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Downed interface vs. deleted interface - routes?

Unanswered Question
Feb 23rd, 2006
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I have a question regarding what happens to a route when an interface is "disabled" vs. "deleted"? If you disable the interface, and routes are there - when a packet comes in, another interface, and needs to go out the "downed" interface, what happens?

My thought -- "ICMP Unreachable" is generated.

I have a scenaria where we are replacing a router interface with a firewall. So we don't have to change all the workstations, we are using the same IP of the router interface - it is dual-addressed on a single interface btw.

A route is then set on the router saying go to the IP of the FW (which is the same as the "downed" interface) to get to net x. I believe the router will send back ICMP unreachables as long as the interface is NOT deleted - only disabled. Your thoughts?


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pkhatri Thu, 02/23/2006 - 12:37
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It is important to understand that an ICMP unreachable will be sent when a route to a network is missing. The router itself does not care whether an interface is disabled or deleted. Either case will result in routes reachable through the interface being removed from the routing table.

So in your case, if the route that previously belonged to the disabled interface is now available through some other interface, an ICMP unreachable will not be sent.

Hope that helps - pls rate the post if it does.



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