EIGRP route summarization
By default EIGRP has auto summarization enabled. This means that it will summarize to classfull addresses at network boundaries. Leaving auto summarization enabled could cause a lot of troubles in our network. EIGRP has the ability to create summary routes at arbitrary boundaries by using manual summarization. Manual summarization can be applied anywhere in EIGRP domain, on every router, on every interface. Summary route will exist in routing table as long as at least one more specific route will exist. If the last specific route will disappear, summary route also will fade out.
If you want to use autosummarization you have to do nothing because it is enbaled by default, otherwise use no auto-summary eigrp command to disable it. To do manual summarization go under interface and use ip summary-address eigrp as-number address mask [administrative-distance ] command. Next, you see the topology I’ve used in this lab.
Unlike OSPF where we can summarize only on ABR or ASBR in EIGRP we can summarize anywhere. I will do this on R2’s S0/0 interface, so a summary route will be sent to R1. First will configure EIGRP to run in this network.
R1
R1(config)#interface serial 0/0 R1(config-if)#ip address 192.168.0.2 255.255.255.0 R1(config-if)#no shutdown R1(config)#router eigrp 1 R1(config-router)#no auto-summary R1(config-router)#network 192.168.0.0
R2
R2(config)#interface serial 0/0 R2(config-if)#ip address 192.168.0.1 255.255.255.0 R2(config-if)#no shutdown R2(config-if)#interface fa0/0 R2(config-if)#ip address 192.168.10.1 255.255.255.0 R2(config-if)#no shutdown R2(config-if)#interface fa0/1 R2(config-if)#ip address 192.168.8.1 255.255.255.0 R2(config-if)#no shutdown R2(config)#router eigrp 1 R2(config-router)#no auto-summary R2(config-router)#network 192.168.0.0 R2(config-router)#network 192.168.8.0 R2(config-router)#network 192.168.10.0
R3
R3(config)#interface fastEthernet 0/0 R3(config-if)#ip address 192.168.10.2 255.255.255.0 R3(config-if)#no shutdown R3(config-if)#interface loopback 0 R3(config-if)#ip address 192.168.11.1 255.255.255.0 R3(config)#router eigrp 1 R3(config-router)#no auto-summary R3(config-router)#network 192.168.10.0 R3(config-router)#network 192.168.11.0
R4
R4(config)#interface fastEthernet 0/0 R4(config-if)#ip address 192.168.8.2 255.255.255.0 R4(config-if)#no shutdown R4(config-if)#interface loopback 0 R4(config-if)#ip address 192.168.9.1 255.255.255.0 R4(config)#router eigrp 1 R4(config-router)#no auto-summary R4(config-router)#network 192.168.8.0 R4(config-router)#network 192.168.9.0
Let’s check now R1’s routing table to see what we’ve got
R1#sh ip route
... output ommited ...
Gateway of last resort is not set
D 192.168.8.0/24 [90/2195456] via 192.168.0.1, 00:00:12, Serial0/0
D 192.168.9.0/24 [90/2323456] via 192.168.0.1, 00:00:12, Serial0/0
D 192.168.10.0/24 [90/2195456] via 192.168.0.1, 00:00:12, Serial0/0
D 192.168.11.0/24 [90/2323456] via 192.168.0.1, 00:00:12, Serial0/0
C 192.168.0.0/24 is directly connected, Serial0/0
R1#
Pay attention to the metrics of learned routes, and after we do summarization, will compare this one with that summary metric. Now I will do summarization on R2’s S0/0 interafce.
R2(config-if)#ip summary-address eigrp 1 192.168.8.0 255.255.248.0
And again will check routing table on R1
R1#sh ip route ... output ommited ... Gateway of last resort is not set C 192.168.0.0/24 is directly connected, Serial0/0 D 192.168.8.0/21 [90/2195456] via 192.168.0.1, 00:01:42, Serial0/0 R1#
Do you see? All routes learned from R1 are now summarized and the metric is 2195456, the smallest metric of the specific routes
R2#show ip route ... output ommited ... Gateway of last resort is not set C 192.168.8.0/24 is directly connected, FastEthernet0/1 D 192.168.9.0/24 [90/409600] via 192.168.8.2, 01:04:43, FastEthernet0/1 C 192.168.10.0/24 is directly connected, FastEthernet0/0 D 192.168.11.0/24 [90/409600] via 192.168.10.2, 01:04:43, FastEthernet0/0 C 192.168.0.0/24 is directly connected, Serial0/0 D 192.168.8.0/21 is a summary, 00:14:46, Null0 R2#
Comments (14)
Ducky
| #
Going to put this acrtile to good use now.
Reply
Rado
| #
Thank you!!!!
Reply
Emma
| #
Awesome!
Reply
Sagar
| #
Very helpful.. Thank you..!!
Reply
starcoss
| #
the lab doesn’t work ? wich image do you use or wich setting ?
thanks !
Reply
admin
| #
Hi,
The image is for 3725 routers. Probably, on your machine it’s in a different location than on mine.
Reply
Net
| #
The AD should be 5 right? for eigrp summary-route
Reply
darren
| #
to be more efficient, shouldn’t the summary mask be 255.255.252.0?
only 4 networks need summarising
nice document. thanks
Reply
admin
| #
You are right darren, in this specific case it will be more efficient if you use 255.255.252.0
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Rodrigo
| #
Great, thanks!
Reply
JOEL OGBECHIE
| #
I just stumbled on this site. Im currently studying for my CCNP Route. This EIGRP summary is very simplified. Thumbs up to the owner of this blog
Reply
vikrant bhutani
| #
null0 is for preventing forward plane loops and AS-5 is used to prevent from control plane loops
Reply
Ruwan
| #
Cheers!!!
Reply
Unknown
| #
Thanks!!!
Reply