MPLS ICMP Tunneling

This feature introduces ICMP tunneling over MPLS LSPs. Without ICMP tunneling, when a transit MPLS router receives an MPLS packet with MPLS TTL equal to 1, it decreases the MPLS TTL to 0, drops the packet, and generates an ICMP "TTL Expired" message destined to the source IP address of the dropped packet. However, in an MPLS network, transit routers are not required to maintain routing information for MPLS-encapsulated IP packets. A typical example is Layer 3 VPN deployment, where transit routers do not maintain any IP state for Layer 3 VPNs. This means that the transit router might not be able to send the ICMP "TTL Expired" message directly to the source IP address of the original packet. To overcome this issue, when ICMP tunneling is enabled, the ICMP "TTL Expired" packet is sent along the original LSP, toward the LSP endpoint (egress PE), where IP lookup, including the IP lookup in VRF context, can be performed. Based on this lookup, the ICMP "TTL Expired" packet can be routed to the source IP address of the originally dropped packet.