IP Header TTL field supports two featuers
- method to identify looping packets – TTL decrements to 0 and gets discarded
- method for traceroute to find the IP address on a network path
MPLS requires a TTL field that ignores the IP header, and only decrements the MPLS TTL as opposed to the IP TTL.
The following describes how TTL from the IP header is translated to the MPLS TTL and transported across the network, this is called MPLS TTL Propagation:
Ingress Edge Label Switch Routers (E-LSR) decrements the IP TTL field and then pushes an MPLS label onto the packet copying the the IP TTL into the MPLS TTL field.
Label Switch Router – When the frames is passed through other MPLs routers, the MPLS TTL field is decremented, the IP TTL field is ignored.
Egress E-LSR – After egress LSR decrements the MPLS TTL field, it pops the MPLS label and copies the TTL field to the IP TTL field.
Routers can be configured to disable MPLS TTL propagation, this causes the ingress E-LSR to set the MPLS header TTL field to 255, and the IPv4 TTL is unchanged, this results in the MPLS network from being seen on traceroutes.