EIGRP-Prefer External routes vs internal routes

[PJ]

Hi All,

Is there a way in EIGRP to prefer external routes versus internal routes. EIGRP always picks up internal routes as long as they are available, no matter if external routes have better metric. Our Scenario is that we have DMVPN hub and spoke topology running EIGRP 101. The Core routers also on EIGRP 101 prefer EIGRP 101 routes. We have the new MPLS network running BGP and redistributing these BGP routes into EIGRP 101. The core routers prefer EIGRP 101 routes (internal) to redistributed BGP (external) routes.

Thanks.

[manish arora]

Hi,

I have seen a similar issue with one of my client and the way I resolved it was by running two different processes of EIGRP. You can run a process for DMVPN and assign it higher AD all the routes for that process and this will make the router install routes from the eigrp process running for your MPLS connection. Manish

Sent from Cisco Technical Support iPhone App

[mohAmed khAdr]

Have you tried changing the delay on the external routes?

If you can lower the delay for the ex routes on the distribute statement....that might do it

Gave it a shot.

HTH

[milan.kulik · Accepted solution]

Hi,

have you tried to set the AD od the external EIGRP better than internal?

See distance eigrp command details at

http://www.cisco.com/en/US/docs/ios/12_2/iproute/command/reference/1rfeigrp.html#wp1017590

HTH,

Milan

[PJ]

Thanks everyone for replying back to my post. I tried reducing delay, reducing external AD vs internal AD for EIGRP, it didn't help at all. EIGRP always picks up the internal routes vs external routes even though external routes have better metrics. I verified it using " show ip eigrp topology x.x.x.x"

Mainsh's recommendation of running two EIGRP processes looks doable to me. I want to clarify one thing here. I am looking at the core routers EIGRP view. These routers need to make a decision now, whether to pick MPLS network routes or DMVPN network. MPLS will have to be primary, DMVPN as secondary.

EIGRP 101 is currently running on DMVPN, MPLS and Core routers.

[u2637ps]

Have you tried offset list to prefer the route?

http://www.cisco.com/en/US/docs/ios/12_2/ip/configuration/guide/1cfeigrp.html#wp1001036

I presume you control the eigrp routers so can alter what the core sees?

[Richard Burts · Accepted solution]

The suggestions to change the delay or to configure an offset list are well intentioned suggestions. But neither of them has any chance of working if a single EIGRP process on the core is receiving advertisement with default AD of a prefix as internal from one neighbor and as external from the other neighbor.

I like the suggestion from Milan about changing the AD so that external becomes more favorable than internal. It should work. Can you post what you did in the config to change AD, and specify on what router(s) you made changes?

I also like the suggestion from Manish to run multiple processes. If there is one EIGRP process that receives advertisement of internal routes and that process is redistributed into a second process, then the second process will see both MPLS and DMVPN as external and will then choose based on the better metric. (Of course the challenge in doing this is to make sure that routes chosen by the second process are chosen for the IP routing table and not routes from the first process.)

HTH

Rick

[PJ]

Hi Rick,

Thanks for your reply. Your explanation looks exact to what I see. I am going to try out some config changes in the lab. As I said before, until there are internal routes learnt by EIGRP, it will not try to install any external learned routes even though external routes have better metric.

I lowered the distance of external EIGRP routes to 80 on the core router as below. I understand that distance configuration is local to the router where it is configured and not advertised to other routers. is that correct ?

router eigrp 101

distance 80 10.140.139.241 255.255.255.255 (vlan 10-mpls rtr)
distance 80 10.140.140.241 255.255.255.255 (vlan 140-mpls rtr)

[samavedula_rama · Accepted solution]

Praful,

Here is the definition of AD directly from Cisco.

Administrative distance - This is the measure of trustworthiness of the source of the route. If a router learns about a destination from more than one routing protocol, administrative distance is compared and the preference is given to the routes with lower administrative distance. In other words, it is the believability of the source of the route.

So, I don't think AD would come into picture until a route comparision from a different protocol/different instance of the same protocol occurs. Until then internal route is still preferred than external.

I would say combine both, i mean, create eigrp 102 and then redistribute bgp into it and then apply distance command. I am sure it will work. Atleast for OSPF it did for me.

Thanks

[milan.kulik]

Hi,

you are correct, I just found

"There are two caveats with EIGRP/IGRP redistribution within the same autonomous system:

Internal EIGRP routes are always preferred over external EIGRP or IGRP routes.
External EIGRP route metrics are compared to scaled IGRP metrics (the administrative distance is ignored)."

in http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094cb7.shtml#sameauto

So it's not a bug, it's a feature! :-)

BR,

Milan