An NHG of resolution type direct supports uniform failover either within an NHG or across NHGs of the same policy. These uniform failover behaviors are mutually exclusive on a per-NHG basis depending on whether it has a single primary next hop or it has both a primary and backup next hops.
When an NHG has both a primary and a backup next hop, the forwarding policy database assigns a Protect-Group ID (PG-ID) to each and programs both in data path. The primary next hop is the preferred active path in the absence of any failure of the NHG.
During a failure affecting the active next hop, or the primary or backup next hop, CPM signals the corresponding PG-ID switch to the data path which then immediately begins using the NHLFE of the other next hop for flow packets mapped to NHGs of all forwarding polices which share the failed next hop.
An interface down event sent by CPM to the data path causes the data path to switch the PG-ID of all next hops associated with this interface and perform the uniform failover procedure for NHGs of all policies which share these PG-IDs.
Any subsequent network event causing a failure of the newly active next hop while the originally active next hop is still down, blackholes traffic of this NHG until CPM updates the policy to redirect the affected flows to the remaining NHGs of the forwarding policy.
When the NHG has only a primary next hop and it fails, CPM signals the corresponding PG-ID switch to the data path which then uses the uniform failover procedure to immediately re-assign the affected flows to the other NHGs of the policy.
A subsequent failure of the active next hop of a NHG the affected flow was re-assigned to in the first failure event, causes the data path to use the uniform failover procedure to immediately switch the flow to the other next hop within the same NHG.
Figure 1 illustrates the failover behavior for the flow packets assigned to an NHG with both a primary and backup next hop and to an NHG with a single primary next hop.
The notation NHGi{Pi,Bi} refers to NHG "i" which consists of a primary next hop (Pi) and a backup next hop (Bi). When an NHG does not have a backup next hop, it is referred to as NHGi{Pi,Bi=null}.
