Information Technology Reference
In-Depth Information
The Restart State indicates that the router restarted. This prevents two adjacent restarting
routers from deadlocking while waiting for an end-of-RIB marker. After a router restarts
and reforms its BGP sessions, it waits until it has received the initial routing update from
each of its peers before running the best-path algorithm and sending its own updates. If the
Restart State is set to 1, they don't wait for that peer to send the initial routing update.
The Restart Time indicates how long a peer of the restarting router should maintain prefix
information received from the restarting router. For example, suppose Router A indicates a
Restart Time of 180 seconds to Router B. If Router A restarts but does not reform the
session to Router B within 180 seconds, Router B assumes that there was a problem with
Router A's restarting. Router B then removes all the stale prefix information in its RIB
from Router A, removing Router A from the forwarding path.
The Forwarding State indicates if a router successfully maintained forwarding state over
the restart. Not all platforms can maintain state information over a route processor restart.
This must be indicated in the graceful restart capability after a router restarts. If a BGP
router is capable of BGP-GR, but it cannot maintain forwarding state over a restart, it
cannot restart gracefully itself. It only participates in a supporting role. This means that if
the router that is incapable of maintaining forwarding state fails, it does not gracefully
restart. However, if one of its peers that is capable of maintaining forwarding state restarts,
it participates in updating the restarting router's BGP RIB.
A BGP router advertises the BGP-GR capability using BGP's dynamic capability negotia-
tion feature at session initiation. The reception of a BGP-GR capability with no AFI/SAFI
information indicates that the sending peer supports the end-of-RIB marker and can support
peers that can maintain forwarding state and that want to utilize BGP-GR. The reception of
a BGP-GR capability with AFI/SAFI information indicates that the sending peer wants to
perform BGP-GR for the included AFI/SAFIs.
The restart time should be less than the holdtime for the BGP peer. In the following
sections, the “restarting BGP router” is the router on which BGP has been restarted, and
the “receiving BGP router” is the BGP router that is peered with the restarting BGP router.
When the BGP router restarts, the forwarding information base (FIB) should be marked as
stale. The stale forwarding information is used to forward packets. The “stale” designation
allows the router to update the forwarding information after restart.
After the BGP router restarts, the Restart State must be set to 1 to reestablish the peering
session with the receiving BGP router. After the BGP session has reformed, the restarting
BGP router waits until it has received the full initial routing update, as denoted by the end-
of-RIB marker from the receiving BGP router. The restarting BGP router then runs the BGP
decision process, refreshes the forwarding table, and updates the receiving BGP router with
the Adj-RIB-Out terminated by the end-of-RIB marker.
When the BGP router restarts, the receiving BGP router might or might not detect the
session failure. The receiving BGP router still might have the BGP session in an Established
state when the restarting BGP router attempts to initiate a new BGP session with the
