RIP Route Summarization

Problem

You want to decrease the size of your routing tables to improve the stability and efficiency of the routing process.

Solution

You can manually configure address summarization on an individual interface with the ip summary-address rip configuration command:

Router1#configure terminal 
Enter configuration commands, one per line. End with CNTL/Z.
Router1(config)#interface Serial0/0.2 
Router1(config-subif)#ip summary-address rip 172.25.0.0 255.255.0.0
Router1(config-subif)#exit
Router1(config)#end
Router1#

By default, RIP will summarize groups of subnets into classful network routes. You can disable this automatic summarization with the no auto-summary configuration command:

Router1#configure terminal 
Enter configuration commands, one per line. End with CNTL/Z.
Router1(config)#router rip
Router1(config-router)#no auto-summary
Router1(config-router)#exit
Router1(config)#end
Router1#

 

Discussion

RIP automatically summarizes along classful network boundaries. So if your router sees that several subnets of the same network all use the same path, and there are no subnets of this network using a different path, it will automatically summarize this information. The routes to the individual subnets are suppressed. Then any downstream devices from this router will see only a summary route, such as 172.25.0.0/16 instead of all of the individual subnets like 172.25.1.0/24, 172.25.2.16/28, and so forth.

The downstream devices don't need to know that they are seeing summary routes instead of the actual individual routes. So this works well even with legacy equipment.

Note, however, that the auto-summary feature works only along classful network boundaries. So, for example, it will not summarize a group of networks such as 192. 168.16.0/24 through 192.168.31.0/24 into 192.168.16.0/20 because these are all separate Class C networks. RIP cannot advertise supernets like this.

The interface specific summarization command became available in IOS Version 12.0(6)T. Configuring a summary addresses on an interface tells the router to send this summary information out through this interface. Then all of the routers that are downstream from the configured interface will see only the summary route, and none of the constituent subnet routes. As long as any of these subnet routes is valid, the router will propagate the summary information. But when the last subnet that is part of the summarized range disappears, the router stops sending the summary route through this interface.

You cannot configure more than one summary address from the same classful network address on an interface. For example, the following code is invalid because both of the summary addresses are subnets of 172.25.0.0/16:

Router1#configure terminal 
Enter configuration commands, one per line. End with CNTL/Z.
Router1(config)#interface Serial0/0.2 
Router1(config-subif)#ip summary-address rip 172.25.16.0 255.255.240.0
Router1(config-subif)#ip summary-address rip 172.25.35.0 255.255.255.0
IP-RIP:No summary for 172.25.35.0/24.Summary 172.25.16.0/20 already on interface
Router1(config-subif)#end
Router1#

However, note that either of these summarizations would be fine on its own. You can configure several different summary addresses as long as they are for different networks:

Router1#configure terminal 
Enter configuration commands, one per line. End with CNTL/Z.
Router1(config)#interface Serial0/0.2 
Router1(config-subif)#ip summary-address rip 172.25.16.0 255.255.240.0
Router1(config-subif)#ip summary-address rip 172.26.35.0 255.255.255.0
Router1(config-subif)#ip summary-address rip 10.101.0.0 255.255.0.0
Router1(config-subif)#end
Router1#

Route summarization has two important advantages. First, it reduces the size and complexity of the routing table. This in turn reduces the amount of memory and processing required on the downstream routers. The second advantage is that because all information about the individual subnets is suppressed, the downstream routers will not know or care about flapping interfaces in another part of the network. These two factors combine to improve overall network stability.

Suppose you have a routing table that looks like this before summarization:

Router2#show ip route rip 
R 172.21.0.0/16 [120/1] via 172.25.2.1, 00:00:01, Serial0.1
R 172.22.0.0/16 [120/1] via 172.25.2.1, 00:00:01, Serial0.1
 172.25.0.0/16 is variably subnetted, 6 subnets, 3 masks
R 172.25.25.6/32 [120/2] via 172.25.2.1, 00:00:01, Serial0.1
R 172.25.25.1/32 [120/1] via 172.25.2.1, 00:00:01, Serial0.1
R 172.25.1.0/24 [120/1] via 172.25.2.1, 00:00:01, Serial0.1
R 172.25.0.0/16 [120/1] via 172.25.2.1, 00:00:01, Serial0.1
R* 0.0.0.0/0 [120/1] via 172.25.2.1, 00:00:02, Serial0.1
Router2#

If you then enable autosummarization, the router will replace all of the highlighted subnets with a single summary for the entire classful network address:

Router2#show ip route rip 
R 172.21.0.0/16 [120/1] via 172.25.2.1, 00:00:01, Serial0.1
R 172.22.0.0/16 [120/1] via 172.25.2.1, 00:00:01, Serial0.1
 172.25.0.0/16 is variably subnetted, 3 subnets, 3 masks
R 172.25.0.0/16 [120/1] via 172.25.2.1, 00:00:01, Serial0.1
R* 0.0.0.0/0 [120/1] via 172.25.2.1, 00:00:01, Serial0.1
Router2#

You can see if a router is doing any summarization by looking at the show ip protocols command. This command shows both automatic summarization that is enabled by default as well as any interface-specific manually configured summarization:

Router1#show ip protocols 
Routing Protocol is "rip"
 Sending updates every 30 seconds, next due in 14 seconds
 Invalid after 180 seconds, hold down 180, flushed after 240
 Outgoing update filter list for all interfaces is not set
 Incoming update filter list for all interfaces is not set
 Redistributing: static, rip
 Default version control: send version 2, receive version 2
 Interface Send Recv Triggered RIP Key-chain
 FastEthernet0/0.1 2 2 ORA 
 Serial0/0.2 2 2 
 FastEthernet0/1 2 2 
 Automatic network summarization is in effect
 Address Summarization:
 172.25.0.0/16 for Serial0/0.2
 Maximum path: 4
 Routing for Networks:
 172.22.0.0
 172.25.0.0
 Routing Information Sources:
 Gateway Distance Last Update
 172.25.1.7 120 00:00:23
 172.25.2.2 120 00:00:00
 172.22.1.4 120 08:00:53
 Distance: (default is 120)
Router1#

The only thing you have to be careful of with summarization is when you have discontiguous network addressing. If you configure a router to advertise a summary route for downstream routers, then you have to ensure that no subnets in the summarized range exist in the downstream part of the network. The problem that can result from summarizing discontiguous networks is that any router in the middle of your network will see two or more identical summaries, pointing to two different RIP neighbors. Because the summary routes suppress the individual routes, there is no way to tell which way to send traffic for each individual subnet.