Topic: Describe routing concepts

Let’s begin with a discussion of how routing really works. You know a packet enters a router, and a routing lookup is done, but what does this really mean? What are the details of the packet-handling process?

When an IPv4 packet arrives on a router interface, the router de-encapsulates the Layer 2 frame and examines the Layer 3 IPv4 header. The router identifies the destination IPv4 address and proceeds through the route lookup process. The router scans the routing table to find a best match for the destination IPv4 address. The best match is the longest match in the table. For example, if the destination IPv4 address is 172.16.0.10, and the entries in the routing table are for 172.16.0.0/12, 172.16.0.0/18, and 172.16.0.0/26, the longest match and the entry used for the packet is 172.16.0.0/26. Remember that for any of these routes to be considered a possible match, there must be at least the number of matching bits indicated by the subnet mask of the routing table prefix. For example, the routing entries 192.168.5.0/24 and 192.168.0.0/16 are both matches for destination address 192.168.5.55, while routing entry 10.65.0.0/16 is not.

Another critical aspect to understand is the frame rewrite procedure used by the router. For the router to do its job, it needs to encapsulate the IP packet without substantially modifying the IP header (except the TTL value) into a new Layer 2 frame. The source MAC address in the outgoing frame is the MAC address of the forwarding interface of the local router. The destination MAC address is the MAC address of the receiving interface of the next hop device. An FCS is also added as the trailer. This process continues from hop to hop on Ethernet networks until the packet reaches the destination host. Even though this process essentially involves discarding the inbound L2 header and creating a brand-new L2 header, it is called an L2 header rewrite.

Topic: Interpret the components of a routing table

It is time to examine in great detail the vast amount of information communicated in a key network data structure: the IP routing table of a Cisco router. Example 14.1 shows a sample table.

Example 14.1 IP Routing Table on a Cisco Router

Click here to view code image

R1# show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B – BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
        i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
        ia - IS-IS inter area, * - candidate default, U - per-user static route
        o - ODR, P - periodic downloaded static route, + - replicated route

Gateway of last resort is not set

     10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C      10.10.10.0/24 is directly connected, FastEthernet0/0
L      10.10.10.1/32 is directly connected, FastEthernet0/0
    172.16.0.0/24 is subnetted, 3 subnets
R      172.16.1.0 [120/1] via 10.10.10.3, 00:00:19, FastEthernet0/0
R      172.16.2.0 [120/1] via 10.10.10.3, 00:00:19, FastEthernet0/0
R      172.16.3.0 [120/1] via 10.10.10.3, 00:00:19, FastEthernet0/0
    192.168.1.0/32 is subnetted, 1 subnets
O      192.168.1.2 [110/2] via 10.10.10.2, 00:00:37, FastEthernet0/0
    192.168.2.0/32 is subnetted, 1 subnets
O      192.168.2.2 [110/2] via 10.10.10.2, 00:00:37, FastEthernet0/0
R1#

Specifically, in this output, it is important to master the meanings and locations of the following components:

• ▸ Prefix: Notice that the routing table lists the parent and child prefixes that are reachable in the table. For example, in the table shown in Example 14.1, the entry 172.16.0.0/24 is subnetted, 3 subnets lists the classful network prefix, the subnet length for all subnets, and the specific subnet prefixes 172.16.1.0, 172.16.2.0, and 172.16.3.0. In the case where a classful network is variably subnetted, the classful network prefix length is shown, using CIDR notation (which is the case with the first network shown: 10.0.0.0/8).

• ▸ Network mask: Notice that the parent prefix lists the network mask in prefix notation. So for the 172.16.0.0 example above, the network mask is /24. Remember that in subnet mask notation, this is 255.255.255.0.

• ▸ Next hop: The next hop IP address follows the via word for a subnet prefix entry. Note that it is 10.10.10.3 for the 172.16.0.0/24 entries. The next hop refers to the IP address of the next router in the path when forwarding packets to a remote destination.

• ▸ Routing protocol code: Located at the very beginning of a routing table entry is the routing protocol code. Cisco IOS is kind enough to provide a legend at the beginning of the show output to explain what each value indicates:

  • ▸ L: Local

  • ▸ C: Connected

  • ▸ S: Static

  • ▸ R: RIP

  • ▸ M: Mobile

  • ▸ B: BGP

  • ▸ D: EIGRP

  • ▸ EX: EIGRP External

  • ▸ O: OSPF

  • ▸ IA: OSPF inter-area

  • ▸ N1: OSPF NSSA External Type 1

  • ▸ N2: OSPF NSSA External Type 2

  • ▸ E1: OSPF External Type 1

  • ▸ E2: OSPF External Type 2

  • ▸ i: IS-IS

  • ▸ su: IS-IS summary

  • ▸ L1: IS-IS Level 1

  • ▸ L2: IS-IS Level 2

  • ▸ ia: IS-IS inter-area

  • ▸ *: Candidate default

  • ▸ U: Per-user static route

  • ▸ o: ODR

  • ▸ P: Periodic downloaded static route

  • ▸ +: Replicated route

• ▸ Administrative distance: The routing table shows the administrative distance (AD) for the prefix. (AD is covered in more detail later in this chapter.) In Example 14.1, the AD associated with the 172.16.0.0/24 prefixes is 120. This is because these routes were learned through RIP, and 120 is the default administrative distance for RIP.

• ▸ Metric: The metric varies depending on the dynamic routing protocol involved. It is a measure of the “distance” to reach the prefix. In the 172 prefixes, the metric is a hop count. RIP uses this simple metric, which indicates how many routers you must cross to reach the destination prefix in question.

• ▸ Gateway of last resort: Notice that Example 14.1 indicates that there is no gateway of last resort set. This means there is no default route 0.0.0.0/0 that allows the router to send traffic somewhere if it does not have a specific prefix entry for the destination IP address. The gateway of last resort can be dynamically learned, or it can be set using any of three different commands: ip default-gateway, ip default-network, and ip route 0.0.0.0 0.0.0.0.

Topic: Describe how a routing table is populated by different routing information sources

What happens when multiple different routing sources indicate that they know how to reach a network/prefix? The router needs to be able to break this “tie” between routing information sources. To do so, the router uses administrative distance (AD). This can be a bit of a misleading term since the value has nothing to do with actual distance of any kind. Some administrators like to call it administrative trustworthiness. Cisco ranks the trustworthiness of the various routing information sources. A lower score is better, just as in golf and routing protocol metrics.

Table 14.1 shows the default administrative distance values. As you can see, Cisco rated its own protocols, IGRP and EIGRP, as being very trustworthy.

Review Questions

1. If a router cannot find a better match, what might the router use to route traffic?

A. 255.255.255.255/0

B. 127.0.0.1/32

C. 0.0.0.0/32

D. 0.0.0.0/0

2. When a router forwards packets onto Ethernet, what is rewritten?

A. The source and destination IP addresses

B. Only the source IP address

C. Only the source MAC address

D. The source and destination MAC addresses

3. What does the routing protocol code B indicate in the routing table?

A. EIGRP

B. IGRP

C. RIP

D. OSPF

E. BGP

4. What is the administrative distance of Internal EIGRP routes?

A. 5

B. 20

C. 90

D. 100

5. What is the unreachable AD?

A. 0

B. 100

C. 200

D. 255

Answers to Review Questions

1. D is correct. Routers use the default route (0.0.0.0/0) to send packets that have no other better match in the routing table.

2. D is correct. The router must rewrite the source and destination MAC addresses in the Ethernet header before forwarding a frame.

3. E is correct. B indicates BGP.

4. C is correct. Internal EIGRP routes are installed in the routing table with an AD of 90.

5. D is correct. An AD of 255 indicates an unreachable prefix.

Additional Resources

Routing Tables

https://youtu.be/zeArDrC2xPg

What Is Administrative Distance?

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/15986-admin-distance.html