Question

Given a network topology of 3 or more routers with bandwidth designations, compare the metrics used during the route selection process and the resulting routing tables generated by the dual algorithm and static routing.

Answer 1

Routes are chosen and built in the routing table based on the routing protocol's administrative distance. The routes learned from the routing protocol with the lowest administrative distance are installed in the routing table. If there are multiple paths to the same destination from a single routing protocol, then the multiple paths would have the same administrative distance and the best path is selected based on the metrics. Metrics are values associated with specific routes, ranking them from most preferred to least preferred. The parameters used to determine the metrics differ for different routing protocols. The path with the lowest metric is selected as the optimal path and installed in the routing table. If there are multiple paths to the same destination with equal metrics, load balancing is done on these equal-cost paths. For more information on load balancing see How Does Load Balancing Work?

Prefix Lengths

Let's look at another scenario to see how the router handles another common situation: varying prefix lengths. Assume, again, that a router has four routing processes running on it, and each process has received these routes:

• EIGRP (internal): 192.168.32.0/26

• RIP: 192.168.32.0/24

• OSPF: 192.168.32.0/19

Which of these routes will be installed in the routing table? Since EIGRP internal routes have the best administrative distance, it's tempting to assume the first one will be installed. However, since each of these routes has a different prefix length (subnet mask), they're considered different destinations, and they will all be installed in the routing table.

Let's see how the forwarding engine uses the information from the routing table to make forwarding decisions.

Making Forwarding Decisions

Let's look at the three routes we just installed in the routing table, and see how they look on the router.

router# show ip route
     ....
     D   192.168.32.0/26 [90/25789217] via 10.1.1.1
     R   192.168.32.0/24 [120/4] via 10.1.1.2
     O   192.168.32.0/19 [110/229840] via 10.1.1.3
     ....

If a packet arrives on a router interface destined for 192.168.32.1, which route would the router choose? It depends on the prefix length or the number of bits set in the subnet mask. Longer prefixes are always preferred over shorter ones when forwarding a packet.

In this case, a packet destined to 192.168.32.1 is directed toward 10.1.1.1, because of 192.168.32.1 falls within the 192.168.32.0/26 network (192.168.32.0 to 192.168.32.63). It also falls within the other two routes available, but 192.168.32.0/26 has the longest prefix within the routing table (26 bits versus 24 or 19 bits).

Likewise, if a packet destined for 192.168.32.100 arrives on one of the router's interfaces, it's forwarded to 10.1.1.2, because 192.168.32.100 doesn't fall within 192.168.32.0/26 (192.168.32.0 through 192.168.32.63), but it does fall within the 192.168.32.0/24 destination (192.168.32.0 through 192.168.32.255). Again, it also falls into the range covered by 192.168.32.0/19, but 192.168.32.0/24 has a longer prefix length.

IP Classless

Where the IP classless configuration command falls within the routing and forwarding processes is often confusing. In reality, IP classless only affects the operation of the forwarding processes in IOS; it doesn't affect the way the routing table is built. If IP classless isn't configured (using the no IP classless command), the router won't forward packets to supernets. As an example, let's again place three routes in the routing table and route packets through the router.

Note: If the supernet or default route is learned via IS-IS or OSPF, the no IP classless configuration command is ignored. In this case, packet switching behavior works as though IP classless were configured.

router# show ip route
....
     172.30.0.0/16 is variably  subnetted, 2 subnets, 2 masks
D        172.30.32.0/20 [90/4879540] via  10.1.1.2
D       172.30.32.0/24  [90/25789217] via 10.1.1.1
S*   0.0.0.0/0 [1/0] via 10.1.1.3  

Remembering that the 172.30.32.0/24 network includes the addresses 172.30.32.0 through 172.30.32.255, and the 172.30.32.0/20 network includes the addresses 172.30.32.0 through 172.30.47.255, we can then try switching three packets through this routing table and see what the results are.

• A packet destined to 172.30.32.1 is forwarded to 10.1.1.1 since this is the longest prefix match.

• A packet destined to 172.30.33.1 is forwarded to 10.1.1.2 since this is the longest prefix match.

• A packet destined to 192.168.10.1 is forwarded to 10.1.1.3; since this network doesn't exist in the routing table, this packet is forwarded to the default route.

• A packet destined to 172.30.254.1 is dropped.

The surprising answer out of these four is the last packet, which is dropped. It's dropped because its destination, 172.30.254.1, is within a known major network, 172.30.0.0/16, but the router doesn't know about this particular subnet within that major network.

This is the essence of classful routing: If one part of a major network is known, but the subnet toward which the packet is destined within that major network is unknown, the packet is dropped.

The most confusing aspect of this rule is that the router only uses the default route if the destination major network doesn't exist in the routing table at all.

This can cause problems in a network where a remote site, with one connection back to the rest of the network, is running no routing protocols, as illustrated.

The remote site router is configured like this:

interface Serial 0
     ip address 10.1.2.2 255.255.255.0
   !
   interface Ethernet 0
     ip address 10.1.1.1 255.255.255.0
   !
   ip route 0.0.0.0 0.0.0.0 10.1.2.1
   !
   no ip classless