2 Identical EIGRP Processes

Answered Question
Oct 6th, 2009

I have to change from EIGRP AS 1 to EIGRP AS 10. I figured that I'd simply create AS 10, wait for the routes to be advertised, and then turn off AS1. Therefore no outage to users and ensuring a seamless transition. Unfortunately, AS10 is not advertising routes unless I turn off AS1. Is this by design, or do I need to do something special? NOTE - I am not doing a redistribution and the 2 AS processes are identical with the same network statement ("network 10.0.0.0").

Correct Answer by Giuseppe Larosa about 7 years 4 months ago

Hello Bob,

this is probably by design lowest AS number preferred by IP routing table mantainer

or because EIGRP routes of AS 1 are older and already installed in routing table.

after having deployed EIGRP AS 10 everywhere you can check correct propagation of routes with

sh ip eigrp 10 topology

compare it with

sh ip eigrp 1 topology

actually in the routing table you can see only the routes of the process with the lowest ID.

What you want to do has been done by other collegues for different reasons: for example to introduce authentication.

If everything is fine and you have a match of eigrp topology tables in several routers in your network you can then remove EIGRP AS1.

the fact that you don't see EIGRP AS 10 routes in routing table doesn't mean that its routes are not propagated unless you see an almost empty EIGRP AS 10 topology table.

We used in the past a similar approach in moving from EIGRP to OSPF for MPLS needs.

We had to compare EIGRP topology table with OSPF link state DB before removing EIGRP.

Edit:

I've found confirmation in EIGRP FAQ

Q. If there are two EIGRP processes that run and two equal paths are learned, one by each EIGRP process, do both routes get installed?

A. No, only one route is installed. The router installs the route that was learned through the EIGRP process with the lower Autonomous System (AS) number. In Cisco IOS releases earlier than 12.2(7)T, the router installed the path with the latest timestamp received from either of the EIGRP processes. The change in behavior is tracked by Cisco bug ID CSCdm47037.

see

http://www.cisco.com/en/US/tech/tk365/technologies_q_and_a_item09186a008012dac4.shtml#eight

Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Tue, 10/06/2009 - 09:15

Hello Bob,

this is probably by design lowest AS number preferred by IP routing table mantainer

or because EIGRP routes of AS 1 are older and already installed in routing table.

after having deployed EIGRP AS 10 everywhere you can check correct propagation of routes with

sh ip eigrp 10 topology

compare it with

sh ip eigrp 1 topology

actually in the routing table you can see only the routes of the process with the lowest ID.

What you want to do has been done by other collegues for different reasons: for example to introduce authentication.

If everything is fine and you have a match of eigrp topology tables in several routers in your network you can then remove EIGRP AS1.

the fact that you don't see EIGRP AS 10 routes in routing table doesn't mean that its routes are not propagated unless you see an almost empty EIGRP AS 10 topology table.

We used in the past a similar approach in moving from EIGRP to OSPF for MPLS needs.

We had to compare EIGRP topology table with OSPF link state DB before removing EIGRP.

Edit:

I've found confirmation in EIGRP FAQ

Q. If there are two EIGRP processes that run and two equal paths are learned, one by each EIGRP process, do both routes get installed?

A. No, only one route is installed. The router installs the route that was learned through the EIGRP process with the lower Autonomous System (AS) number. In Cisco IOS releases earlier than 12.2(7)T, the router installed the path with the latest timestamp received from either of the EIGRP processes. The change in behavior is tracked by Cisco bug ID CSCdm47037.

see

http://www.cisco.com/en/US/tech/tk365/technologies_q_and_a_item09186a008012dac4.shtml#eight

Hope to help

Giuseppe

bocarrier Tue, 10/06/2009 - 09:50

Very good! I found it in the topo table, which I overlooked previously. In the lab, I have both ASes. I deleted AS1 and there were no connection losses.

Thanks.

Bob

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