At a high level, the major difference in the behavior of the UDP traceroute when ICMP tunneling is enabled at an LSR node is that the LSR node tunnels the ICMP reply packet toward the egress of the LSP without looking up the traceroute sender's address. When ICMP tunneling is disabled, the LSR looks it up and replies if the sender is reachable. However there are additional differences in the two behaviors and they are summarized in the following.
icmp-tunneling disabled/IPv4 LSP/IPv4 traceroute
Ingress LER, egress LER, and LSR attempt to reply to the UDP traceroute of both IPv4 and VPN-IPv4 routes.
For VPN-IPv4 routes, the LSR attempts to reply but it may not find a route and in such a case the sender node times out. In addition, the ingress and egress ASBR nodes in VPRN inter-AS option B do not respond as in current implementation and the sender times out.
icmp-tunneling disabled/IPv4 LSP/IPv6 traceroute
Ingress LER and egress LER reply to traceroute of both IPv6 and VPN-IPv6 routes. LSR does not reply.
icmp-tunneling enabled/IPv4 LSP/IPv4 traceroute
Ingress LER and egress LER reply directly to the UDP traceroute of both IPv4 and VPN-IPv4 routes. LSR tunnels the reply to endpoint of the LSP to be forwarded from there to the source of the traceroute.
For VPN-IPv4 routes, the ingress and egress ASBR nodes in VPRN inter-AS option B also tunnels the reply to the endpoint of the LSP and therefore there is no timeout at the sender node like in the case when icmp-tunneling is disabled.
icmp-tunneling enabled/IPv4 LSP/IPv6 traceroute
Ingress LER and egress LER reply directly to the UDP traceroute of both IPv6 and VPN-IPv6 routes. LSR tunnels the reply to endpoint of the LSP to be forwarded from there to the source of the traceroute.
For VPN-IPv6 routes, the ingress and egress ASBR nodes in VPRN inter-AS option B also tunnels the reply to the endpoint of the LSP like in the case when icmp-tunneling is disabled.
In the presence of ECMP, CPM generated UDP traceroute packets are not sprayed over multiple ECMP next-hops. The first outgoing interface is selected. In addition, a LSR ICMP reply to a UDP traceroute is also forwarded over the first outgoing interface regardless if ICMP tunneling is enabled or not. When ICMP tunneling is enabled, it means the packet is tunneled over the first downstream interface for the LSP when multiple next-hops exist (LDP FEC or BGP label route). In all cases, the ICMP reply packet uses the outgoing interface address as the source address of the reply packet.