EIGRP auto-summary

hclisschennai · ‎02-12-2009

Hi Everybody,

I am configuring and testing EIGRP. I facing instance as described below:

I have 3 Router configured as shown in diagram runing EIGRP.

I have 3 Subnet 172.16.1.0 / 24, 172.16.2.0 / 24, 172.30.1.0 / 24 in Router A

1. When I configure "no auto-summary" in Router A, I am seeing the Routing table of Router C that 172.30.1.0/24, 172.16.2.0 / 24, 172.30.1.0 / 24.

2. When I configure "auto-summary" in Router A, I am seeing the Routing table of Router C that 172.30.0.0/16, 172.16.2.0/24, 172.30.1.0/24.

Why 172.16.1.0/24 and 172.16.2.0/24 is not getting summarized as like 172.30.1.0/24.

I Expected 172.16.1.0/24 & 172.16.2.0/24 will also be summarized to 172.16.0.0/16 like it happended in 172.30.1.0/24 changed to 172.30.0.0/16

Can you please help me to know about this behaviour

R.B.Kumar

royalblues · ‎02-12-2009

How are you getting the route installed at RouterA - network statements or redistribution ?

EIGRP will not auto-summarize external routes unless there is a component of the same major network that is an internal route

HTH

Narayan

hclisschennai · ‎02-13-2009

Hi Narayan,

I am not redistributing. Networks 172.30.1.1/24, 172.16.1.0/24,172.16.2.0/24 are all directly connected

Any clues..?

R.B.KUMAR

royalblues · ‎02-13-2009

can you post the relevant configurations of the routers?

Narayan

Peter010101 · ‎02-15-2009

I configured this in my lab and here are my results.

no auto-summary

C(config-router)#do sho ip route

Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

E1 - OSPF external type 1, E2 - OSPF external type 2

i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

ia - IS-IS inter area, * - candidate default, U - per-user static route

o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 3 subnets

D 172.16.30.0 [90/2809856] via 10.1.3.1, 00:00:14, Serial0/0

D 172.16.1.0 [90/2809856] via 10.1.3.1, 00:00:14, Serial0/0

D 172.16.2.0 [90/2809856] via 10.1.3.1, 00:00:14, Serial0/0

10.0.0.0/24 is subnetted, 2 subnets

C 10.1.3.0 is directly connected, Serial0/0

D 10.1.2.0 [90/2681856] via 10.1.3.1, 00:00:14, Serial0/0

with auto-summary

Gateway of last resort is not set

D 172.16.0.0/16 [90/2809856] via 10.1.3.1, 00:00:07, Serial0/0

10.0.0.0/24 is subnetted, 2 subnets

C 10.1.3.0 is directly connected, Serial0/0

D 10.1.2.0 [90/2681856] via 10.1.3.1, 00:01:44, Serial0/0

Peter010101 · ‎02-15-2009

Post your config's.

moses12315 · ‎02-16-2009

Actually i face the similar problem. So at the begining i was configuring manual summarization. Then a CCIE told me that summarization is not so importand if you have a network less than 100 routers. Also he adviced me that i should avoid auto-summary because sometimes it causes problems . I do not know. They know better.

Good luck

Moses.

Peter010101 · ‎02-16-2009

Auto-sum will cause problems.

For example you have 10.2.1.1/24, 10.3.1.1/24 and 10.4.1.1/24.

With auto-sum your routing tables will show 10.0.0.0/8.

With no auto

you will see this

10.0.0.0/24 is subnetted, 3 subnets

10.2.0.0

10.3.0.0

10.4.0.0