Hello,

The ICMP TTL expired packets from a P router will have to be sent to the destination CE and back. Thus the first hop should be seen with approx. round trip time to the destination.

Imagine you traceroute to a remote CE. Up to the ingress PE/VRF everything is IP only. The time measured is the roundtrip time between the traceroute host and the answering IP router.

To send a packet across the MPLS core, two labels are needed, top label to send to the BGP next hop, bottom label is VPN label. A P router discarding this packet with labelstack, will create an ICMP "TTL expired in transit" packet. The question is, how to send it back to the originating host. As a P router does not have customer routes any direct delivery of the ICMP packet is impossible. Thus the ICMP packet is sent further on through the same LSP as the original discarded IP packet. This means, that the ICMP reply of a P router will travel to the remote CE, who does an IP lookup and sends it back. You can easily verify this with an access-list denying all ICMP packets in at the remote CE in a lab environment. This will discard the ICMP packets from the core and thus every core/P router will show up as "*" in your traceroute output.

So even the first hop appears with the complete round trip time to the remote CE.

In addition there might be variations due to overload conditions and the like.

Hope this helps! Please rate all posts.

Regards, Martin