Ldp Shortcut For Igp Route Resolution - Alcatel-Lucent 7450 Configuration Manual

Bi-directional Forwarding Detection
neighbor advertisement received, or periodic traffic destined for it and the normal NS/NA
procedure, the static route becomes valid/active and is installed.

LDP Shortcut for IGP Route Resolution

This feature enables you to forward user IP packets and specified control IP packets using LDP
shortcuts over all network interfaces in the system that participate in the IS-IS and OSPF routing
protocols. The default is to disable the LDP shortcut across all interfaces in the system.
config>router>ldp-shortcut [ipv4][ipv6]
IGP Route Resolution
When LDP shortcut is enabled, LDP populates the RTM with next-hop entries corresponding to all
prefixes for which it activated an LDP FEC. For a given prefix, two route entries are populated in
RTM. One corresponds to the LDP shortcut next-hop and has an owner of LDP. The other one is
the regular IP next-hop. The LDP shortcut next-hop always has preference over the regular IP
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 ldp-interarea-prd.
Note that the LDP next-hop entry is not exported to LDP control plane or to any other control
plane protocols except OSPF, IS-IS, and specific OAM control plane as specified in
Control Packets on page
This feature is not restricted to /32 IPv4 prefixes or /128 IPv6 FEC prefixes. However only /32
IPv4 and /128 IPv6 FEC prefixes will be populated in the Tunnel Table for use as a tunnel by
services.
All user and specified control packets for which the longest prefix match in RTM yields the FEC
prefix will be forwarded over the LDP LSP. 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.
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7450 ESS Router Configuration Guide
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