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I am trying to play (in GNS3 if that matters) with a very simple topology of three routers connected via hub. The time I am trying to ping from one of the router to another say R1 to R2. The R3 replies with ICMP redirect message causing R1 to re issue the ping request to R2. The loop continues infinitely wrecking havoc on simulated network. The question is why R3 replies to R1 for ICMP message not directed to it (ping is from R1 to R2). enter image description here

R3 routing table :-

R3>enable
Password:
R3#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       + - replicated route, % - next hop override

Gateway of last resort is not set

      10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
O        10.1.0.0/16 [110/2] via 192.168.0.1, 00:58:17, FastEthernet1/0
O        10.2.0.0/16 [110/2] via 192.168.0.2, 00:58:17, FastEthernet1/0
C        10.3.0.0/16 is directly connected, FastEthernet0/0
L        10.3.0.1/32 is directly connected, FastEthernet0/0
C     192.168.0.0/16 is directly connected, FastEthernet1/0
      192.168.0.0/32 is subnetted, 1 subnets
L        192.168.0.3 is directly connected, FastEthernet1/0
R3#

UPDATE: The problem is not ICMP redirect but the fact that any router will place the ICMP ping packet it cannot handle back to the interface it arrived from flooding the network till TTL expires.

Update2: replacing hub with switch solves the problem.

  • You should edit your question to include the router configurations. Also, you should not cross-post the same question to multiple SE sites. Pick one site and delete the question on the other sites. – Ron Maupin Jul 22 '16 at 4:23
  • Please also post R1's route and arp tables – Ron Trunk Jul 22 '16 at 11:52
  • "192.168.0.0/32 is subnetted, 1 subnets" what's your mask on the LAN interfaces of these routers? – marctxk Jul 22 '16 at 12:26
  • R3 could be proxy APing if the subnet masks are misconfigured? – Karl Billington Jul 22 '16 at 14:49
  • if you could post the config there may be something in there... – Ron Royston Jul 22 '16 at 16:42
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This is expected behaviour. As you were using a hub originally (which sends the packets out all interfaces except the one they were received on), the routers were receiving packets destined for other routers on the same subnet. As a result, these routers were sending redirects to the ping originator telling it "don't send your packets to me, send them directly to the router you're trying to ping."

As you mentioned, using a switch instead of a hub resolves the issue. This is because the switch ensures only the router which the ping is destined for receives the packet. As the other routers on the subnet do not receive these packets, they do not send redirects anymore.

  • Good catch. Sounds right. – Ron Royston Jul 23 '16 at 1:26
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The issue is that CISCO router assumes that it is sitting behind a switch and not a hub so by default when receiving packet to wrong MAC address it tries to "fix" the situation (assuming switch has wrong port/mac mappings) by replacing the the original SRC MAC with his own and resending the packet (with TTL-1). Of cause in case of hub not only it does not fix the situation it makes it worse because now other routers (at least one in our case) receive messages to wrong MAC, causing "infinite" flooding.

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R3 can deffinetly handle the packet. R3 has a route to R2 out f1/0.

You will get ICMP Redirects when a router's next hop for a packet goes out the same interface it was received on.

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