You want to decrease the size of your routing tables to improve the stability and efficiency of the routing process.
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)#end Router1#
By default, RIP Version 2 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)#end Router1#
RIP Version 2 automatically summarizes along classful network boundaries. If your router sees that several subnets of the same network all use the same path, and that there are no subnets of this network using a different path, it automatically summarizes this information. The routes to the individual subnets are also suppressed. Any devices downstream from this router will see only a summary route (such as 172.25.0.0/16) instead of all of the individual subnets (such as 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 summarizing works well even with legacy equipment.
However the auto-summary feature only works along classful network boundaries. 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 they are all separate Class C networks. RIP cannot advertise such supernets because they are not classful.
The interface-specific summarization command became available in IOS Version 12.0(6)T. Configuring summary address on an interface tells the router to send summary information out through a specific interface. All 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 one of these subnet routes are valid, the router will propagate the summary information. But, when the last subnet that is part of the summarized range disappears, the router will stop sending the summary route through the configured interface.
You cannot configure more than one summary address from the same classful network address on an interface. For example, the following 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, 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, which in turn reduces the amount of memory and processing required on the downstream routers. Second, 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 enable automatic summarization, 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 0, 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 when using summarization is discontiguous networks. If you configure a router to advertise a summary route for downstream routers, ensure that no subnets in the summarized range exist in the downstream part of the network.
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