To enable the mLDP fast upstream switchover feature, configure the following option in the CLI:
config>router>ldp>mcast-upstream-frr
When mcast-upstream-frr is enabled and LDP is resolving an mLDP FEC received from a downstream LSR, LDP checks for the existence of an ECMP next hop or a loop-free alternate (LFA) next hop to the root LSR node. If LDP finds one, it programs a primary incoming label map (ILM) on the interface corresponding to the primary next hop and a backup ILM on the interface corresponding to the ECMP or LFA next hop. LDP then sends the corresponding labels to both upstream LSR nodes. In normal operation, the primary ILM accepts packets and the backup ILM drops them. If the interface or the upstream LSR of the primary ILM goes down, causing the LDP session to go down, the backup ILM starts accepting packets.
To use the ECMP next hop, configure the ecmp max-ecmp-routes value in the system to be at least 2, using the following command:
config>router>ecmp max-ecmp-routes
To use the LFA next hop, enable LFA using the following commands (as needed):
config>router>isis>loopfree-alternates
or
config>router>ospf>loopfree-alternates
Enabling IP FRR or LDP FRR is not strictly required, because LDP only needs to know the location of the alternate next hop to the root LSR 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:
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.
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.
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 likely downloads a new set of primary and backup ILMs.