Anatomy of a Routing Table | CCNP BSCI Exam Cram 2 (Exam Cram 642-801)

To summarize the process of propagating routing tables, standard distance vector protocols broadcast the entire routing table to directly connected routing devices in regular update intervals as well as in flash updates. This makes the bandwidth overhead higher with traditional distance vector algorithms. Link-state protocols, on the other hand, use triggered multicast advertisement packets to notify other routers about changes in the topology. Only the specific topology changes are sent, not the entire routing table.

OSPF is a little different from the rest in that it also sends a required announcement every 30 minutes to maintain the synchronization of the topology database among all the routers in the area. The IP routing table, similar to Listing 2.3, can be displayed by simply typing show ip route at the Cisco IOS EXEC prompt.

Remember that if the information in the routing table seems outdated , you can refresh the table by issuing the clear ip route command from the Cisco IOS EXEC prompt.

Listing 2.3 A Sample IP Routing Table Using RIPv2

 RouterA#  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  external  type 2        E1  OSPF external type 1, E2  OSPF external type 2, E  EGP        I  IS-IS, L1  IS-IS level-1, L2  IS-IS level-2, * - candidate def        U  per-user static route, o  ODR        T  traffic engineered route Gateway of last resort is not set        172.16.0.0/16 is variably subnetted,  9 subnets,  2 masks R          172.16.9.0/24 [120/1] via 172.16.3.2,  00:00:23,  Serial0/1 R          172.16.8.0/24 [120/1] via 172.16.3.2,  00:00:23,  Serial0/1 R          172.16.7.0/24 [120/1] via 172.16.3.2,  00:00:23,  Serial0/1 C          172.16.6.0/24 is directly connected,  Loopback2 C          172.16.5.0/24 is directly connected,  Loopback1 C          172.16.4.0/24 is directly connected,  Loopback0 C          172.16.3.0/30 is directly connected,__Serial0/1 R          172.16.2.0/24 [120/1] via 172.16.3.2,  00:00:00,  Serial0/1 C          172.16.1.0/24_is directly connected,  Ethernet0/0

The routing table in Listing 2.3 stores all the information needed to properly perform the routing functions. It also contains a number of pieces of information, including the following:

The manner in which the route was learned as indicated by one of the provided codes
The destination network or subnet (and a host address on rare occasion)
The AD number
The metric as a hop count or aggregate cost
The logical address of the next hop device
The amount of time the information has been in the routing table since last updated
The interface used to reach the destination network

In Listing 2.4, you can see that the manner in which the route was learned is through the RIP routing protocol (the letter R beginning the entry) and the destination network or subnet is 172.16.9.0/24. The administrative distance for RIP is 120 with a metric (hop count) of 1. The logical address of the next hop device is via 172.16.3.2, and the age of the entry since the last update is 23 seconds. Finally, the interface used to reach the destination is Serial0/0.

Listing 2.4 A Snippet from Listing 2.3

 RouterA#  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  external  type 2        E1  OSPF external type 1, E2  OSPF external type 2, E  EGP        I  IS-IS, L1  IS-IS level-1, L2  IS-IS level-2, * - candidate def        U  per-user static route, o  ODR        T  traffic engineered route Gateway of last resort is not set        172.16.0.0/16 is variably subnetted,  9 subnets,  2 masks  R          172.16.9.0/24 [120/1] via 172.16.3.2,  00:00:23,  Serial0/1

The routing table always displays the optimal path to all network destinations. As mentioned, however, some routing algorithms support multiple equal-cost paths to a destination. Listing 2.5 shows the result of the show ip route command on an OSPF router.

Listing 2.5 A Sample IP Routing Table Using the OSPF Protocol

 RouterB#  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  external  type 2     E1  OSPF external type 1, E2  OSPF external type 2, E  EGP     I  IS-IS, L1  IS-IS level-1, L2  IS-IS level-2, * - candidate def     U  per-user static route, o  ODR     T  traffic engineered route Gateway of last resort is 10.50.5.5 to network 0.0.0.0     172.16.0.0/24 is subnetted,  2 subnets C       172.16.10.0 is directly connected, Loopback100 C       172.16.11.0 is directly connected, Loopback101  O   E2       172.22.0.0/16 [110/20] via 10.30.3.3,  01:01:13,  Serial1/2   [110/20] via 10.40.4.4,  01:01:13,  Serial1/3   [110/20] via 10.50.5.5,  01:01:13,  Serial1/4  O   E2    192.168.4.0/24 [110/20] via 10.40.4.4, 01:01:13, Serial1/3 C   E2    192.168.5.0/24 [110/20] via 10.50.5.5, 01:01:13, Serial1/4     10.0.0.0/24 is subnetted, 4 subnets C       10.50.5.0 is directly connected,  Serial1/4 C       10.40.4.0 is directly connected,  Serial1/3 C       10.30.3.0 is directly connected,  Serial1/2 C       10.10.1.0 is directly connected,  Serial1/0 O   E2    192.168.3.0/24_[110/20] via 10.30.3.3, 01:01:15, Serial1/2 S*       0.0.0.0/0_[1/0] via 10.50.5.5

The previous sample routing table shows multiple equal-cost paths to network 172.22.0.0 via three separate serial interfaces in bold. Look at the following snippet:

 O   E2       172.22.0.0/16 [110/20] via 10.30.3.3,  01:01:13,  Serial1/2 [110/20] via 10.40.4.4,  01:01:13,  Serial1/3 [110/20] via 10.50.5.5,  01:01:13,  Serial1/4

Although the order of entries displayed in the routing table seems arbitrary, the Cisco router actually has a purpose behind the order. The router calculates the order of entries in the routing table to optimize the lookups using the subnet mask length value.

Additional information about a particular route in the routing table can be extracted by specifying an individual network number using the show ip route [ network ] command.