MPLS Guide Label Distribution Protocol
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Enabling IP FRR or LDP FRR is not strictly required, since LDP only needs to know
the location of the alternate next hop to the root LSR in order to send the label
mapping message and program the backup ILM during the initial signaling of the
tree. That is, enabling the LFA option is sufficient for providing the backup ILM
information. However, if unicast IP and LDP prefixes need to be protected, then IP
FRR and LDP FRR—and the mLDP fast upstream switchover—can be enabled
concurrently using the following commands:
config>router>ip-fast-reroute
or
config>router>ldp>fast-reroute
An mLDP FRR fast switchover relies on the fast detection of a lost LDP session to
the upstream peer to which the primary ILM label had been advertised. To ensure
fast detection of a lost LDP session, do the following:
Step 1. Enable BFD on all LDP interfaces to upstream LSR nodes. When BFD
detects the loss of the last adjacency to the upstream LSR, it brings down
the LDP session immediately, which causes the CSM to activate the
backup ILM.
Step 2. If there is a concurrent T-LDP adjacency to the same upstream LSR node,
enable BFD on the T-LDP peer in addition to enabling it on the interface.
Step 3. Enable the ldp-sync-timer option on all interfaces to the upstream LSR
nodes. If an LDP session to the upstream LSR to which the primary ILM is
resolved goes down for any reason other than a failure of the interface or
the upstream LSR, then routing and LDP go out of synchronization. This
means that the backup ILM remains activated until the next time SPF is run
by IGP.
By enabling the IGP-LDP synchronization feature, the advertised link
metric changes to the maximum value as soon as the LDP session goes
down. This, in turn, triggers an SPF, and LDP will likely download a new set
of primary and backup ILMs.
5.3.2 mLDP Fast Upstream Switchover Behavior
This feature allows a downstream LSR to send a label binding to two upstream LSR
nodes, but only accept traffic as follows:
• for normal operation, traffic is accepted from the ILM on the interface to the
primary next hop of the root LSR for the point-to-multipoint FEC
• for failure operation, traffic is accepted from the ILM on the interface to the
backup next hop
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