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amaksimenko1
Beginner

EIGRP best path selection two AS

Hello.

i have a question about EIGRP.

there are two EIGRP processes that run and two equal paths are learned, one by each EIGRP process. which routes get installed to routing table? how can i control that process?

Aleksey.

1 ACCEPTED SOLUTION

Accepted Solutions

Hello Aleksey,

with two different EIGRP processes (EIGRP AS numbers) you cannot achieve to install both routes in the routing table. As you have noted in another post in the thread, the new behaviour is that the IPv4 routing table daemon would pick the route coming from the lower AS number, with older IOS versions the timestamp of the route is looked and the more recent route is installed.

If you want to have both routes installed you have to change the design to use a single EIGRP domain (AS). In that case two equal cost routes are both installed in the IP routing table by default.

This is called "ships in the night" and it is the normal behaviour of IOS when dealing with multiple instances of the same routing protocol. The same happens for OSPF for example,where if two OSPF processes offer the same prefix to the IPv4 routing table manager this one takes the first regardless of the OSPF route type.

If you need to keep two EIGRP processes in your design you need to tweak the administrative distance in order to decide coherently on all routers which EIGRP domain is preferred (the same choice has to be done everywhere).

Hope to help

Giuseppe

View solution in original post

11 REPLIES 11
Mark Malone
VIP Mentor

As an option you could try go back to where the routes are being advertised from and set a cost on one route like delay so its less preferred than the other that way there not equal cost anymore wqhen advertised in and only 1 will be learned into the actual table that may effect other routes too depending on topology or use an offset list to alter the metric and match it against a an acl for that specific host route

example

http://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/13673-14.html

Hi!

this technique don't work for different AS.

can you provide more information on the design and setup then ,  is there redistribution going on , your saying the routes are both equal cost going into a routing table ? can you show exactly whats involved what you see may help

for example

have two AS on same router.

from AS one (1) learned 192.168.1.0/24, FD is 1000 via 192.168.2.1
from AS two (2) learned 192.168.1.0/24, FD is 1000 via 192.168.3.1

have result: now AS one (1) is successor always
i wanna get next result: AS two (2) will be successor, and AS one (1) will be feasible successor

If one is a successor and the others a feasible then there not equal cost routes to the local router , somewhere along the path the metric is different as the router has only chosen one

if they were equal when you do a show ip route 192.168.1.0 they would both be in the routing table is that the case now ? or are you looking at the eigrp topology table

follow the path from the source for route 192.168.1.0 in each AS and check the metrics along it they cant be equal if the router your looking at has them as S and FS

Philip D'Ath
Advisor

I don't know the answer - but I would expect both to be installed into the routing table, as equal cost paths.

thanx for reply

there are Cisco FAQ about this issue. but this not what i want.

http://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/13681-eigrpfaq.html#eight2

---

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 Software 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.

Philip D'Ath
Advisor

What about if you use a prefix list and the "distribute-list in" on one EIGRP process to drop the route you don't want installed?

http://www.cisco.com/c/en/us/td/docs/ios/12_2/iproute/command/reference/fiprrp_r/1rfeigrp.html#wp1030091

amaksimenko1
Beginner

The route that is installed into the EIGRP topology database first gets placed into the routing table.

P.S. lowest AS  number have effect also in some IOS version

Hello Aleksey,

with two different EIGRP processes (EIGRP AS numbers) you cannot achieve to install both routes in the routing table. As you have noted in another post in the thread, the new behaviour is that the IPv4 routing table daemon would pick the route coming from the lower AS number, with older IOS versions the timestamp of the route is looked and the more recent route is installed.

If you want to have both routes installed you have to change the design to use a single EIGRP domain (AS). In that case two equal cost routes are both installed in the IP routing table by default.

This is called "ships in the night" and it is the normal behaviour of IOS when dealing with multiple instances of the same routing protocol. The same happens for OSPF for example,where if two OSPF processes offer the same prefix to the IPv4 routing table manager this one takes the first regardless of the OSPF route type.

If you need to keep two EIGRP processes in your design you need to tweak the administrative distance in order to decide coherently on all routers which EIGRP domain is preferred (the same choice has to be done everywhere).

Hope to help

Giuseppe

View solution in original post

Hello Giuseppe!

Thanks for help! your solution with AD change working normaly. But we will change design to use single AS.

router eigrp 10
distance eigrp 190 170