When it comes to EIGRP or any other Interior Gateway Protocol (IGP), route summarization is an important tool that is used to establish a boundary for query propagation. It is also required in order to generate a single route entry that has a prefix and prefix length that is more than one or less component routes.
The quantity of routing information that routers are required to communicate, process, and keep is reduced as a result of route summarization. This, in turn, enables faster topology convergence and minimizes the amount of CPU load that the router experiences inside the network. The extent of a region that is affected by changes to the network can also be limited through the use of summarization. This is accomplished by concealing the modifications that have been made to the different networks behind a single advertised summary route that incorporates all of the component routes.
Two methods of route summarization are supported by EIGRP:
- Automatic summarization
- Manual summarization
Commands for EIGRP Route Summarization:
- Automatic summarization: Since IOS update 15.0(1)M, automatic summarization has been turned off by default, making networks almost useless. In older versions of IOS, to turn it off, the following command should be used:
Device(config-router)#no auto-summary
- Manual summarization: We can do it at any router and its unique network interface. The following command can be used to set up manual summary for each interface:
Device(config-if)#ip summary-address eigrp autonomous-system address netmask [ distance] [ leak-map name ]
The router that is doing the summarization automatically adds a discard route for the advertised summary route to its routing table whenever it is advertised. Both the subnet and netmask in this discard route and the published summary have the same subnet and netmask. The outgoing interface is set to Null0.
Discard route stops suboptimal routing or routing loops when a router posts a summary route without knowing a more specific subnet that the incoming traffic will fit.
EIGRP Summarization should be configured as follows:
Step 1: Let’s set up the two routers as seen in the following image:
Step 2: Use the command router eigrp <1-65535 Autonomous system number> to set up the EIGRP process on both routers:
R1(config)#router eigrp 1
R1(config-router)#network 10.0.0.0
R1(config-router)#network 172.168.5.0
R1(config-router)#network 172.168.6.0
R2(config)#router eigrp 1
R2(config-router)#network 10.0.0.0
R2(config-router)#network 8.8.8.8
Step 3: Verifying the configuration of EIGRP
R1#show ip protocols
Routing Protocol is "eigrp 1"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 1
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is in effect
Automatic address summarization:
172.168.0.0/16 for FastEthernet0/0
Summarizing with metric 128256
10.0.0.0/8 for Loopback0, Loopback1
Summarizing with metric 281600
Maximum path: 4
Routing for Networks:
10.0.0.0
172.168.0.0
Routing Information Sources:
Gateway Distance Last Update
(this router) 90 00:01:08
Gateway Distance Last Update
10.0.0.2 90 00:00:32
Distance: internal 90 external 170
R2#show ip protocols
Routing Protocol is "eigrp 1"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 1
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is in effect
Automatic address summarization:
10.0.0.0/8 for Loopback0
Summarizing with metric 281600
8.0.0.0/8 for FastEthernet0/0
Summarizing with metric 128256
Maximum path: 4
Routing for Networks:
8.0.0.0
10.0.0.0
Routing Information Sources:
Gateway Distance Last Update
(this router) 90 00:01:09
Gateway Distance Last Update
10.0.0.1 90 00:01:27
Distance: internal 90 external 170
Step 4: Examining the EIGRP route entry in the routers’ IPv4 routing table
