tracert and traceroute

[carlo.agapito]

Hi,

I hope everyone is doing well! One of the things that I'm doing right now is updating our network diagrams and I ran into an odd situation.

We're running an MPLS network and I've been curious why the transit paths that a packet is taking in our LAN to our MPLS networks do not show on tracert(from my PC) or traceroute(one of our Cores). It only shows host's gateway-jumps to mpls router ip address facing the lan-mpls router ip address on the ISP-another mpls router ip address on the ISP-remote host's IP.

But when I'm tracing the paths of a packet that is destined to the Internet, it shows the transit paths or IP addresses of the LAN devices before reaching its destination.

Thanks!

[Richard Burts]

I do not know the topology of your network and therefore do not understand what you are trying to describe about what addresses you are seeing when traffic is going through the MPLS. But let me offer a suggestion based on what I believe that you may be trying to describe.

Let us start by remembering that both tracert and traceroute work on the basis of sending probe packets and using the decrementing of Time To Live in the packet to find the path through the network. When you are sending traffic to the Internet then all the routers are using normal IP routing and will decrement the TTL and you will see all of them in the output of tracert/traceroute. But when you are sending to MPLS then the routers are not making normal IP routing which will decrement the TTL but they are forwarding based on the tags used by MPLS and usually they are not decrementing the TTL of your packet and therefore they do not show up in the output of tracert/traceroute.

HTH

Rick