BGP paths received
from neighbor are removed immediately when it detects that the session is down.
This behavior is to keep BGP table and forwarding table updated with current
network state. Eventually this avoids traffic blackhole and routing loops. But
in some scenarios keeping the routes for longer time during control plane
failure helps the services that are less IP sensitive to continue uninterrupted
for longer duration. The traffic flow does not get affected in the following
scenarios, even if the BGP routes are stored for longer time during the BGP
advertisement path is different than the forwarding path that is, through MPLS
tunnels. For example, VPN routes.
When the purpose
of route advertisement is to push configuration that is, filter programming on
the router. For example, flow-spec, route-targets.
advertisement is used for auto-discovery. For example, VPLS.
enables the local router to retain routes that it has learnt from the
configured neighbor even when the neighbor session is down. BGP persistence is
also referred as long lived graceful restart (LLGR). LLGR comes into effect
after graceful restart (GR) ends. LLGR ends either when the LLGR stale timer
expires or when the neighbor sends the end-of-RIB marker after it has sent its
routes. When LLGR for a neighbor ends, all routes from that neighbor that are
still LLGR stale gets deleted. The LLGR capability is signaled to a neighbor in
the BGP OPEN message, if configured. With BGP persistence the paths are held
for very long time (days), and unlike graceful-restart behavior the paths are
de-preferenced so that a non-stale path is chosen over a stale path.
advertises LLGR capability including all address-families configured with LLGR.
LLGR stale time is per address-family. The BGP persistence feature is supported
on the following address family indicators (AFIs):
persistence, the paths are held for a very long time (days), but unlike basic
graceful-restart behavior, the paths are de-preferenced so that a non-stale
path is always chosen over a stale path. When the neighbor goes down, it first
performs the classic graceful restart which consists of the following steps:
Persistence is executed by the helper router only after the
graceful-restart is completed. Persistence ends when neighbor sends end-of-row
(EoR) or persistence timer expires.