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 completely 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
considered unreachable, and therefore the affected routes will not be installed
in the appropriate Routing Information Base (RIB).
For a BFD session to
be successfully established, BFD must be configured on the interface on the
peer and 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 usually 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 BFD.
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
static routes to be BFD-tracked. 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.