Tracerouting in MPLS Networks !
Tracerouting in an MPLS network is a bit different compared to tracerouting in an IP network. The
probes sent are the same, but the routers that are running MPLS treat the ICMP messages
differently. 

The traceroute is done on the
left CE router toward the right CE router. The packets that the CE router sends are still UDP
packets with high destination UDP ports. The first probe has IP TTL set to 1 and is received on the
VRF interface on the ingress PE router. The TTL of the packet expires on the ingress PE, and it
sends an ICMP message “TTL exceeded” back to the CE. This is the same behavior as tracerouting
in an IP network.

The CE sends the second probe. The ingress PE receives it, lowers the TTL by 1, and adds two
labels: the VPN label, followed by the IGP label on top. The TTL of both labels is set to 1, in
compliance with the rules mentioned in the previous section. The ingress PE forwards this packet
to the next LSR, a P router. This P router sees the TTL in the top label expiring. It removes the
label stack, examines the IP header of the underlying packet, and creates an ICMP message “TTL
expired.” Because this is a new packet, the IP TTL and the TTL in the labels are set to a value of
255. The source IP address is the IP address on the incoming interface of the P router. This ICMP
message is not sent directly to the originator of the packet. Instead, a label stack is put onto the
ICMP message—as if this were the original packet being forwarded according to the LFIB—and
forwarded along the original LSP toward the egress LSR. On the egress PE router, the labels are
stripped off, and the IP packet is forwarded. Because the IP destination of the original packet was
not directly connected to the egress PE router, the outgoing label on the egress PE is “No Label.”
As such, the unlabeled IP packet is directly forwarded—without an IP lookup—toward the CE
router. That CE router has the destination IP address of the ICMP message in its routing table and
returns the ICMP message. Eventually, the CE router on the left receives the ICMP message.

This behavior is the same for all probes: All ICMP messages are forwarded along the original LSP
and are not sent back directly to the originator.