Ldp Fast-Reroute For Is-Is And Ospf Prefixes - Alcatel-Lucent 7950 SR User Configuration Manual

LDP Fast-Reroute for IS-IS and OSPF Prefixes

LDP Fast Re-Route (FRR) is a feature which allows the user to provide local protection for an
LDP FEC by pre-computing and downloading to XCM both a primary and a backup NHLFE for
this FEC.
The primary NHLFE corresponds to the label of the FEC received from the primary next-hop as
per standard LDP resolution of the FEC prefix in RTM. The backup NHLFE corresponds to the
label received for the same FEC from a Loop-Free Alternate (LFA) next-hop.
The LFA next-hop pre-computation by IGP is described in RFC 5286 – "Basic Specification for IP
Fast Reroute: Loop-Free Alternates". LDP FRR relies on using the label-FEC binding received
from the LFA next-hop to forward traffic for a given prefix as soon as the primary next-hop is not
available. This means that a node resumes forwarding LDP packets to a destination prefix without
waiting for the routing convergence. The label-FEC binding is received from the loop-free
alternate next-hop ahead of time and is stored in the Label Information Base since LDP on the
router operates in the liberal retention mode.
This feature requires that IGP performs the Shortest Path First (SPF) computation of an LFA next-
hop, in addition to the primary next-hop, for all prefixes used by LDP to resolve FECs. IGP also
populates both routes in the Routing Table Manager (RTM).
LDP FRR Configuration
The user enables Loop-Free Alternate (LFA) computation by SPF under the IS-IS or OSPF routing
protocol level:
config>router>isis>loopfree-alternate
config>router>ospf>loopfree-alternate.
The above commands instruct the IGP SPF to attempt to pre-compute both a primary next-hop and
an LFA next-hop for every learned prefix. When found, the LFA next-hop is populated into the
RTM along with the primary next-hop for the prefix.
Next the user enables the use by LDP of the LFA next-hop by configuring the following option:
config>router>ldp>fast-reroute
When this command is enabled, LDP will use both the primary next-hop and LFA next-hop, when
available, for resolving the next-hop of an LDP FEC against the corresponding prefix in the RTM.
This will result in LDP programming a primary NHLFE and a backup NHLFE into the XCM for
each next-hop of a FEC prefix for the purpose of forwarding packets over the LDP FEC.
7950 SR OS MPLS Configuration Guide
Label Distribution Protocol
Page 349