Multiple routes to the same destination

Unanswered Question
Jul 16th, 2008

Dear all,

I have a question regarding how Cisco IOS behaves when multiple routing protocols are present.

Suppose a router is running two different routing processes (say, EIGRP 100 and RIP) on two different interfaces, and receives two routes for the same destination (say, 192.168.1.0/24), one from each routing protocol. Assume that no redistribution between EIGRP and RIP (or vice-versa) is configured in the router and that the router knows of no other way to reach 192.168.1.0/24.

Since the default administrative distance (AD) of EIGRP internal routes is higher than RIP's, the router will naturally choose the route received via EIGRP and place it in the routing table.

The question is: which route to 192.168.1.0/24 is announced by the router to its neighbors on the RIP side (if any)? Since the active route comes from EIGRP but no redistribution is configured, it doesn't seem right to announce the route learned via EIGRP. But announcing the route learned via RIP doesn't make sense either, since it is not the route actually used to forward data packets to 192.168.1.0/24.

What does Cisco IOS do in this situation? Would the answer be different if the protocols in question were any other than EIGRP and RIP?

Thanks in advance for your help.

Best regards,

Tiago

I have this problem too.
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Paolo Bevilacqua Wed, 07/16/2008 - 06:58

Hi, routing protocols do not take into any account if a route is being used or not.

Once a route is in the database of the protocol, it will be announced unless otherwise configured. In your example, it seems that the same route will be announced by both protocols.

milan.kulik Wed, 07/16/2008 - 12:18

Hi Paolo,

when you say "Once a route is in the database of the protocol, it will be announced unless otherwise configured." you mean the route received by RIP will be advertised to RIP neighbours and the route received by EIGRP (can have a different next-hop!) will be advertised to EIGRP neighbours?

IMHO, one routing protocol does take into any account if a route is being used or not: BGP.

It will advertise a route only in a case it's present in the routing table.

BR,

Milan

tiagoquelhas Wed, 07/16/2008 - 14:11

"In your example, it seems that the same route will be announced by both protocols."

The same route? Or rather: the route learned via RIP is announced via RIP, and the route learned via EIGRP is announced via EIGRP? (They may have different next-hops, corresponding to different paths on the network).

If this is true, doesn't it introduce the potential for forwarding loops? I ask because looking at the RIP side, the router is advertising a route it is not actually going to use! Any packets arriving at the router will be forwarded using the route learned via EIGRP --- not the one learned via RIP (because forwarding is destination-based, and there is only a single active route, the one learned through EIGRP).

Best regards,

Tiago

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