Ldp Shortcut Forwarding Plane - Alcatel-Lucent 7450 Manual

Label Distribution Protocol
next-hop for forwarding user packets and specified control packets over a given outgoing interface
to the route next-hop.
The prior activation of the FEC by LDP is done by performing an exact match with an IGP route
prefix in RTM. It can also be done by performing a longest prefix-match with an IGP route in
RTM if the aggregate-prefix-match option is enabled globally in LDP.
This feature is not restricted to /32 FEC prefixes. However only /32 FEC prefixes will be
populated in the CPM Tunnel Table for use as a tunnel by services.
All user packets and specified control packets for which the longest prefix match in RTM yields
the FEC prefix will be forwarded over the LDP LSP. Currently, the control packets that could be
forwarded over the LDP LSP are ICMP ping and UDP-traceroute. The following is an example of
the resolution process.
Assume the egress LER advertised a FEC for some /24 prefix using the fec-originate command.
At the ingress LER, LDP resolves the FEC by checking in RTM that an exact match exists for this
prefix. Once LDP activated the FEC, it programs the NHLFE in the egress data path and the LDP
tunnel information in the ingress data path tunnel table.
Next, LDP provides the shortcut route to RTM which will associate it with the same /24 prefix.
There will be two entries for this /24 prefix, the LDP shortcut next-hop and the regular IP next-
hop. The latter was used by LDP to validate and activate the FEC. RTM then resolves all user
prefixes which succeed a longest prefix match against the /24 route entry to use the LDP LSP.
Assume now the aggregate-prefix-match was enabled and that LDP found a /16 prefix in RTM to
activate the FEC for the /24 FEC prefix. In this case, RTM adds a new more specific route entry of
/24 and has the next-hop as the LDP LSP but it will still not have a specific /24 IP route entry.
RTM then resolves all user prefixes which succeed a longest prefix match against the /24 route
entry to use the LDP LSP while all other prefixes which succeed a longest prefix-match against
the /16 route entry will use the IP next-hop.

LDP Shortcut Forwarding Plane

Once LDP activated a FEC for a given prefix and programmed RTM, it also programs the ingress
Tunnel Table in forwarding engine with the LDP tunnel information.
7450 ESS MPLS Guide Page 579