Label Distribution Protocol
7210 SAS M, T, X, R6, Mxp MPLS Configuration Guide Page 243
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 IOMXCM
for each next-hop of a FEC prefix for the purpose of forwarding packets over the LDP FEC.
Note that because LDP can detect the loss of a neighbor/next-hop independently, it is possible that
it switches to the LFA next-hop while IGP is still using the primary next-hop. In order to avoid this
situation, it is recommended to enable IGP-LDP synchronization on the LDP interface:
config>router>interface>ldp-sync-timer seconds
Reducing the Scope of the LFA Calculation by SPF
The user can instruct IGP to not include all interfaces participating in a specific IS-IS level or
OSPF area in the SPF LFA computation. This provides a way of reducing the LFA SPF calculation
where it is not needed.
config>router>isis>level>loopfree-alternate-exclude
config>router>ospf>area>loopfree-alternate-exclude
Note that if IGP shortcut are also enabled in LFA SPF, LSPs with destination address in that IS-IS
level or OSPF area are also not included in the LFA SPF calculation.
The user can also exclude a specific IP interface from being included in the LFA SPF computation
by IS-IS or OSPF:
config>router>isis>interface> loopfree-alternate-exclude
config>router>ospf>area>interface> loopfree-alternate-exclude
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