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New Member

How are routing decisions made in this scenario?

Hello,

In the following scenario, can anyone tell me what decision will be made by the router?

server-->CAT4000(layer3)-->3620-->to 172.16.x.x

The cat4000 has a static route to the 172.16.x.x network via the 3620 and a default route out to a PIX firewall. The 3620 is plugged into the cat4000. If the 3620 goes down, does the cat4000 then remove that route to the 172.16.x.x network via the 3620 from its routing table and send packets destined for 172.16.x.x then to the default gateway or do they just get a destination unreachable from the 4000?

thanks..

3 REPLIES
New Member

Re: How are routing decisions made in this scenario?

I believe they'll be routed to the default gateway.

If you need dest unreachables, you could always put a static route with a higher distance to null 0(ip route 172.16.0.0 255.255.255.0 null 0 5). I think that would do the trick.

Someone correct me if I'm wrong.

Cisco Employee

Re: How are routing decisions made in this scenario?

If the static route is removed from the routing table then the default route is used. The static route is not necesseraly taken out of the routing table just because the 3620 goes down though. Depending on whether the static route points to the interface as the next-hop or to the IP address of the next-hop router, the conditions for that route to be deleted from the routing table change. In the first case, the route is removed if the interface goes down. In the second case, the route is removed if the next-hop ip address is no longer resolvable.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
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México
New Member

Re: How are routing decisions made in this scenario?

the static route is still there but can't be used because the 3620 is down. Since the cat4000 doesn't know where to send the packet it use's the default route which is the pix, which means that if the pix can't find the route you won't be able to get there.

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