/0/0 toward R2.

A router using a distance vector routing protocol does not have the knowledge of the entire path to a destination network. Distance vector protocols use routers as sign posts along the path to the final destination. The only information a router knows about a remote network is the distance or metric to reach that network and which path or interface to use to get there. Distance vector routing protocols do not have an actual map of the network topology.

There are four distance vector IPv4 IGPs:

RIPv1: First generation legacy protocol

RIPv2: Simple distance vector routing protocol

IGRP: First generation Cisco proprietary protocol (obsolete and replaced by EIGRP)

EIGRP: Advanced version of distance vector routing

To continue our analogy of sign posts, using a link-state routing protocol is like having a complete map of the network topology. The sign posts along the way from source to destination are not necessary, because all link-state routers are using an identical map of the network. A link-state router uses the link-state information to create a topology map and to select the best path to all destination networks in the topology.

RIP-enabled routers send periodic updates of their routing information to their neighbors. Link-state routing protocols do not use periodic updates. After the network has converged, a link-state update is only sent when there is a change in the topology. For example, in Figure 3-12, the link-state update is sent when the 172.16.3.0 network goes down.

Link-state protocols work best in situations where:

The network design is hierarchical, usually occurring in large networks

Fast convergence of the network is crucial

The administrators have good knowledge of the implemented link-state routing protocol

There are two link-state IPv4 IGPs:

OSPF: Popular standards-based routing protocol

IS-IS: Popular in provider networks

The two original IPv4 routing protocols developed were RIPv1 and IGRP. They were created when network addresses were allocated based on classes (i.e., class A, B, or C). At that time, a routing protocol did not need to include the subnet mask in the routing update, because the network mask could be determined based on the first octet of the network address.

The fact that RIPv1 and IGRP do not include subnet mask information in their updates means that they cannot provide variable-length subnet masks (VLSMs) and Classless Inter-Domain Routing (CIDR).

Classful routing protocols also create problems in discontiguous networks. A discontiguous network is when subnets from the same classful major network address are separated by a different classful network address.

To illustrate the shortcoming of classful routing, refer to the topology in Figure 3-13.

Notice that the LANs of R1 (172.16.1.0/24) and R3 (172.16.2.0/24) are both subnets of the same class B network (172.16.0.0/16). They are separated by different classful network addresses (192.168.1.0/30 and 192.168.2.0/30).

When R1 forwards an update to R2, RIPv1 does not include the subnet mask information with the update; it only forwards the class B network address 172.16.0.0.

R2 receives and processes the update. It then creates and adds an entry for the class B 172.16.0.0/16 network in the routing table, as shown in Figure 3-14.

When R3 forwards an update to R2, it also does not include the subnet mask information and therefore only forwards the classful network address 172.16.0.0.

R2 receives and processes the update and adds another entry for the classful network address 172.16.0.0/16 to its routing table, as shown in Figure 3-15. When there are two entries with identical metrics in the routing table, the router shares the load of the traffic equally among the two links. This is known as load balancing.

Discontiguous networks have a negative impact on a network. For example, a ping to 172.16.1.1 would return “U.U.U” because R2 would forward the first ping out its Serial 0/0/1 interface toward R3, and R3 would return a Destination Unreachable (U) error code to R2. The second ping would exit out of R2’s Serial 0/0/0 interface toward R1, and R1 would return a successful code (.). This pattern would continue until the ping command is done.

IPv6 routing protocols are classless. The distinction whether a routing protocol is classful or classless typically only applies to IPv4 routing protocols. All IPv6 routing protocols are considered classless because they include the prefix-length with the IPv6 address.

Figures 3-16 through 3-18 illustrate how classless routing solves the issues created with classful routing.

In the discontiguous network design of Figure 3-16, the classless protocol RIPv2 has been implemented on all three routers. When R1 forwards an update to R2, RIPv2 includes the subnet mask information with the update 172.16.1.0/24.

In Figure 3-17, R2 receives, processes, and adds two entries in the routing table. The first line displays the classful network address 172.16.0.0 with the /24 subnet mask of the update. This is known as the parent route. The second entry displays the VLSM network address 172.16.1.0 with the exit and next-hop address. This is referred to as the child route. Parent routes never include an exit interface or next-hop IP address.

When R3 forwards an update to R2, RIPv2 includes the subnet mask information with the update 172.16.2.0/24.

R2 receives, processes, and adds another child route entry 172.16.2.0/24 under the parent route entry 172.16.0.0, as shown in Figure 3-18.

A ping from R2 to 172.16.1.1 would now be successful.

Speed of convergence: Speed of convergence defines how quickly the routers in the network topology share routing information and reach a state of consistent knowledge. The faster the convergence, the more preferable the protocol. Routing loops can occur when inconsistent routing tables are not updated due to slow convergence in a changing network.

Scalability: Scalability defines how large a network can become, based on the routing protocol that is deployed. The larger the network is, the more scalable the routing protocol needs to be.

Classful or classless (use of VLSM): Classful routing protocols do not include the subnet mask and cannot support variable-length subnet mask (VLSM). Classless routing protocols include the subnet mask in the updates. Classless routing protocols support VLSM and better route summarization.

Resource usage: Resource usage includes the requirements of a routing protocol such as memory space (RAM), CPU utilization, and link bandwidth utilization. Higher resource requirements necessitate more powerful hardware to support the routing protocol operation, in addition to the packet forwarding processes.

Implementation and maintenance: Implementation and maintenance describes the level of knowledge that is required for a network administrator to implement and maintain the network based on the routing protocol deployed.

Table 3-4 summarizes the characteristics of each routing protocol.

A metric is a measurable value that is assigned by the routing protocol to different routes based on the usefulness of that route. In situations where there are multiple paths to the same remote network, the routing metrics are used to determine the overall “cost” of a path from source to destination. Routing protocols determine the best path based on the route with the lowest cost.

Different routing protocols use different metrics. The metric used by one routing protocol is not comparable to the metric used by another routing protocol. Two different routing protocols might choose different paths to the same destination.

For example, assume that PC1 wants to send a packet to PC2. In Figure 3-19, the RIP routing protocol has been enabled on all routers and the network has converged. RIP makes a routing protocol decision based on the least number of hops. Therefore, when the packet arrives on R1, the best route to reach the PC2 network would be to send it directly to R2 even though the link is much slower that all other links.

In Figure 3-20, the OSPF routing protocol has been enabled on all routers and the network has converged. OSPF makes a routing protocol decision based on the best bandwidth. Therefore, when the packet arrives on R1, the best route to reach the PC2 network would be to send it to R3, which would then forward it to R2.

Some distance vector routing protocols send periodic updates. For example, RIP sends a periodic update to all of its neighbors every 30 seconds. RIP does this even if the topology has not changed; it continues to send updates. RIPv1 reaches all of its neighbors by sending updates to the all-hosts IPv4 address of 255.255.255.255, a broadcast.

The broadcasting of periodic updates is inefficient because the updates consume bandwidth and consume network device CPU resources. Every network device has to process a broadcast message. RIPv2 and EIGRP, instead, use multicast addresses so that only neighbors that need updates will receive them. EIGRP can also send a unicast message to only the affected neighbor. Additionally, EIGRP only sends an update when needed, instead of periodically.

As shown in Figure 3-21, the two modern IPv4 distance vector routing protocols are RIPv2 and EIGRP. RIPv1 and IGRP are listed only for historical accuracy.

The algorithm used for the routing protocols defines the following processes:

Mechanism for sending and receiving routing information

Mechanism for calculating the best paths and installing routes in the routing table

Mechanism for detecting and reacting to topology changes

In the animation in the online course, R1 and R2 are configured with the RIP routing protocol. The algorithm sends and receives updates. Both R1 and R2 then glean new information from the update. In this case, each router learns about a new network. The algorithm on each router makes its calculations independently and updates the routing table with the new information. When the LAN on R2 goes down, the algorithm constructs a triggered update and sends it to R1. R1 then removes the network from the routing table.

Different routing protocols use different algorithms to install routes in the routing table, send updates to neighbors, and make path determination decisions. For example:

RIP uses the Bellman-Ford algorithm as its routing algorithm. It is based on two algorithms developed in 1958 and 1956 by Richard Bellman and Lester Ford, Jr.

IGRP and EIGRP use the Diffusing Update Algorithm (DUAL) routing algorithm developed by Dr. J.J. Garcia-Luna-Aceves at SRI International.

RIPv1 has the following key characteristics:

Routing updates are broadcasted (255.255.255.255) every 30 seconds.

The hop count is used as the metric for path selection.

A hop count greater than 15 hops is deemed infinite (too far). That 15th hop router would not propagate the routing update to the next router.

In 1993, RIPv1 evolved to a classless routing protocol known as RIP version 2 (RIPv2). RIPv2 introduced the following improvements:

Classless routing protocol: It supports VLSM and CIDR, because it includes the subnet mask in the routing updates.

Increased efficiency: It forwards updates to multicast address 224.0.0.9, instead of the broadcast address 255.255.255.255.

Reduced routing entries: It supports manual route summarization on any interface.

Secure: It supports an authentication mechanism to secure routing table updates between neighbors.

Table 3-5 summarizes the differences between RIPv1 and RIPv2.

RIP updates are encapsulated into a UDP segment, with both source and destination port numbers set to UDP port 520.

In 1997, the IPv6-enabled version of RIP was released. RIPng is based on RIPv2. It still has a 15-hop limitation and the administrative distance is 120.

Bandwidth, delay, load, and reliability are used to create a composite metric.

Routing updates are broadcast every 90 seconds, by default.

In 1992, IGRP was replaced by Enhanced IGRP (EIGRP). Like RIPv2, EIGRP also introduced support for VLSM and CIDR. EIGRP increases efficiency, reduces routing updates, and supports secure message exchange.

Table 3-6 summarizes the differences between IGRP and EIGRP.

EIGRP also introduced:

Bounded triggered updates: It does not send periodic updates. Only routing table changes are propagated, whenever a change occurs. This reduces the amount of load the routing protocol places on the network. Bounded triggered updates means that EIGRP only sends to the neighbors that need it. It uses less bandwidth, especially in large networks with many routes.

Hello keepalive mechanism: A small Hello message is periodically exchanged to maintain adjacencies with neighboring routers. This means a very low usage of network resources during normal operation, instead of the periodic updates.

Maintains a topology table: Maintains all the routes received from neighbors (not only the best paths) in a topology table. DUAL can insert backup routes into the EIGRP topology table.

Rapid convergence: In most cases, it is the fastest IGP to converge because it maintains alternate routes, enabling almost instantaneous convergence. If a primary route fails, the router can use the alternate route identified. The switchover to the alternate route is immediate and does not involve interaction with other routers.

Multiple network layer protocol support: EIGRP uses Protocol Dependent Modules (PDM), which means that it is the only protocol to include support for protocols other than IPv4 and IPv6, such as legacy IPX and AppleTalk.

Refer to the reference topology in Figure 3-22 and the addressing table in Table 3-7.

In this scenario, all routers have been configured with basic management features and all interfaces identified in the reference topology are configured and enabled. There are no static routes configured and no routing protocols enabled; therefore, remote network access is currently impossible. RIPv2 is used as the dynamic routing protocol.

To enable RIP, use the router rip command to enter router configuration mode, as shown in the following output. This command does not directly start the RIP process. Instead, it provides access to the router configuration mode where the RIP routing settings are configured.

Click here to view code image

R1# conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)# router rip
R1(config-router)#

To disable and eliminate RIP, use the no router rip global configuration command. This command stops the RIP process and erases all existing RIP configurations.

Figure 3-23 displays a partial list of the various RIP commands that can be configured. This section covers the two highlighted commands as well as network, passive-interface, and version.

To enable RIP routing for a network, use the network network-address router configuration mode command. Enter the classful network address for each directly connected network. This command:

Enables RIP on all interfaces that belong to a specific network. Associated interfaces now both send and receive RIP updates.

Advertises the specified network in RIP routing updates sent to other routers every 30 seconds.

In the following command sequence, the network command is used to advertise the R1 directly connected networks.

Click here to view code image

R1(config)# router rip
R1(config-router)# network 192.168.1.0
R1(config-router)# network 192.168.2.0
R1(config-router)#

This output confirms that:

1. RIP routing is configured and running on router R1.

2. The values of various timers; for example, the next routing update is sent by R1 in 16 seconds.

3. The version of RIP configured is currently RIPv1.

4. R1 is currently summarizing at the classful network boundary.

5. The classful networks are advertised by R1. These are the networks that R1 includes in its RIP updates.

6. The RIP neighbors are listed, including their next-hop IP address, the associated AD that R2 uses for updates sent by this neighbor, and when the last update was received from this neighbor.

The show ip route command displays the RIP routes installed in the routing table. In Figure 3-25, R1 now knows about the highlighted networks.

Click here to view code image

R1# show ip protocols
*** IP Routing is NSF aware ***

Routing Protocol is "rip"
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  Sending updates every 30 seconds, next due in 16 seconds
  Invalid after 180 seconds, hold down 180, flushed after 240
  Redistributing: rip
  Default version control: send version 1, receive any version
    Interface             Send  Recv  Triggered RIP  Key-chain
    GigabitEthernet0/0    1     1 2
    Serial0/0/0           1     1 2
  Automatic network summarization is in effect
  Maximum path: 4
  Routing for Networks:
    192.168.1.0
    192.168.2.0
  Routing Information Sources:
    Gateway         Distance      Last Update
    192.168.2.2          120      00:00:15
  Distance: (default is 120)

R1#

However, even though the router only sends RIPv1 messages, it can interpret both RIPv1 and RIPv2 messages. A RIPv1 router ignores the RIPv2 fields in the route entry.

Use the version 2 router configuration mode command to enable RIPv2, as shown in Figure 3-26.

Notice how the show ip protocols command verifies that R2 is now configured to send and receive version 2 messages only. The RIP process now includes the subnet mask in all updates, making RIPv2 a classless routing protocol.

The following output verifies that there are no RIP routes still in the routing table:

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R1# show ip route | begin Gateway
Gateway of last resort is not set

      192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.1.0/24 is directly connected, GigabitEthernet0/0
L        192.168.1.1/32 is directly connected, GigabitEthernet0/0
      192.168.2.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.2.0/24 is directly connected, Serial0/0/0
L        192.168.2.1/32 is directly connected, Serial0/0/0
R1#

There are no RIP routes because R1 is now only listening for RIPv2 updates. R2 and R3 are still sending RIPv1 updates. Therefore, the version 2 command must be configured on all routers in the routing domain.

To modify the default RIPv2 behavior of automatic summarization, use the no auto-summary router configuration mode command as shown in the following command sequence:

Click here to view code image

R1(config)# router rip
R1(config-router)# no auto-summary
R1(config-router)# end
R1#
*Mar 10 14:11:49.659: %SYS-5-CONFIG_I: Configured from console by console
R1# show ip protocols | section Automatic
  Automatic network summarization is not in effect
R1#

This command has no effect when using RIPv1. When automatic summarization has been disabled, RIPv2 no longer summarizes networks to their classful address at boundary routers. RIPv2 now includes all subnets and their appropriate masks in its routing updates. The show ip protocols output now states that automatic network summarization is not in effect.

For instance, refer to the topology in Figure 3-22. RIP sends updates out of its Gigabit Ethernet 0/0 interface even though no RIP device exists on that LAN. R1 has no way of knowing this and, as a result, sends an update every 30 seconds. Sending out unneeded updates on a LAN impacts the network in three ways:

Wasted bandwidth: Bandwidth is used to transport unnecessary updates. Because RIP updates are either broadcasted or multicasted, switches also forward the updates out all ports.

Wasted resources: All devices on the LAN must process the update up to the transport layers, at which point the devices will discard the update.

Security risk: Advertising updates on a broadcast network is a security risk. RIP updates can be intercepted with packet sniffing software. Routing updates can be modified and sent back to the router, corrupting the routing table with false metrics that misdirect traffic.

To address these problems, an interface can be configured to stop sending routing updates. This is referred to as configuring a passive interface. Use the passive-interface router configuration command to prevent the transmission of routing updates through a router interface but still allow that network to be advertised to other routers. The command stops routing updates out the specified interface. However, the network that the specified interface belongs to is still advertised in routing updates that are sent out other interfaces.

There is no need for R1, R2, and R3 to forward RIP updates out of their LAN interfaces. The configuration in Figure 3-28 identifies the R1 Gigabit Ethernet 0/0 interface as passive.

The show ip protocols command is then used to verify that the Gigabit Ethernet interface was passive. Notice that the Gigabit Ethernet 0/0 interface is no longer listed as sending or receiving version 2 updates, but instead is now listed under the Passive Interface(s) section. Also notice that the network 192.168.1.0 is still listed under Routing for Networks, which means that this network is still included as a route entry in RIP updates that are sent to R2.

As an alternative, all interfaces can be made passive using the passive-interface default command. Interfaces that should not be passive can be re-enabled using the no passive-interface command.

Similar default static routes could be configured on R2 and R3, but it is much more scalable to enter it one time on the edge router R1 and then have R1 propagate it to all other routers using RIP. To provide Internet connectivity to all other networks in the RIP routing domain, the default static route needs to be advertised to all other routers that use the dynamic routing protocol.

To propagate a default route, the edge router must be configured with:

A default static route using the ip route 0.0.0.0 0.0.0.0 exit-intf next-hop-ip command.

The default-information originate router configuration command. This instructs R1 to originate default information, by propagating the static default route in RIP updates.

The example in Figure 3-30 configures a fully specified default static route to the service provider, and then the route is propagated by RIP. Notice that R1 now has a Gateway of Last Resort and default route installed in its routing table.

Refer to the reference topology in Figure 3-31.

In this scenario, all routers have been configured with basic management features and all interfaces identified in the reference topology are configured and enabled. There are no static routes configured and no routing protocols enabled; therefore, remote network access is currently impossible.

To enable an IPv6 router to forward IPv6 packets, ipv6 unicast-routing must be configured.

Unlike RIPv2, RIPng is enabled on an interface and not in router configuration mode. In fact, there is no network network-address command available in RIPng. Instead, use the ipv6 rip domain-name enable interface configuration command.

In the following output, IPv6 unicast routing is enabled and the Gigabit Ethernet 0/0 and Serial 0/0/0 interfaces are enabled for RIPng using the domain name RIP-AS:

Click here to view code image

R1(config)# ipv6 unicast-routing
R1(config)#
R1(config)# interface gigabitethernet 0/0
R1(config-if)# ipv6 rip RIP-AS enable
R1(config-if)# exit
R1(config)#
R1(config)# interface serial 0/0/0
R1(config-if)# ipv6 rip RIP-AS enable
R1(config-if)# no shutdown
R1(config-if)#

The process to propagate a default route in RIPng is identical to RIPv2 except that an IPv6 default static route must be specified. For example, assume that R1 had an Internet connection from a Serial 0/0/1 interface to IP address 2001:DB8:FEED:1::1/64. To propagate a default route, R1 would have to be configured with:

A default static route using the ipv6 route 0::/0 2001:DB8:FEED:1::1 global configuration command.

The ipv6 rip domain-name default-information originate interface configuration mode command. For example, the Serial 0/0/1 interface of R1 would have to be configured with the ipv6 rip RIP-AS default-information originate command. This would instruct R1 to be the source of the default route information and propagate the default static route in RIPng updates sent out of the RIPng-enabled interfaces.

However, the command does confirm the following parameters:

1. That RIPng routing is configured and running on router R1.

2. The interfaces configured with RIPng.

The show ipv6 route command displays the routes installed in the routing table as shown in Figure 3-33. The output confirms that R1 now knows about the highlighted RIPng networks.

Notice that the R2 LAN is advertised as two hops away. This is because there is a difference in the way RIPv2 and RIPng calculate the hop counts. With RIPv2 (and RIPv1), the metric to the R2 LAN would be one hop. This is because the metric (hop count) that is displayed in the IPv4 routing table is the number of hops required to reach the remote network (counting the next-hop router as the first hop). In RIPng, the sending router already considers itself to be one hop away; therefore, R2 advertises its LAN with a metric of 1. When R1 receives the update, it adds another hop count of 1 to the metric. Therefore, R1 considers the R2 LAN to be two hops away. Similarly it considers the R3 LAN to be three hops away.

Appending the rip keyword to the command as shown in Figure 3-34 only lists RIPng networks.

The IPv4 link-state routing protocols are shown Figure 3-35:

Open Shortest Path First (OSPF)

Intermediate System-to-Intermediate System (IS-IS)

Link-state routing protocols have the reputation of being much more complex than their distance vector counterparts. However, the basic functionality and configuration of link-state routing protocols is equally straightforward.

Just like RIP and EIGRP, basic OSPF operations can be configured using the:

router ospf process-id global configuration command

network command to advertise networks

In Figure 3-36, each path is labeled with an arbitrary value for cost.

The cost of the shortest path for R2 to send packets to the LAN attached to R3 is 27. Specifically, the cost is R2 to R1 (20) plus R1 to R3 (5) plus R3 to LAN (2). Each router determines its own cost to each destination in the topology. In other words, each router calculates the SPF algorithm and determines the cost from its own perspective.

The shortest path is not necessarily the path with the least number of hops. For example, look at the path to the R5 LAN. It might be assumed that R1 would send directly to R4 instead of to R3. However, the cost to reach R4 directly (22) is higher than the cost to reach R4 through R3 (17).

Observe the shortest path for each router to reach each of the LANs, as shown in Tables 3-8 through 3-11.

All routers in an OSPF area will complete the following generic link-state routing process to reach a state of convergence:

1. Each router learns about its own links and its own directly connected networks. This is done by detecting that an interface is in the up state.

2. Each router is responsible for meeting its neighbors on directly connected networks. Link-state routers do this by exchanging Hello packets with other link-state routers on directly connected networks.

3. Each router builds a link-state packet (LSP)containing the state of each directly connected link. This is done by recording all the pertinent information about each neighbor, including neighbor ID, link type, and bandwidth.

4. Each router floods the LSP to all neighbors. Those neighbors store all LSPs received in a database. They then flood the LSPs to their neighbors until all routers in the area have received the LSPs. Each router stores a copy of each LSP received from its neighbors in a local database.

5. Each router uses the database to construct a complete map of the topology and computes the best path to each destination network. Like having a road map, the router now has a complete map of all destinations in the topology and the routes to reach them. The SPF algorithm is used to construct the map of the topology and to determine the best path to each network.

Refer to the topology in Figure 3-38. For purposes of this discussion, assume that R1 was previously configured and had full connectivity to all neighbors. However, R1 lost power briefly and had to restart.

During boot up R1 loads the saved startup configuration file. As the previously configured interfaces become active, R1 learns about its own directly connected networks. Regardless of the routing protocols used, these directly connected networks are now entries in the routing table.

As with distance vector protocols and static routes, the interface must be properly configured with an IPv4 address and subnet mask, and the link must be in the up state before the link-state routing protocol can learn about a link. Also, like distance vector protocols, the interface must be included in one of the network router configuration statements before it can participate in the link-state routing process.

Figure 3-38 shows R1 linked to four directly connected networks:

FastEthernet 0/0: 10.1.0.0/16

Serial 0/0/0: 10.2.0.0/16

Serial 0/0/1: 10.3.0.0/16

Serial 0/1/0: 10.4.0.0/16

As shown in Figures 3-39 through 3-42, the link-state information includes:

The interface’s IPv4 address and subnet mask

The type of network, such as Ethernet (broadcast) or Serial point-to-point link

The cost of that link

Any neighbor routers on that link

Routers with link-state routing protocols use a Hello protocol to discover any neighbors on their links. A neighbor is any other router that is enabled with the same link-state routing protocol.

In Figure 3-43, R1 sends Hello packets out its links (interfaces) to discover if there are any neighbors.

In Figure 3-44, R2, R3, and R4 reply to the Hello packet with their own Hello packets because these routers are configured with the same link-state routing protocol. There are no neighbors out the FastEthernet 0/0 interface. Because R1 does not receive a Hello on this interface, it does not continue with the link-state routing process steps for the FastEthernet 0/0 link.

When two link-state routers learn that they are neighbors, they form an adjacency. These small Hello packets continue to be exchanged between two adjacent neighbors and serve as a keepalive function to monitor the state of the neighbor. If a router stops receiving Hello packets from a neighbor, that neighbor is considered unreachable and the adjacency is broken.

After a router has established its adjacencies, it can build its LSPs that contain the link-state information about its links. A simplified version of the LSP from R1 displayed in Figure 3-45 would contain the following:

1. R1; Ethernet network 10.1.0.0/16; Cost 2

2. R1 -> R2; Serial point-to-point network; 10.2.0.0/16; Cost 20

3. R1 -> R3; Serial point-to-point network; 10.3.0.0/16; Cost 5

4. R1 -> R4; Serial point-to-point network; 10.4.0.0/16; Cost 20

Each router floods its link-state information to all other link-state routers in the routing area as shown in Figure 3-46.

Whenever a router receives an LSP from a neighboring router, it immediately sends that LSP out all other interfaces except the interface that received the LSP. This process creates a flooding effect of LSPs from all routers throughout the routing area.

In the animation, the LSPs are flooded almost immediately after being received without any intermediate calculations. Link-state routing protocols calculate the SPF algorithm after the flooding is complete. As a result, link-state routing protocols reach convergence very quickly.

Remember that LSPs do not need to be sent periodically. An LSP only needs to be sent:

During initial startup of the routing protocol process on that router (e.g., router restart)

Whenever there is a change in the topology (e.g., a link going down or coming up, a neighbor adjacency being established or broken)

In addition to the link-state information, other information is included in the LSP, such as sequence numbers and aging information, to help manage the flooding process. This information is used by each router to determine if it has already received the LSP from another router or if the LSP has newer information than what is already contained in the link-state database. This process allows a router to keep only the most current information in its link-state database.

Eventually, all routers receive an LSP from every other link-state router in the routing area. These LSPs are stored in the link-state database.

Table 3-12 displays the link-state database content of R1.

As a result of the flooding process, R1 has learned the link-state information for each router in its routing area. Notice that R1 also includes its own link-state information in the link-state database.

With a complete link-state database, R1 can now use the database and the shortest path first (SPF) algorithm to calculate the preferred path or shortest path to each network, resulting in the SPF tree.

For example, using the link-state information from all other routers, R1 can now begin to construct an SPF tree of the network. To begin, the SPF algorithm interprets each router’s LSP to identify networks and associated costs.

The SPF algorithm then calculates the shortest paths to reach each individual network, resulting in the SPF tree as shown in Figure 3-47. R1 now has a complete topology view of the link-state area.

Each router constructs its own SPF tree independently from all other routers. To ensure proper routing, the link-state databases used to construct those trees must be identical on all routers.

The routing table also includes all directly connected networks and routes from any other sources, such as static routes. Packets are now forwarded according to these entries in the routing table.

Builds a topological map: Link-state routing protocols create a topological map, or SPF tree of the network topology. Because link-state routing protocols exchange link-states, the SPF algorithm can build an SPF tree of the network. Using the SPF tree, each router can independently determine the shortest path to every network.

Fast convergence: When receiving an LSP, link-state routing protocols immediately flood the LSP out all interfaces except for the interface from which the LSP was received. In contrast, RIP needs to process each routing update and update its routing table before flooding the routing update out other interfaces.

Event-driven updates: After the initial flooding of LSPs, link-state routing protocols only send out an LSP when there is a change in the topology. The LSP contains only the information regarding the affected link. Unlike some distance vector routing protocols, link-state routing protocols do not send periodic updates.

Hierarchical design: Link-state routing protocols use the concept of areas. Multiple areas create a hierarchical design to networks, allowing for better route aggregation (summarization) and the isolation of routing issues within an area.

Link-state protocols also have a few disadvantages compared to distance vector routing protocols:

Memory requirements: Link-state protocols require additional memory to create and maintain the link-state database and SPF tree.

Processing requirements: Link-state protocols can also require more CPU processing than distance vector routing protocols. The SPF algorithm requires more CPU time than distance vector algorithms such as Bellman-Ford, because link-state protocols build a complete map of the topology.

Bandwidth requirements: The flooding of link-state packets can adversely affect the available bandwidth on a network. This should only occur during initial startup of routers, but can also be an issue on unstable networks.

For example, in Figure 3-49, there are three separate routing domains: area 1, area 0, and area 51.

If a network in area 51 goes down, the LSP with the information about this downed link is only flooded to other routers in that area. Only those routers in area 51 need to update their link-state databases, rerun the SPF algorithm, create a new SPF tree, and update their routing tables. Routers in other areas learn that this route is down, but this is done with a type of LSP that does not cause them to rerun their SPF algorithm. Routers in other areas can update their routing tables directly.

Open Shortest Path First (OSPF) is the most popular implementation. It was designed by the Internet Engineering Task Force (IETF) OSPF Working Group. The development of OSPF began in 1987 and there are two current versions in use:

OSPFv2: OSPF for IPv4 networks (RFC 1247 and RFC 2328)

OSPFv3: OSPF for IPv6 networks (RFC 2740)

IS-IS was designed by International Organization for Standardization (ISO) and is described in ISO 10589. The first incarnation of this routing protocol was developed at Digital Equipment Corporation (DEC) and is known as DECnet Phase V. Radia Perlman was the chief designer of the IS-IS routing protocol.

IS-IS was originally designed for the OSI protocol suite and not the TCP/IP protocol suite. Later, Integrated IS-IS, or Dual IS-IS, included support for IP networks. Although IS-IS has been known as the routing protocol used mainly by ISPs and carriers, more enterprise networks are beginning to use IS-IS.

OSPF and IS-IS share many similarities and also have many differences. There are many pro-OSPF and pro-IS-IS factions who discuss and debate the advantages of one routing protocol over the other. Both routing protocols provide the necessary routing functionality.

Notice that in the topology:

R1 is the edge router that connects to the Internet. Therefore, it is propagating a default static route to R2 and R3.

R1, R2, and R3 contain discontiguous networks separated by another classful network.

R3 is also introducing a 192.168.0.0/16 supernet route.

Figure 3-51 displays the IPv4 routing table of R1 with directly connected, static, and dynamic routes.

Figure 3-53 displays one of the routing table entries on R1 for the directly connected network 172.16.1.0. These entries were automatically added to the routing table when the GigabitEthernet 0/0 interface was configured and activated.

The entries contain the following information:

Route source: Identifies how the route was learned. Directly connected interfaces have two route source codes. C identifies a directly connected network. Directly connected networks are automatically created whenever an interface is configured with an IP address and activated. L identifies that this is a local route. Local routes are automatically created whenever an interface is configured with an IP address and activated.

Destination network: The address of the remote network and how that network is connected.

Outgoing interface: Identifies the exit interface to use when forwarding packets to the destination network.

A router typically has multiple interfaces configured. The routing table stores information about both directly connected and remote routes. As with directly connected networks, the route source identifies how the route was learned. For instance, common codes for remote networks include:

S: Identifies that the route was manually created by an administrator to reach a specific network. This is known as a static route.

D: Identifies that the route was learned dynamically from another router using the EIGRP routing protocol.

O: Identifies that the route was learned dynamically from another router using the OSPF routing protocol.

R: Identifies that the route was learned dynamically from another router using the RIP routing protocol.

The entry identifies the following information:

Route source: Identifies how the route was learned.

Destination network: Identifies the address of the remote network.

Administrative distance: Identifies the trustworthiness of the route source.

Administrative Distance

Metric: Identifies the value assigned to reach the remote network. Lower values indicate preferred routes.

Metric

Next hop: Identifies the IPv4 address of the next router to forward the packet to.

Next Hop Address

Route timestamp: Identifies from when the route was last heard.

Outgoing interface: Identifies the exit interface to use to forward a packet toward the final destination.

Lesson 7: Routing Protocol Characteristics

The Cisco IP routing table is not a flat database. The routing table is actually a hierarchical structure that is used to speed up the lookup process when locating routes and forwarding packets. Within this structure, the hierarchy includes several levels.

Routes are discussed in terms of:

Ultimate route

Level 1 route

Level 1 parent route

Level 2 child routes

In Figure 3-56, the highlighted areas are examples of ultimate routes. Notice that all of these routes specify either a next-hop IPv4 address or an exit interface.

Network route: A network route has a subnet mask equal to that of the classful mask.

Supernet route: A supernet route is a network address with a subnet mask less than the classful mask, for example, a summary address.

Default route: A default route is a static route with the address 0.0.0.0/0.

The source of the level 1 route can be a directly connected network, static route, or a dynamic routing protocol.

Figure 3-57 highlights how level 1 routes are also ultimate routes.

Figure 3-58 highlights level 1 routes.

Figure 3-60 highlights the level 1 parent routes in the routing table of R1. The routing table basically provides a heading for the specific subnets it contains. Each entry displays the classful network address, the number of subnets, and the number of different subnet masks that the classful address has been subdivided into.

A level 1 parent route contains level 2 child routes, as shown in Figure 3-62.

Like a level 1 route, the source of a level 2 route can be a directly connected network, a static route, or a dynamically learned route. Level 2 child routes are also ultimate routes.

Figure 3-63 highlights the level 2 child routes in the routing table of R1.

In Figure 3-64, the router examines level 1 network routes for the best match with the destination address of the IPv4 packet.

Specifically, the router proceeds as follows:

1. If the best match is a level 1 ultimate route, then this route is used to forward the packet.

2. If the best match is a level 1 parent route, proceed to the next step.

In Figure 3-65, the router examines child routes (the subnet routes) of the parent route for a best match.

3. If there is a match with a level 2 child route, that subnet is used to forward the packet.

4. If there is not a match with any of the level 2 child routes, proceed to the next step.

In Figure 3-66, the router continues searching level 1 supernet routes in the routing table for a match, including the default route, if there is one.

5. If there is now a lesser match with a level 1 supernet or default routes, the router uses that route to forward the packet.

6. If there is not a match with any route in the routing table, the router drops the packet.

For there to be a match between the destination IPv4 address of a packet and a route in the routing table, a minimum number of far left bits must match between the IPv4 address of the packet and the route in the routing table. The subnet mask of the route in the routing table is used to determine the minimum number of far left bits that must match. Remember that an IPv4 packet only contains the IPv4 address and not the subnet mask.

The best match is the route in the routing table that has the most number of far left matching bits with the destination IPv4 address of the packet. The route with the greatest number of equivalent far left bits, or the longest match, is always the preferred route.

In Figure 3-67, a packet is destined for 172.16.0.10.

The router has three possible routes that match this packet: 172.16.0.0/12, 172.16.0.0/18, and 172.16.0.0/26. Of the three routes, 172.16.0.0/26 has the longest match and is therefore chosen to forward the packet. Remember, for any of these routes to be considered a match there must be at least the number of matching bits indicated by the subnet mask of the route.

Because IPv6 is classless by design, all routes are effectively level 1 ultimate routes. There is no level 1 parent of level 2 child routes.

The topology displayed in Figure 3-68 is used as the reference topology for this section.

Notice that in the topology:

R1, R2, and R3 are configured in a full mesh topology. All routers have redundant paths to various networks.

R2 is the edge router and connects to the ISP; however, a default static route is not being advertised.

EIGRP for IPv6 has been configured on all three routers.

Figure 3-70 highlights the connected network and local routing table entries of the directly connected interfaces. The three entries were added when the interfaces were configured and activated.

As shown in Figure 3-71, directly connected route entries display the following information:

Route source: Identifies how the route was learned. Directly connected interfaces have two route source codes (C identifies a directly connected network while L identifies that this is a local route).

Directly connected network: The IPv6 address of the directly connected network.

Administrative distance: Identifies the trustworthiness of the route source. IPv6 uses the same distances as IPv4. A value of 0 indicates the best, most trustworthy source.

Metric: Identifies the value assigned to reach the remote network. Lower values indicate preferred routes.

Outgoing interface: Identifies the exit interface to use when forwarding packets to the destination network.

Figure 3-73 displays a routing table entry on R1 for the route to remote network 2001:DB8:CAFE:3::/64 on R3.

The entry identifies the following information:

Route source: Identifies how the route was learned. Common codes include O (OSPF), D (EIGRP), R (RIP), and S (Static route).

Destination network: Identifies the address of the remote IPv6 network.

Administrative distance: Identifies the trustworthiness of the route source. IPv6 uses the same distances as IPv4.

Metric: Identifies the value assigned to reach the remote network. Lower values indicate preferred routes.

Next hop: Identifies the IPv6 address of the next router to forward the packet to.

Outgoing interface: Identifies the exit interface to use to forward a packet toward the final destination.

When an IPv6 packet arrives on a router interface, the router examines the IPv6 header and identifies the destination IPv6 address. The router then proceeds through the following router lookup process.

The router examines level 1 network routes for the best match with the destination address of the IPv6 packet. Just like IPv4, the longest match is the best match. For example, if there are multiple matches in the routing table, the router chooses the route with the longest match. A match is made by matching the far left bits of the packet’s destination IPv6 address with the IPv6 prefix and prefix-length in the IPv6 routing table.

Dynamic routing protocols are used by routers to facilitate the exchange of routing information between routers. The purpose of dynamic routing protocols includes: discovery of remote networks, maintaining up-to-date routing information, choosing the best path to destination networks, and ability to find a new best path if the current path is no longer available. While dynamic routing protocols require less administrative overhead than static routing, they do require dedicating part of a router’s resources for protocol operation, including CPU time and network link bandwidth.

Networks typically use a combination of both static and dynamic routing. Dynamic routing is the best choice for large networks and static routing is better for stub networks.

Routing protocols are responsible for discovering remote networks, as well as maintaining accurate network information. When there is a change in the topology, routing protocols propagate that information throughout the routing domain. The process of bringing all routing tables to a state of consistency, where all of the routers in the same routing domain or area have complete and accurate information about the network, is called convergence. Some routing protocols converge faster than others.

Routing protocols can be classified as either classful or classless, as distance vector or link-state, and as an Interior Gateway Protocol or an Exterior Gateway Protocol.

Distance vector protocols use routers as “sign posts” along the path to the final destination. The only information a router knows about a remote network is the distance or metric to reach that network and which path or interface to use to get there. Distance vector routing protocols do not have an actual map of the network topology.

A router configured with a link-state routing protocol can create a complete view or topology of the network by gathering information from all of the other routers.

Metrics are used by routing protocols to determine the best path or shortest path to reach a destination network. Different routing protocols may use different metrics. Typically, a lower metric means a better path. Metrics can be determined by hops, bandwidth, delay, reliability, and load.

Routers sometimes learn about multiple routes to the same network from both static routes and dynamic routing protocols. When a router learns about a destination network from more than one routing source, Cisco routers use the administrative distance value to determine which source to use. Each dynamic routing protocol has a unique administrative value, along with static routes and directly connected networks. The lower the administrative value, the more preferred the route source. A directly connected network is always the preferred source, followed by static routes and then various dynamic routing protocols.

The show ip protocols command displays the IPv4 routing protocol settings currently configured on the router. For IPv6, use show ipv6 protocols.

With link-state routing protocols such as OSPF, a link is an interface on a router. Information about the state of those links is known as link-states. All link-state routing protocols apply Dijkstra’s algorithm to calculate the best path route. The algorithm is commonly referred to as the shortest path first (SPF) algorithm. This algorithm uses accumulated costs along each path, from source to destination, to determine the total cost of a route.

Class Activity 3.6.1.1: IPv6 Details, Details...

Packet Tracer Activity 3.2.2.4: Comparing RIP and EIGRP Path Selection

Packet Tracer Activity 3.3.1.8: Configuring RIPv2

Packet Tracer Activity 3.3.2.3: Configuring RIPng

1. What are two advantages of static routing over dynamic routing? (Choose two.)

A. The configuration is less error prone.

B. Static routing is more secure because routers do not advertise routes.

C. Growing the network usually does not present a problem.

D. No computing overhead is involved.

E. The administrator has less work maintaining the configuration.

2. Match the description to the proper routing protocol.

Routing protocols:

RIP

IGRP

OSPF

EIGRP

BGP

Description:

A. Path vector exterior routing protocol:

B. Cisco advanced interior routing protocol:

C. Link-state interior routing protocol:

D. Distance vector interior routing protocol:

E. Cisco distance vector interior routing protocol:

3. Which statement best describes convergence on a network?

A. The amount of time required for routers to share administrative configuration changes, such as password changes, from one end of a network to the other end

B. The time required for the routers in the network to update their routing tables after a topology change has occurred

C. The time required for the routers in one autonomous system to learn routes to destinations in another autonomous system

D. The time required for routers running disparate routing protocols to update their routing tables

4. Dynamic routing protocols perform which two tasks? (Choose two.)

A. Assign IP addressing

B. Discover hosts

C. Network discovery

D. Propagate host default routes

E. Update and maintain routing tables

5. Which of the following parameters are used to calculate metrics? (Choose two.)

A. Hop count

B. Uptime

C. Bandwidth

D. Convergence time

E. Administrative distance

6. Which routing protocol has the most trustworthy administrative distance by default?

A. EIGRP internal routes

B. IS-IS

C. OSPF

D. RIPv1

E. RIPv2

7. Which command will show the administrative distance of routes?

A. show interfaces

B. show ip route

C. show ip interfaces

D. debug ip routing

8. When do directly connected networks appear in the routing table?

A. When they are included in a static route

B. When they are used as an exit interface

C. As soon as they are addressed and operational at Layer 2

D. As soon as they are addressed and operational at Layer 3

E. Always when a no shutdown command is issued

9. Router R1 is using the RIPv2 routing protocol and has discovered multiple unequal paths to reach a destination network. How will Router R1 determine which path is the best path to the destination network?

A. Lowest metric

B. Highest metric

C. Lowest administrative distance

D. Highest administrative distance

E. By load-balancing between up to four paths

10. Which of the following will trigger the sending of a link-state packet by OSPF? (Choose two.)

A. A change in the topology

B. A link to a neighbor router has become congested

C. The initial startup of the routing protocol process

D. The router update timer expiring

11. After examining its routing table for a best match with the destination address, which route will a router use to forward an IPv4 packet?

A. A level 1 child route

B. A level 1 parent route

C. A level 1 ultimate route

D. A level 2 supernet route

E. A level 2 parent route

12. What is different between IPv6 routing table entries and IPv4 routing table entries?

A. By design IPv6 is classless, so all routes are effectively level 1 ultimate routes.

B. Unlike IPv4, IPv6 does not use static routes to populate the routing table.

C. IPv6 routing tables include local route entries, which IPv4 routing tables do not.

D. The selection of IPv6 routes is based on the shortest matching prefix, unlike IPv4 route selection, which is based on the longest matching prefix.

13. Enter the proper administrative distance for each routing protocol.

A. eBGP:

B. EIGRP (Internal):

C. EIGRP (External):

D. IS-IS:

E. OSPF:

F. RIP:

14. Designate the following characteristics as belonging to either a classful routing protocol or a classless routing protocol.

A. Does not support discontiguous networks:

B. EIGRP, OSPF, and BGP:

C. Sends subnet mask information in routing updates:

D. Supports discontiguous networks:

E. RIP version 1 and IGRP:

F. Does not send subnet mask in its routing updates:

G. Allows for use of both 172.16.1.0/26 and 172.16.1.128/27 subnets in the same topology:

15. Explain why static routing might be preferred over dynamic routing.

16. What are four ways of classifying dynamic routing protocols?

17. What are the most common metrics used in IP dynamic routing protocols?

18. What is administrative distance, and why is it important?

19. What is the purpose of a passive interface?