Does traceroute show the IP address of the entry port? or exit port? For example, if there is an intermediary router in the path with facing the source (entry port), and facing the destination (exit port), which of these IP addresses will I see in the output of Tracert?


It is the address on the interface which responds back to with ICMP timeout. Which, by default, would be the interface closest to the source.

  • 2
    It's worth noting that the source address can be overridden on the intermediate routers by doing something like 'ip source loopback0 icmp-errors' Jan 6 '16 at 19:54
  • do you mean the router may, in some cases, return the address of the interface furthest from the source? Jan 6 '16 at 20:01
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    Some routers, like Cisco, can have sources for various thing set to specific ports which can differ from the default. This is not common for ICMP, but is an option.
    – Ron Maupin
    Jan 6 '16 at 20:04
  • Specifically this can be useful if your router loopbacks are on public IPs but your point to point links are on private IPs. Sourcing ICMP errors from private IPs can cause them to be dropped by ingress filters. Nov 5 '16 at 0:31

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