Failure detection can be achieved by using either of the following:
IGP failure detection
Enabling BFD is recommended for IGP protocols or static route (if static route is used for IGP forwarding). This enables faster IGP failure detection.
IGP can detect P router failures for IGP MoFRR (single AS).
If the ASBR fails, IGP can detect the failure and converge the route table to the local leaf. The local leaf in an AS can be either the ASBR or the actual leaf.
IGP routes to the ASBR address must be deleted for IGP failure to be handled.
BGP failure detection
BGP neighboring must be established between the local leaf and each ASBR. Using multi-hop BFD for ASBR failure is recommended.
Each local leaf attempts to calculate a primary ASBR or backup ASBR. The local leaf sets up a primary LSP to the primary ASBR and a backup LSP to the backup ASBR. If the primary ASBR has failed, the local leaf removes the primary ASBR from the next-hop list and allows traffic to be processed from the backup LSP and the backup ASBR.
BGP MoFRR can offer faster ASBR failure detection than IGP MoFRR.
BGP MoFRR can also be activated via IGP changes, such as if the node detects a direct link failure, or if IGP removes the BGP neighbor system IP address from the routing table. These events can cause a switch from the primary ASBR to a backup ASBR. It is recommended to deploy IGP and BFD in tandem for fast failure detection.