ICMP Time Exceeded message?

Node Man

Hi Everybody,
Can anyone explain how to tell if a "icmp time exceeded message" will occur just by reading the results of "show ip route rip" and "show ip interface brief". I cant find it in my ICND1 book.


Thanks

iamme4eva

Didn't know you could.

Awaits with interest.........

MrBrian

Not sure what you mean? A device can send or receive ICMP time exceeded messages.. this most frequent way this occurs (well the only way I know of) is when the TTL in the IP header is decremented to 0.

This is done every time you do a traceroute. First it will send out three packets with a TTL of 1 in the IP header.. the first device that it hits will decrement the TTL to 0 and send an ICMP time exceeded message back to the originator.. and the traceroute program will show this as the first hop. Then the traceroute program will send out another three packets with TTL of 2, in which the second hop will send back ICMP time exceeded messages. That's how traceroute works.

If you do "sho ip route rip" you'll just see all the RIP routes that you router has learned, and doing "sho ip int bri" just shows your connected interfaces. Can you elaborate on the question?

Node Man

i think i can narrow it down. i think my confusion is with pings&tracerts:

I see that a ping returns a"." if an echo takes more than 2 seconds. But what i dont understand is how to know which routers in a path will respond with a time out message when pinging another host on the other side of a network.

Does this info help you help me?

iamme4eva

I'm pretty sure the originator of the ping decides the time out. The routers in the middle are just passing packets. Unless the TTL counter reaches 0, then the router the made it 0 responds with a time exceeded. That's different to no response at all though.

I might be wrong - but that's my two cents.

Timeout in seconds [2]:
Timeout interval. Default: 2 (seconds). The ping is declared successful only if the ECHO REPLY packet is received before this time interval.

Using the Extended ping and Extended traceroute Commands - Cisco Systems