This feature configures the TTL propagation for transit packets at a router acting as an LSR for a BGP label route.
When an LSR swaps the BGP label for a IPv4 prefix packet, therefore acting as a ABR, ASBR, or data-path Route-Reflector (RR) in the base routing instance, or swaps the BGP label for a vpn-IPv4 or vpn-IPv6 prefix packet, therefore acting as an inter-AS Option B VPRN ASBR or VPRN data path Route-Reflector (RR), the all value of the following command enables TTL propagation of the decremented TTL of the swapped BGP label into all LDP or RSVP transport labels.
configure router ttl-propagate lsr-label-route [none | all]
When an LSR swaps a label or stitches a label, it always writes the decremented TTL value into the outgoing swapped or stitched label. What the above CLI controls is whether this decremented TTL value is also propagated to the transport label stack pushed on top of the swapped or stitched label.
The none value reverts to the default mode which disables TTL propagation. This changes the existing default behavior which propagates the TTL to the transport label stack. When a customer upgrades, the new default becomes in effect. The above commands do not have a no version.
The following describes the behavior of LSR TTL propagation in a number of other use cases and indicates if the above CLI command applies or not.
When an LSR stitches an LDP label to a BGP label, the decremented TTL of the stitched label is propagated or not to the LDP or RSVP transport labels as per the above configuration.
When an LSR stitches a BGP label to an LDP label, the decremented TTL of the stitched label is automatically propagated into the RSVP label if the outgoing LDP LSP is tunneled over RSVP. This behavior is not controlled by the above CLI.
When an LSR pops a BGP label and forwards the packet using an IGP route (IGP route to destination of prefix wins over the BGP label route), it pushes an LDP label on the packet and the TTL behavior is as described in the previous bullet when stitching from a BGP label to an LDP label.
When an ingress Carrier Supporting Carrier (CsC) PE swaps the incoming EBGP label into a VPN-IPv4 label. The reverse operation is performed by the egress CsC PE. In both cases, the decremented TTL of the swapped label is or is not passed on to the LDP or RSVP transport labels as per the above configuration.