ICMP Redirects

[rooniix]

What happends when a router sends an ICMP redirect message to another router. How dose the router react? Do it discard the packet or dose it change anything in the route table, if there are a better route to the destination?

[steve.barlow]

The router that sends the redirect still forwards the original packet to the destination. Even after receiving an ICMP Redirect message, some devices might continue using the less-efficient route.

Routes sometimes can become less than optimal. For example, it is possible for the protocol translator to be forced to resend a packet through the same interface on which it was received. If this happens, the protocol translator sends an ICMP Redirect message to the packet's originator that it is on a subnet directly connected to the protocol translator when the protocol translator would otherwise forward the packet to another system on the same subnet. It does this because the originating host presumably could have sent that packet to the next hop without involving the protocol translator at all. The Redirect message instructs the sender to remove the protocol translator from the route and substitute a specified device representing a more direct path.

[rooniix]

Thanks for the reply :)

What I'm wondering is, how dose a router react when it recives an ICMP-redirect? I know how an end-host reacts, because that's the usual scenario, but what if there's 3 routers connected to a common network (lets call them A, B and C).Due to a misconfigured static route, router A routes packets to router B when the destination of the packets is a network behind router C, so router B would have to send the packets back out the same interface to router C and that would generate a ICMP-redirect message back to router A. How dose router A react when it recives the ICMP-redirect message?

(sorry about the spelling, I'm swedish) :)

[efrahim · Accepted solution]

Cisco router is the routing device , it check outed packets against its routing table, if the packet will be routed back to the same interface, then it just send the packet to the same interface plus the redirect. But if it receive icmp redirects, it won't change anything in its routing table-

Hope this helps

[rooniix]

Yepp :)

Thanks!