The user enables the mLDP fast upstream switchover feature by configuring the following option in CLI:
config>router>ldp>mcast-upstream-frr
When this command is enabled and LDP is resolving a mLDP FEC received from a downstream LSR, it checks if an ECMP next-hop or a LFA next-hop exist to the root LSR node. If LDP finds one, it programs a primary 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 while 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 then starts accepting packets.
To make use of the ECMP next-hop, the user must configure the ecmp value in the system to at least two (2) using the following command:
config>router>ecmp
To make use of the LFA next-hop, the user must enable LFA using the following commands:
config>router>isis>loopfree-alternates
config>router>ospf>loopfree-alternates
Enabling IP FRR or LDP FRR using the following commands is not strictly required because 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
The mLDP FRR fast switchover relies on the fast detection of loss of **LDP session** to the upstream peer to which the primary ILM label had been advertised. We strongly recommend that you perform 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 immediately the LDP session which causes the IOM to activate the backup ILM.
If there is a concurrent TLDP adjacency to the same upstream LSR node, enable BFD on the T-LDP peer in addition to enabling it on the interface.
Enable 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 other reason than a failure of the interface or of the upstream LSR, routing and LDP goes out of sync. This means the backup ILM remains activated until the next time SPF is rerun by IGP. By enabling IGP-LDP synchronization feature, the advertised link metric is changed to max 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.