Examining the Cisco IP Routing Table

Routing tables are generated by devices learning new remote networks using some form of a routing protocol. Routing tables are used by routers, for example, to make intelligent decisions regarding where packets should be sent so that user data is sent as efficiently as possible. Hence, one of the most common IOS commands used on a Cisco router is to display a routing table. The command to display the IP routing table on a Cisco router is:

show ip route

This command can also be written as:

sh ip ro

The Internet routing table consists of almost 70,000 IP routing entries. Listing 2.3 displays a sample IP routing table.

Listing 2.3 An IP routing table.

R1>show ip route

Codes: C - connected, S - static, I - IGRP, 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, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate defaultU - per-user static route, o - ODR Gateway of last resort is not set

10.0.0.0/32 is subnetted, 1 subnets C 10.1.1.1 is directly connected, Loopback0

137.10.0.0/16 is variably subnetted, 4 subnets, 3 masks D 137.10.255.0/24 [90/2681856] via 137.10.253.2, 1w1d,S0 C 137.10.253.0/24 is directly connected, Serial0 D 137.10.17.0/28 [90/688128] via 137.10.253.2, 1w1d, S0 D 137.10.16.0/27 [90/793600] via 137.10.253.2, 1w1d, S0

The first half of Listing 2.3 summarizes the codes used to identify how networks have been learned dynamically, statically, or from directly connected networks (for example, those networks assigned directly to a router's interface).

Note that directly connected networks are identified on the left side as C, and D represents EIGRP discovered networks. IPX and AppleTalk maintain similar tables. The IP table lists the remote network, the next hop and metric, and how long the route has been valid. No layer 2 information, like MAC addresses, is listed in the routing table.

You must familiarize yourself with IP routing tables. An IP routing table displays how remote networks are reachable. A switch or bridge will maintain a layer 2 table called a bridge table or content addressable memory (CAM) table, which lists layer 2 information only, such as MAC addresses.

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