IS-IS BFD-enabled sub-TLV

The strict BFD feature allows IS-IS to converge only when BFD is UP, and it sends the UP notification to IS-IS. We have introduced a new state in the IS-IS adjacency state machine (InitStrictBFD state) to indicate the agreement between routers to support strict BFD. We have also added a new BFD IS-IS Hello PDU (IIH) TLV in IS-IS IIH hello packets. 

If BFD is unstable, to prevent the traffic from being compromised until the IS-IS adjacency timer expires, we have added the option to configure the hold down interval in milliseconds that delays sending the UP notification to the BFD client. Once the BFD is UP, the hold down interval starts, and upon expiration, a BFD UP notification is sent to IS-IS so that IS-IS can start forming an adjacency. 

We have added the following command statements: 

• strict-bfd at the [edit protocols isis interface level ] hierarchy. 
• holddown-interval at the [edit protocols isis interface family inet bfd-liveness-detection] hierarchy.