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As I understand ICMP redirect message is sent to host to "shorten" the routing path of the packet .
If i have the example here and the path from host in network 1 to network 3 is equal using both R2 and R3 .
Questions :
1 - which one would ICMP (from R1) message redirect to R2 or R3 ?
2 - Is it possible that the traffic from network 1 to network 3 be split among R2 and R3
using an ICMP message from R1 ?

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    Is this a real-world question, general learning, homework? ...this stack is mostly aimed at answering real-world questions. – Craig Constantine Nov 20 '13 at 18:09
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    shouldn't there be a router (like R4) between N2 and N3? – Mike Pennington Nov 20 '13 at 18:17
  • yeah should be , but thats not the problem – saeed hardan Nov 20 '13 at 18:35
  • and this is a real world question , as i know ospf uses multiple paths with the same cost to route packets – saeed hardan Nov 20 '13 at 18:35
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    As far as i know the ICMP Redirect messages are only sent if the interface on which the packet comes into the router is the same interface on which the packet gets routed out. If i understand correctly, for N1 network, R1 have 2 routes via R2 and R3? – t3mp Nov 21 '13 at 0:38
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For the sake of this question, I'm assuming that R1 uses a passive-interface to N2, and forms routing protocol adjacencies to R2 and R3 via N1. Assume PCs on N1 default through R1, which then issues ICMP redirects for the routes it learns from R2 and R3.

You're trying to solve routing problems with ICMP redirects; however, ICMP redirects are the wrong way to solve routing problems.

The right answer: GLBP

I'm reading between the lines, but it looks like you're trying to load-balance outbound from N1 through multiple routers.

The right way to solve this problem is with GLBP. GLBP is an improvement over HSRP because it automatically load-balances hosts on a subnet. Like HSRP, GLBP takes over for unreachable gateways if one goes down. There are also show commands directly on the router to understand which gateway is load-balancing a specific PC. Incidentally, there is GLBP for IPv6.

The wrong answer: ICMP Redirects to different gateways

1 - which one would ICMP (from R1) message redirect to R2 or R3 ?

The answer is implementation dependent, but relying on ICMP redirects to correct routing problems is asking for trouble, and is very unsupportable.

  • Think about what happens if either R2 or R3 goes down. Now, you have clients that are black-holed until someone goes to each PC and clears the ICMP redirects from the cache.
  • How do you troubleshoot problems by looking at show commands in the router? It's impossible, because you can't know which router any given PC has cached a redirect for. Furthermore, these entries will be cached per destination IP address and will be a nightmare to support.

If you design the network correctly, and you don't have to deal with these issues.

2 - Is it possible that the traffic from network 1 to network 3 be split among R2 and R3 using an ICMP message from R1?

This is also implementation dependent.

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  • ok thanks , i knew its a bad idea to rely on icmp. – saeed hardan Nov 21 '13 at 11:02

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