Unlike dynamic routing protocols, such as OSPF and BGP, static routing has no method of peer discovery. Therefore, when BFD
is configured, the reachability of the gateway is dependent on the state of the BFD session to the specified neighbor. Unless
the BFD session is up, the gateway for the static route is unreachable, and the affected routes are not installed in the appropriate
Routing Information Base (RIB).
To successfully establish a BFD session, BFD must be configured on the interface on the peer. There must be a BFD client registered
on the peer for the address of the BFD neighbor. When an interface is used by dynamic routing protocols, the latter requirement
is met by configuring the routing protocol instances on each neighbor for BFD. When an interface is used exclusively for static
routing, this requirement must be met by configuring static routes on the peers.
If a BFD configuration is removed from the remote peer while the BFD session is in the up state, the updated state of the
BFD session is not signaled to IPv4 static. This will cause the static route to remain in the RIB. The only workaround is
to remove the IPv4 static BFD neighbor configuration so that the static route no longer tracks BFD session state. Also, if
you change the encapsulation type on a serial interface to one that is unsupported by BFD, BFD will be in a down state on
that interface. The workaround is to shut down the interface, change to a supported encapsulation type, and then reconfigure
A single BFD session can be used by an IPv4 static client to track the reachability of next hops through a specific interface.
You can assign a BFD group for a set of BFD-tracked static routes. Each group must have one active static BFD configuration,
one or more passive BFD configurations, and the corresponding BFD tracked static routes. Nongroup entries are BFD-tracked
static routes for which a BFD group is not assigned. A BFD group must accommodate static BFD configurations that can be part
of different VRFs. Effectively, the passive static BFD configurations need not be in the same VRF as that of the active configuration.
For each BFD group, there can be only one active static BFD session. You can configure the active BFD session by adding a
static BFD configuration and a corresponding static route that uses the BFD configuration. The BFD session in a group is created
only when there is an active static BFD configuration and the static route that uses the static BFD configuration. When the
active static BFD configuration or the active static route is removed from a BFD group, all the passive static routes are
withdrawn from the RIB. Effectively, all the passive static routes are inactive until an active static BFD configuration and
a static route to be tracked by the active BFD session are configured in the group.
Similarly, for each BFD group, there can be one or more passive static BFD configurations and their corresponding static routes
to be BFD-tracked. Passive static session routes take effect only when the active BFD session state is reachable. Though the
active BFD session state of the group is reachable, the passive static route is added to the RIB only if the corresponding
interface state is up. When a passive BFD session is removed from a group, it will not affect the active BFD session if one
existed, or the BFD group reachability status.