Traceroute and ICMP echo timeouts

LBC90805

I've been using the Boson practice exam test engine that comes with the Cisco Press certification guides. Studying for the ICND1, CCENT, cert I ran into a question concerning Traceroute. The question showed a network with Five interconnected routers with redundant links from one host to the other. When issuing a "Tracert" from one host to figuring out the path to the other the correct answer states that an ICMP echo request will timeout on two of the routers.

Now I'm just trying to grasp this concept but how do the ICMP echo request work with Traceroute? Does Traceroute issue ICMPs with a small value in the TTL field thus causing the echo's to die enroute? That would be the only explanation I could think of.

Also according to the correct answer will Traceroute map multiple routes if they exist within a small network. The correct answer listed ICMP echo timeouts on multiple routes. Does that make sense?

dtlokee

It sort of depends on the version of "traceroute" that is in use. Some versions use ICMP echo, some versions use UDP and some use TCP as the protocol. They all use the idea of TTL to find the hops along the path. It will send 3 packets typically all with a TTL of 1 which will get dropped by the first hop router(s) and they will send back a TTL expired using their IP address as the source. Then it will send 3 packets with a TTL of 2 which will be dropped by the second hop routers, and they will return a TTL expired message using their IP address as the source. This will continue and the TTL expited messages will tell you the router's IP addresses along the path (since they are sourced from the router that drops the traceroute packets). Once the destination is reached, a "port unavailable" or "protocol unavailable" message will be sent back telling the source of the traceroute that th process is complete.

LBC90805

Any special reason why three ICMPs are issued?

dtlokee

That is what would show different paths if the routers are load-balancing the packets.