Got a traceroute question? When you see output from a traceroute that has ip?s from multiple interfaces on the same device, like the one below, can you tell me what would cause that? In the case below the first and third address under each hop is the same ip?
I am suspecting that each probe took a different route, but am confused by what the effect would be on subsequent hops. Thanks.
Success rate is 100 percent (5/5), round-trip min/avg/max = 64/65/68 ms
Type escape sequence to abort.
Tracing the route to 10.111.1.5
1 10.112.2.141 0 msec
10.112.2.10 0 msec
10.112.2.141 4 msec
2 10.112.2.148 0 msec
10.112.2.17 0 msec
10.112.2.148 4 msec
3 10.119.250.113 [AS 64512] 0 msec 0 msec 0 msec
4 10.119.250.101 [AS 64512] 68 msec 64 msec 64 msec
5 10.119.250.98 [AS 64512] 64 msec 64 msec 64 msec
6 10.111.2.245 [AS 65132] 64 msec 64 msec 64 msec
I have read your follow-up posts and will attempt to answer them.
Let me start by emphasizing that traceroute packets are regular routed IP packets (if it of interest traceroute from *nix boxes and Cisco boxes are UDP packets and tracert from Windows is ICMP packets) and a lot of the answers to your questions depend on the behavior of regular routed IP packets.
So, yes if you see multiple paths in traceroute that is because the ip routing table had multiple entries of how to get to the destination. And if the routing table has multiple entries of how to get to the destination then that means that they were equal cost.
I am less sure of how to answer your next question about how what is actually printed determined by the router. So let me review what happens and hopefully that will be an adequate answer. Your router sends packets toward the destination and manipulates the TTL. As the packets go through the network (processed by normal routing logic) they get to a router where the TTL expires. The router at which they expire will discard your packet and send back an ICMP packet indicating the error. Your router is able to determine the path through the network hop by hop from these ICMP error messages. The source address of the error packet will be the interface address of the router which discarded your packet. So what your router prints is the hop count, the address of the router generating the ICMP error, if it can resolve it the name of the router, and the time that it took to receive the error message.
I believe that the other part of your question here is how your router chooses the path when there are multiple paths possible. Packets generated by the router are inherently process switched. And when the router is process switching it will take turns over all the available paths. After your router makes the decision about how to route to the first hop, then it becomes a routing decision for the next router how to route the second hop, and so forth.
And for the last part of your question: in your example the multiple path responses may have come from a singular device, but it is quite possible that the responses could come from multiple devices.