MPLS Rules for TTL Expiration

MPLS takes the following actions when the TTL in a MPLS label of a received MPLS packet expires:

1. A TTL-expired ICMP packet is constructed.
2. The destination address of ICMP packet is set to the source address of the IP packet that was encapsulated in the MPLS packet.
3. The source address of ICMP packet is set to the router ID of the router on which the TTL expired.
4. The first 128 bytes of the MPLS packet including the IP payload encapsulated in the MPLS packet are copied into the payload of the ICMP packet, followed by the entire label stack of the original packet.

The ICMP packet is label-switched to the end of the LSP. From that location, the packet is forwarded back to the source of the IP packet. This behavior enables IP trace-route to work even when the LSR in the middle of the LSP does not have an IP route to the source address of the IP packet.

Related Topics

•  MPLS Label Distribution Methodology
•  Mapping IP Data Packets onto MPLS LSPs
•  MPLS Forwarding and Next-Hop Tables
•  MPLS Interfaces and Interface Stacking
•  Topology-Driven LSPs

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