MPLS - Loop Detection

MPLS - Loop Detection

- LDP relies on loop detection mechanisms built into IGPs that are used to determine the path.

- If, however, a loop is generated (that is, misconfiguration with static routes), the TTL field in the label header is used to prevent indefinite looping of packets.

- TTL functionality in the label header is equivalent to TTL in the IP headers.

- TTL is usually copied from the IP headers to the label headers (TTL propagation).

Normal TTL Operation

- Cisco routers have TTL propagation enabled by default.
- On ingress: TTL is copied from IP header to label header.
- On egress: TTL is copied from label header to IP header.
- TTL propagation can be disabled.






TTL and Loop Detection

- Labeled packets are dropped when the TTL is decreased to 0.





Disabling TTL Propagation

Cisco command : mpls ip propagate-ttl

- The IP TTL value is not copied into the TTL field of the label, and the label TTL is not copied back into the IP TTL.
- Instead, the value 255 is assigned to the label header TTL field on the ingress LSR.
- Disabling TTL propagation hides core routers in the MPLS domain.
- Traceroute across an MPLS domain does not show any core routers





Traceroute with Disabled TTL Propagation 

- The first traceroute packet (ICMP or UDP) that reaches the network is dropped on router A.
- An ICMP TTL exceeded message is sent to the source from router A.
- Traceroute across an MPLS domain does not show core routers.
- TTL propagation has to be disabled on all label switch routers.
- Mixed configurations (some LSRs with TTL propagation enabled and some with TTL propagation disabled) could result in faulty traceroute output.

- TTL propagation can be enabled for forwarded traffic only—traceroute from LSRs does not use the initial TTL value of 255.



Configuring IP TTL Propagation: Extended Options

Selectively disables IP TTL propagation for:
- Forwarded traffic (Traceroute does not work for transit traffic labeled by this router.)
- Local traffic (Traceroute does not work from the router but works for transit traffic labeled by this router.)

Cisco Command : no mpls ip propagate-ttl [forwarded | local]