Problem: Subnetted Routes Missing from the Routing Table of R2-Cause: Autosummarization Feature Is Enabled

Problem: Subnetted Routes Missing from the Routing Table of R2 ‚ Cause: Autosummarization Feature Is Enabled

In some situations, subnetted routes are not advertised in RIP. Whenever RIP sends an update across a major network boundary, the update will be autosummarized. This is not really a problem; this is done to reduce the size of the routing table.

Figure 3-36 shows a network setup in which R1 has subnets of 155.155.0.0, but R2 shows none of these subnets in its routing table. Either R1 is not advertising them to R2, or R2 is not receiving them. The chances of R1 not advertising more specific subnets of 155.155.0.0/16 is more favorable.

Figure 3-36. RIP Network Vulnerable to Autosummarization Problems

Example 3-98 shows that the subnetted route of 155.155.0.0/16 is missing from the routing table of R2, but the major network route is present. This means that R1 is advertising the routes but is somehow summarizing the subnets to go as 15.155.0.0/16.

Example 3-98 R2's Routing Table Reflects That the Subnetted Route Is Missing

 R2#  show ip route 155.155.155.0 255.255.255.0   % Subnet not in table  R2#show ip route 155.155.0.0  Routing entry for 155.155.0.0/16   Known via "rip", distance 120, metric 1  Redistributing via rip   Advertised by rip (self originated)   Last update from 131.108.1.1 on Ethernet0, 00:00:01 ago   Routing Descriptor Blocks:   * 131.108.1.1, from 131.108.1.1, 00:00:01 ago, via Ethernet0       Route metric is 1, traffic share count is 1 

Figure 3-37 shows the flowchart to fix this problem based on the autosummarization feature being enabled.

Figure 3-37. Flowchart to Solve Why the Sender Is Not Advertising RIP Routes

Debugs and Verification

Example 3-99 shows the configuration of R1 in the case of RIP-1. RIP-1 is a classful protocol and always summarizes to classful boundaries for nondirectly connected major networks.

Example 3-99 R1 Configuration with RIP Version 1

 R1#  interface Loopback1   ip address 131.108.2.1 255.255.255.0   !   interface Loopback3   ip address 155.155.155.1 255.255.255.0   !   interface Ethernet0   ip address 131.108.1.1 255.255.255.0   !   router rip   network 131.108.0.0   network 155.155.0.0  

Example 3-100 shows the routing table in Router R2. Notice that R2 is receiving 155.155.0.0/16, not 155.155.155.0/24, as configured on R1. Also note that R2 is receiving a /24 route of 131.108.2.0, the route of the same major network as that of interface Ethernet 0, which connects R1 to R2.

Example 3-100 R2 Routing Display to Show How Subnetted Routes Are Summarized to Classful Boundaries

 R2#  show ip route RIP   R    155.155.0.0/16 [120/1] via 131.108.1.1, 00:00:22, Ethernet0  131.108.0.0/24 is subnetted, 3 subnets R       131.108.2.0 [120/1] via 131.108.1.1, 00:00:22, Ethernet0 

Solution

In RIP-1, there is no workaround for this problem because RIP-1 is a classful routing protocol. RIP-1 automatically summarizes any update to a natural class boundary when that update goes over an interface configured with a different major network.

As indicated by R2's routing table in Example 3-100, 155.155.155.0/24 is advertised over an interface configured with 131.108.0.0. This summarizes 155.155.155.0/24 to a Class B boundary as 155.155.0.0/16.

In RIP-1, this is not a problem because RIP-1 is a classful protocol and the network should be designed with this understanding. With RIP-2, however, Cisco routers can be configured to stop the autosummarization process.

For example, R1's configurations can be changed to run a RIP-2 process rather than a RIP-1 process.

Example 3-101 shows the configuration that solves this problem for RIP-2.

Example 3-101 Disabling Autosummarization in RIP-2

  router rip    version 2    network 131.108.0.0   network 155.155.0.0    no auto-summary   

Example 3-102 shows the routing table of Router R2, which indicates that it is now receiving desired subnetted route 155.155.155.0/24.

Example 3-102 Router R2's Routing Table Shows That It Is Receiving the Subnetted Route 155.155.155.0/24

 R2#  show ip route 155.155.0.0   155.155.0.0/24  is subnetted, 1 subnets  R       155.155.155.0  [120/1] via 131.108.1.1, 00:00:21, Ethernet0      131.108.0.0/24 is subnetted, 3 subnets R       131.108.2.0 [120/1] via 131.108.1.1, 00:00:21, Ethernet0