When a NSF-capable device begins a BGP session with a BGP peer, it sends an OPEN message to the peer. Included in the message
is a declaration that the NSF-capable device has “graceful restart capability.” Graceful restart is the mechanism by which
BGP routing peers avoid a routing flap following a switchover. If the BGP peer has this capability, it is aware that the device
sending the message is NSF-capable. Both the NSF-capable device and its BGP peer(s) need to exchange the Graceful Restart
Capability in their OPEN messages, at the time of session establishment. If both peers do not exchange the Graceful Restart
Capability, the session is not graceful restart capable.
If the BGP session is lost during the RP switchover, the NSF-aware BGP peer marks all routes associated with the NSF-capable
device as stale; however, it continues to use these routes to make forwarding decisions for a set period of time. This functionality
means that no packets are lost while the newly active RP is waiting for convergence of the routing information with the BGP
After an RP switchover occurs, the NSF-capable device reestablishes the session with the BGP peer. In establishing the new
session, it sends a new graceful restart message that identifies the NSF-capable device as having restarted.
At this point, the routing information is exchanged between two BGP peers. Once this exchange is complete, the NSF-capable
device uses the routing information to update the RIB and the FIB with the new forwarding information. The NSF-aware device
uses the network information to remove stale routes from its BGP table. Following that, the BGP protocol is fully converged.
If a BGP peer does not support the graceful restart capability, it will ignore the graceful-restart capability in an OPEN
message; but will establish a BGP session with the NSF-capable device. This function allows interoperability with non-NSF-aware
BGP peers (and without NSF functionality), but the BGP session with non-NSF-aware BGP peers will not be graceful restart capable.
BGP support in NSF requires that neighbor networking devices be NSF-aware; that is, devices must have the Graceful Restart
Capability and advertise that capability in their OPEN message during session establishment. If an NSF-capable device discovers
that a particular BGP neighbor does not have Graceful Restart Capability, it will not establish an NSF-capable session with
that neighbor. All other neighbors that have Graceful Restart Capability will continue to have NSF-capable sessions with this
NSF-capable networking device.