Feature Behavior - Alcatel-Lucent 7450 Manual

config>router>ospf>loopfree-alternate
Enabling IP FRR or LDP FRR using the following commands is not strictly required since LDP
only needs to know where the alternate next-hop to the root LSR is to be able to send the Label
Mapping message to program the backup ILM at the initial signaling of the tree. Thus enabling the
LFA option is sufficient. If however, unicast IP and LDP prefixes need to be protected, then these
features and the mLDP fast upstream switchover can be enabled concurrently:
config>router>ip-fast-reroute
config>router>ldp>fast-reroute
Note that mLdp FRR fast switchover relies on the fast detection of loss of **LDP session** to the
upstream peer to which primary ILM label had been advertised. As a result, it is strongly
recommended to perform the following:
1. Enable BFD on all LDP interfaces to upstream LSR nodes. When BFD detects the loss of
2. If there is a concurrent TLDP adjacency to the same upstream LSR node, enable BFD on
3. Enable ldp-sync-timer option on all interfaces to the upstream LSR nodes. If an LDP

Feature Behavior

This feature allows a downstream LSR to send a label binding to a couple of upstream LSR nodes
but only accept traffic from the ILM on the interface to the primary next-hop of the root LSR for
the P2MP FEC in normal operation, and accept traffic from the ILM on the interface to the backup
next-hop under failure. Obviously, a candidate upstream LSR node must either be an ECMP next-
hop or a Loop-Free Alternate (LFA) next-hop. This allows the downstream LSR to perform a fast
7450 ESS MPLS Guide
the last adjacency to the upstream LSR, it will bring down immediately the LDP session
which will cause the IOM to activate the backup ILM.
the T-LDP peer in addition to enabling it on the interface.
session to the upstream LSR to which the primary ILM is resolved goes down for any
other reason than a failure of the interface or of the upstream LSR, routing and LDP will
go out of sync. This means the backup ILM will remain activated until the next time SPF
is rerun by IGP. By enabling IGP-LDP synchronization feature, the advertised link metric
will be changed to max value as soon as the LDP session goes down. This in turn will
trigger an SPF and LDP will likely download a new set of primary and backup ILMs.
Label Distribution Protocol
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