This feature provides the option for disabling TTL propagation from a transit or a locally generated IP packet header into the LSP label stack when an LDP LSP is used as a shortcut for BGP next-hop resolution, a static-route next-hop resolution, or for an IGP route resolution.
A transit packet is a packet received from an IP interface and forwarded over the LSP shortcut at ingress LER.
A locally-generated IP packet is any control plane packet generated from the CPM and forwarded over the LSP shortcut at ingress LER.
TTL handling can be configured for all LDP LSP shortcuts originating on an ingress LER using the following global commands:
config>router>ldp>[no] shortcut-transit-ttl-propagate
config>router>ldp>[no] shortcut-local-ttl-propagate
These commands apply to all LDP LSPs which are used to resolve static routes, BGP routes, and IGP routes.
When the no form of the above command is enabled for local packets, TTL propagation is disabled on all locally generated IP packets, including ICMP Ping, traceroute, and OAM packets that are destined for a route that is resolved to the LSP shortcut. In this case, a TTL of 255 is programmed onto the pushed label stack. This is referred to as pipe mode.
Similarly, when the no form is enabled for transit packets, TTL propagation is disabled on all IP packets received on any IES interface and destined for a route that is resolved to the LSP shortcut. In this case, a TTL of 255 is programmed onto the pushed label stack.