Finding Types of IP Routes

Problem

You want to look for a particular type of route in your router's routing tables.

Solution

Often you are more interested in finding all of the directly connected networks, or all of the static routes, rather than in finding a specific route. This is found easily by specifying the type of route in the show command:

Router>show ip route connected 192.168.17.0/27 is subnetted, 1 subnets C 192.168.17.0 is directly connected, Loopback1 172.16.0.0/30 is subnetted, 1 subnets C 172.16.1.0 is directly connected, Async1 172.25.0.0/16 is variably subnetted, 6 subnets, 3 masks C 172.25.25.0/30 is directly connected, Tunnel0 C 172.25.1.0/24 is directly connected, Ethernet0 C 172.25.9.0/24 is directly connected, Ethernet1 C 172.25.10.1/32 is directly connected, Loopback0 Router>show ip route static 192.168.1.0/32 is subnetted, 1 subnets S 192.168.1.1 [1/0] via 172.25.1.4

And another useful variant of the show ip route command summarizes all of the different types of routes in the table:

Router>show ip route summary IP routing table name is Default-IP-Routing-Table(0) Route Source Networks Subnets Overhead Memory (bytes) connected 0 3 328 432 static 1 0 64 144 ospf 55 1 3 256 576 Intra-area: 1 Inter-area: 2 External-1: 1 External-2: 0 NSSA External-1: 0 NSSA External-2: 0 internal 2 2328 Total 4 6 648 3480

Discussion

You can see the full list of possibilities by using a ? on the command line:

Router>show ip route ? Hostname or A.B.C.D Network to display information about or hostname bgp Border Gateway Protocol (BGP) connected Connected egp Exterior Gateway Protocol (EGP) eigrp Enhanced Interior Gateway Routing Protocol (EIGRP) igrp Interior Gateway Routing Protocol (IGRP) isis ISO IS-IS list IP Access list odr On Demand stub Routes ospf Open Shortest Path First (OSPF) profile IP routing table profile rip Routing Information Protocol (RIP) static Static routes summary Summary of all routes supernets-only Show supernet entries only traffic-engineering Traffic engineered routes

This is useful when you want to see all of the routes that the router has learned via a particular routing protocol, or all of the statically configured or directly connected routes. The output format with the different type keywords is exactly the same as for the general show ip route, but it presents only the routes of the required type.

The show ip route summary command gives useful information about the size of the routing table and how much memory the router has allocated to storing this information, conveniently broken down by routing protocol. The example also shows how many routes belong to each of the different OSPF area types.

This has several uses. First, it gives you a convenient way to estimate your routing table's memory requirements. In this case, the routing table is very small, so there is more memory used to store connected routes than OSPF routes. However, in a larger network, you will often want to know if one routing protocol is causing memory problems for your routers. This can help you to decide if you need route filtering or summarization mechanisms. Routers exchanging BGP routing information with the public Internet can have particularly serious memory utilization problems.

Second, because it shows how many routes are learned by each mechanism, you can easily check the stability of the routing table by seeing whether this number changes in time. If you look at the entire routing table, you may not notice that a handful of routes periodically disappear and reappear, but looking at this summary information makes it much easier to spot such problems.

And third, you can easily see whether your routing table is getting its information the way you expect. It can be a very quick and easy way to check if the router is installing floating static routes or external routes in its routing table.