Objectives:

Suppose that in Figure 10.1 the traffic was a ping packet to test connectivity to the remote computer. If for some reason that ping failed, you would need to determine where the problem occurred. One method of testing the failure would be to make sure that you have connectivity to and from the originating PC and the default gateway.

The default gateway on a computer can be assigned manually in the operating system or dynamically from a DHCP server. To ensure that the local computer has its IP and default gateway configured correctly, you need to look at the local PC’s configuration. This step differs depending on the operating system installed on the originating computer. Let’s assume for this example that the computer is using Windows as its operating system. You would need to go to a command prompt and enter ipconfig to see how the computer’s interface(s) are configured for IP. In Figure 10.2, you can see from the output on the computer that the computer has been dynamically assigned an IP address of 192.168.1.2 and a subnet mask of 255.255.255.0. Notice also that the default gateway is also configured correctly, pointing to the local router’s Fast Ethernet 0/0 IP address of 192.168.1.1.

Figure 10.2. Windows computer gateway verification.

To seal the troubleshooting deal, notice that we went so far as to verify that the computer has a correct IP-to-MAC address binding for the default gateway by issuing the arp -a command from the command prompt to display the computer’s ARP table. You can see from the output that the default gateway’s IP address of 192.168.1.1 does correspond to its Fast Ethernet 0/0 MAC address. This ARP entry will continue to remain in the computer’s volatile memory as long as it keeps getting used. If no packets are sent to this address for 5 minutes (default), the ARP entry is removed. If you want to manually clear your ARP table on your computer (useful in cases where you recently changed your default gateway), the command is arp-d followed by the IP address or a wildcard (*) to delete all ARP entries.

Seeing that the configuration of the originating PC is correct, next we should see if we have IP connectivity to the router’s Fast Ethernet interfaces. Because this is a small network, we can simply ping those IP addresses. In larger networks with many routers in between, it might be administratively easier to do a traceroute (the tracert command in Windows) to discover at what point along the routed path the ICMP packet fails.

So what happens if we can ping the two interfaces? At this point, we know that a packet can traverse our network, reach the default gateway, and exit the router’s remote network interface. The only culprit left is the remote PC. It is a pretty sure bet that the remote computer must have an interface problem or configuration error. Keep in mind that it too must return that packet to our network. Therefore, the remote computer must also have a default gateway configured so that it can return that packet to its remote network. Using the same steps as we used for the originating computer, we should be able to determine exactly what is causing this computer to not successfully return packets to our computer.

Now that you are aware of the multiple sources of routing information, you must consider a feasible anomaly that could occur with your routing sources. Namely, if you have several sources of information such as connected interfaces, static routes, and multiple routing protocols, which one are you to trust when more than one source advertises the same network? For example, if a router learns about the 192.168.1.0/24 network from a routing protocol and a static route, how does the router decide which entry to place into its routing table?

The answer lies within a program logic in the IOS called the administrative distance. The administrative distances are values between 1 and 255 that are assigned to routing information sources. These values represent a level of trustworthiness of the information source, in which lower administrative distances are preferred over higher ones.

Table 10.1 lists the Cisco IOS default administrative distances for some of the routing sources.

It should come as no shock that connected interfaces are the most trustworthy sources because they are connected directly to the local router. Static routes have a low administrative distance of 1 because the Cisco IOS assumes that you are competent administrators and any manual entry of a routable network is trusted over any dynamic routing protocols such as EIGRP, OSPF, and RIP.

The general idea behind the static route is to tell the router how to get to a destination network that is not attached to it by going through another router’s interface. It is similar to telling someone, “To go outside, go through that door.” The syntax to configure a static route in global configuration mode is ip route followed by the destination network, destination subnet mask, and the next-hop IP address of the neighbor’s interface. For example, to configure a route to the 10.0.0.0/8 network through the neighbor’s serial interface of 192.168.2.5, the command would look like this:

Router(config)# ip route 10.0.0.0 255.0.0.0 192.168.2.5

In the stub network example shown in Figure 10.4, a static route to Router A and one to Router B were added, telling them about their neighbors’ Ethernet networks. These entries are placed in their routing tables, specifying any packets that are destined for those respective networks must go to the IP of the neighbor’s serial 0/0 interface. From that point, the packets are routed out Router A and Router B’s Ethernet interface because those destination networks are directly connected to the router. This entry remains in the routing table as long as the next-hop address remains valid (the serial network does not go down) or the configuration is not removed.

Figure 10.4. Static route configuration example.

For several reasons such as security, processor resources, and routing path control, you can force the static route to remain in the routing table even if the next-hop interface goes down. This is easily achieved by adding the keyword permanent at the end of the ip route command.

At the end of an ip route static route command, it is possible to add a parameter to assign this particular static route a higher administrative distance than the default administrative distance of 1. These entries, known as floating static routes, are not placed in the routing table if the subnet is being advertised by a routing source with a lower administrative distance. Floating static routes are useful when you have a standby redundant link to another network that will activate in the event of a primary link failure.

For example, consider the example configured in Figure 10.5. Because you have redundant point-to-point links, you can configure the primary static route as usual and include a floating static route to be used if the primary link fails. The 2 at the end of the second static route identifies that route as the floating static route. This entry does not show up in the routing table because the primary route advertises the same subnet with a lower administrative distance (if not specified, the default is 1). If one of the serial 0/0 interfaces on the primary link goes down, the next hop is no longer valid and is removed from the routing table. Because the floating static route has the next lowest administrative distance, that entry is put in the routing table and that link is used until the primary link returns.

Figure 10.5. Floating static route configuration example.

When complex networks contain multiple interconnections, static routes are no longer a practical solution because they cannot adapt or react to changes in the topology. Not to mention, the configuration complexity can grow exponentially as you add more devices to the network.

For example, given the network design in shown Figure 10.8, Router A knows only the three directly connected networks attached to the router. For IP packets to reach the 172.17.0.0 network via static routes, you would have to configure Router A to go through one of its neighbor routers, such as Router B. However, Router B also requires a static route to 172.17.0.0 because that network is not directly connected to it. Supposing that the router is using Router F as its next hop, that router does not require a static route because it is directly connected. Unfortunately, our configuration undertaking does not stop there because we have to configure a static route in Router F and Router B back to the 172.16.0.0 network.

Figure 10.8. Complex internetwork design.

Granted, the configuration scenario shown in Figure 10.8 is not drastically difficult or strenuous, but imagine if the network contained 20 more routers. More importantly, consider what would happen if the link between A and B went down. Because the routes are statically configured in the routers, you must now go back and remove the static routes in Routers A and B and redirect the traffic by configuring static routes to go through Routers C, D, and E. Not to mention, you must remove and reconfigure static routes back to the 172.16.0.0 network in Routers F, E, D, and C.

To alleviate the administrative calamity you might have to encounter with static routes in complex networks, you can use dynamic routing protocols. If you configure routing protocols, the routers advertise their connected networks to the rest of the routers in the network, thus minimizing the amount of configuration required. In addition, routing protocols can detect and adapt to topology changes in the internetwork.

Because one of the core responsibilities of routing protocols is to build routing tables to determine optimal routing paths, you need to have some means of measuring which routes are preferred when there are multiple pathways to a destination. Routing protocols use some measure of metrics to identify which routes are optimal to reach a destination network. The lowest cumulative metric to a destination is the preferred path and the one that ultimately enters the routing table. Different routing protocols use one or several of the following metrics to calculate the best path:

As you will see in the following sections, routing protocols are categorized into several different classifications based on common characteristics and properties that they share. The first of these classifications revolves around the contents of routing updates that Layer 3 devices advertise to their neighbors. Specifically, if the routing updates do not contain the subnet mask along with their respective advertised networks, they are said to be classful routing protocols. Conversely, if the subnet mask is transmitted along with the network information, it is characterized as a classless routing protocol. This may seem like a trivial characteristic to define routing protocols, but as you will see, the results of the subnet mask being present or not in routing updates can affect the routing protocols you choose and how efficiently you can design your entire network.

With classful routing protocols, you assume that your network’s design conforms to the class boundaries of IP subnets. In other words, major networks in your design use their default classful subnet masks as described in Chapter 5, “Implementing IP Addressing” (for example, Class A uses 255.0.0.0, Class B uses 255.255.0.0, and Class C uses 255.255.255.0). If you happen to subnet a major network into smaller subnets, classful routing protocols are disadvantageous, because they do not receive the revised subnet mask. For this reason, classful routing protocols process updates in one of two ways:

For example, when Router B in Figure 10.9 sends an update to Router A using a classful routing protocol (RIPv1 in this example), it summarizes the 172.17.30.0/24 to a default Class B 172.17.0.0 network because it is going out the serial 0/0 interface, which does not contain a subnet in that major network. The 192.168.1.60 network, on the other hand, is in the same major network, so it does not automatically summarize that subnet. When Router A receives that update, it adds the 172.17.0.0 network to its routing table, specifying Router B’s serial interface IP (192.168.1.10) as the next hop to reach that network. In addition, it adds the 192.168.1.60 network as well, using its interface mask because it is in that same major network.

Figure 10.9. Classful routing update scenario.

Now consider if the Ethernet segment of Router B’s 192.168.1.60 network had a /29 subnet mask. The end result would still be the same as before in that Router B would advertise the 192.168.1.60 subnet and Router A would use its interface’s subnet mask of /30. In these instances, classful routing protocols are not the optimal choice because Router A has a route to only a third of the 192.168.1.60 /29 subnet. For this reason, when you subnet a major network, you must be sure that you use the same subnet mask throughout your network design with classful routing protocols. This same subnet design is commonly referred to as a Fixed-Length Subnet Mask (FLSM) network design.

Classful routing protocols can also be problematic when major classful networks are subnetted and are haphazardly dispersed throughout the network, as illustrated in Figure 10.10. When Routers A and C summarize their networks to Router B, Router B thinks that the 172.16.0.0 network is out both of its serial interfaces. This could easily result in traffic destined for 172.16.10.0 and 172.16.50.0 being load balanced out each interface, resulting in 50% packet loss because the packets are sent in a round robin fashion between both interfaces.

Figure 10.10. Discontiguous network example.

Because classless routing protocols advertise the subnet masks in their routing updates, discontiguous networks are no longer an issue because routing devices are aware of the subnetted networks. In addition, the requirement of using the same subnet mask throughout the network ceases to apply because the routers do not automatically summarize the networks to a classful boundary. No longer inhibited by these constraints, you are free to use different subnet masks, known as Variable-Length Subnet Masks (VLSMs) in your network design. In addition, you now have full autonomy to manually summarize networks as you wish to help keep routing tables small to conserve resources. VLSM and route summarization are described in greater detail in the following sections.

Using classless routing protocols affords you the luxury of having support for a VLSM network design. This is advantageous in your network planning because you can allot the appropriate number of IP addresses required for each link. Not to mention, by assigning the minimal number of IP addresses required for a given link, you conserve IP addresses. For example, you can use a /30 subnet mask for point-to-point links because you need only two available IP addresses and a /27 subnet mask on an Ethernet segment to accommodate 30 hosts. If you were using classless routing protocols, you would have to use a /27 for all links, which would inevitably waste 28 IP addresses on the point-to-point links.

Throughout your certification and career, it is quite possible you will have to design your network given a usable subnet and host requirements for all your links. When tackling this designing task, be sure to adhere to the following guidelines:

Challenge

Given the design shown in Figure 10.11, determine how you can use VLSM to ensure that you are using the appropriate subnets given a design scenario. In this example, the zero subnets are available for use. Figure 10.11. VLSM design scenario. Remember to add 2 to each network to accommodate the network ID and broadcast address. Calculate the network and subnet mask for Network D (the largest network). Given the remaining IP addresses, calculate the network and subnet mask for Network C. Given the remaining IP addresses, calculate the network and subnet mask for Network B. Given the remaining IP addresses, calculate the network and subnet mask for Network E. Given the remaining IP addresses, calculate the network and subnet mask for the four point-to-point links.

<division> <title>Challenge Answer</title>

Network D needs to have a subnet that accommodates 128 addresses. A subnet mask of 255.255.255.128 or /25 provides enough hosts for that network. The next subnet for Network C begins at 192.168.1.128 (because you just took 128 IPs) and use a subnet mask of 255.255.255.192 or /26 to accommodate the 64 IP addresses. Network B is assigned the network of 192.168.1.192 with a subnet mask of 255.255.255.224 or /27 to give you 32 IPs. A subnet mask of 255.255.255.240 or /28 with a network ID of 192.168.1.224 is assigned to Network E for the 16 IPs required for that link. The four point-to-point links all use a 255.255.255.252 or /30 subnet mask using the last four networks: 192.168.1.240, 192.168.1.244, 192.168.1.248, 192.168.1.252. To summarize: Network D: 192.168.1.0 /25 Network C: 192.168.1.128 /26 Network B: 192.168.1.192 /27 Network E: 192.168.1.224 /28 Point-to-point links: 192.168.1.240 /30, 192.168.1.244 /30, 192.168.1.248 /30, and 192.168.1.252 /30.

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Route Summarization

As already mentioned, classful routing protocols automatically summarize advertised networks to the classful subnet boundaries. Classless routing protocols, on the other hand, require you to manually control the networks being summarized to your neighbors in the router configuration. By aggregating a contiguous set of networks into an advertised summarized route, you keep the size of the routing tables to a minimum. Neighbors that receive the summarized route do not need to know about the individual subnets you create behind your router because they inevitably have to go through your router to get to them. The additional offshoot of this summarized picture is that your classless routing protocols do not need to notify those neighbors if one of those subnets goes down because they do not even have that subnet in their routing tables. Thus, you can isolate topology changes to be contained behind that summarizing router.

Because you are required to manually specify the networks you are to advertise, you must learn how to accurately summarize smaller subnets into one or several larger networks, or supernets. The rules for supernetting are similar to subnetting, except in this case, you are stealing bits from the network portion of an IP network to create a larger network. The rules for supernetting are as follows:

1. Be sure that the networks are contiguous (otherwise, you would be summarizing networks that you do not have behind the router).

2. Count the number of networks you want to summarize.

3. Determine an increment that is equal to or less than the number of networks.

4. Make sure your base networks start on incremental boundaries (128, 64, 32, and so on) for the number of networks you are summarizing.

5. Calculate the subnet mask by the number of bits you need to steal from the original subnet to equal that incremental value.

The beauty of supernetting is that the resultant network and subnet mask will designate many IP address networks in a single entry. The fact that you are stealing bits from the network portion of an IP address could quite easily violate the traditional barriers of classful addressing, known as Classless Interdomain Routing (CIDR).

For instance, it would not be uncommon to see a summary entry look like the following: 192.168.16.0 /20. This single entry used to be a Class C (/24), but four bits were stolen from the network portion to represent 16 networks (2⁴ = 16). When you advertise this supernet to neighbors, they know that they must go through your router to get to networks 192.168.16.0 through 192.168.31.0 (16 total networks).

Exam Alert

Be prepared to determine the networks being advertised in a given supernet or determine the summary network, given the networks to be summarized.

Figure 10.12 illustrates a typical route summarization example in which Router B is summarizing all its subnetted networks to Router A as one supernetted network. Following the steps outlined previously, you can determine the aggregate network entry to advertise, as follows:

1. The networks are all contiguous, so you can summarize them accurately.

2. A total of 32 networks need to be summarized.

3. 32 conveniently falls on an incremental boundary.

4. Because the network is 192.168.64.0, 64 is an increment of 32 so we can use that as the base network for the summary route.

5. You must steal 5 bits (2⁵ = 32) from the /24 network, so /19 (24 – 5 = 19).

Figure 10.12. Supernetting route summarization example.

By creating the summary route 192.168.64.0 /19, Router A is required to maintain only that one entry in its routing table as opposed to the individual 32 subnets. If a topology change occurs in one of the subnets behind Router B, there is no need to advertise that change to Router A because it knows about only the summarized network.

Note

The number of summarized networks or the base network do not always conveniently fall on incremental boundaries. In those instances, it may take several summary network entries to encompass all the networks you want to summarize.

Routing protocols can fall under two major categories depending on the autonomy of the network on which the routing protocol exists. The identifying characteristic of the category to which the routing protocol belongs ultimately depends on whether the routing protocols exchanges updates within a network that is under your administrative control. When the network is under your control in your own administrative domain, it is known as an autonomous system. Routing protocols used to disseminate information to maintain routing tables and establish pathways inside an autonomous system are categorized as Interior Gateway Protocols (IGPs).

Conversely, the other category of routing protocols is designed to route in between these autonomous systems. For instance, Border Gateway Protocol (BGP) is a routing protocol that is used by ISPs for routing traffic over the Internet. Because the Internet comprises thousands of networks, each under different administrative control, you need to use an Exterior Gateway Protocol such as BGP to route in between these autonomous systems.

In addition to being an IGP/EGP or classful/classless, routing protocols can also fall into one of three classes. Again, the functionality and characteristics of the routing protocol dictate under which class it falls. The most long-standing of these classes is distance vector routing protocols.

Distance vector routing protocols concern themselves with the direction (vector) in which the destination lies and some means of measurement (metric) it takes to reach that destination. Distance vector routing protocols inform their directly connected neighbors of all the connected and learned networks they know about in their routing tables. In fact, they broadcast the contents of the entire routing table to their neighbors periodically, regardless of whether there is a change in the network topology. When the neighbors receive that routing information, they identify and add any new networks to their routing tables and update the metric before eventually passing it on to their neighbors. Because the routing table information is updated before it is sent on to neighbors, downstream routers do not learn that information first hand. For this reason, distance vector routing protocol update processing is often referred to as “routing by rumor.” Distance vector routing protocols are discussed in greater detail in Chapter 11, “Distance Vector Routing Protocols.”

As the name states, link-state routing protocols advertise the state of the links in the network. In fact, they advertise the states and metrics (cost) of all the links they know about for the entire topology to their neighbors, as opposed to just the best routes in your routing table. This detailed overview of the entire routing domain enables each router to calculate and make a decision on the best route from this first-hand information, rather than listen to what its neighbor believes is the best route. In fact, link-state routing protocols keep three tables: a neighbor table of all discovered neighbors, a topology table of all the possible routes to reachable networks learned, and a routing table that contains the best route based on the lowest metric calculated from the topology table.

At first, this may sound like a lot of information to be exchanged between routers; however, link-state routing protocols initially discover their neighbors when they first boot up and synchronize their topology tables. After the neighbor discovery and topology synchronization, they send only periodic hello messages to let their neighbors know they are still functioning. This is significantly different from distance vector routing protocols that periodically exchange the entire routing table, which can contain a large amount of information, depending on the size of the network.

In addition, link-state routing protocols react much faster when a topology change occurs in the network. In fact, these protocols were initially created in response to the slow convergence issues that you typically encounter with distance vector routing protocols. The downfall to these routing protocols is the resources they consume in the router. Namely, maintaining and processing three tables consume quite a bit of memory and processor power.

They say it usually takes three tries to get something absolutely right. The truth behind this saying is that you learn from the mistakes of the previous two attempts. Such is the case with advanced distance vector, often referred to as hybrid or balanced hybrid routing protocols. Because they take the best features and avoid the pitfalls of both distance vector and link-state routing protocols, hybrid routing protocols are a more proficient breed of routing protocols than their predecessors.