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Cisco Employee

why 2 ICMP (U) unreachable messages are missing?

when I ping to an non-exist network, the next hop router reply the icmp reply (unreachable messge) to me. however, I could only find 3 U messages only. Where are the missing 2 U messages?

I also used debug ip packet detail and receive 3 icmp type 3 messages. not 5 messages.

fw#ping 200.0.0.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 200.0.0.1, timeout is 2 seconds:

U.U.U

Success rate is 0 percent (0/5)

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Silver

Re: why 2 ICMP (U) unreachable messages are missing?

Eric

When you ping to a non existant address then the router that receives the ping request and realizes that the destination address is not reachable will generate an ICMP unreachable error message and send it to the originator of the ping. However Cisco routers rate limit their ping responses (as a mechanism to help protect against Denial of Service attacks against the router). The router is receiving 5 requests which can not be forwarded and sends the ICMP error to 3. The other 2 are rate limited.

HTH

Rick

1 REPLY
Hall of Fame Super Silver

Re: why 2 ICMP (U) unreachable messages are missing?

Eric

When you ping to a non existant address then the router that receives the ping request and realizes that the destination address is not reachable will generate an ICMP unreachable error message and send it to the originator of the ping. However Cisco routers rate limit their ping responses (as a mechanism to help protect against Denial of Service attacks against the router). The router is receiving 5 requests which can not be forwarded and sends the ICMP error to 3. The other 2 are rate limited.

HTH

Rick

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