Cisco Systems 4500 BGP Operation

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Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 9 Configuring Cisco NSF with SSO Supervisor Engine Redundancy
About NSF with SSO Supervisor Engine Redundancy
help rebuild the routing tables. Alternately, the IS-IS protocol can be configured to synchronize state
information from the active to the redundant supervisor engine to help rebuild the routing table on the
NSF-capable device in environments where neighbor devices are not NSF-aware. NSF supports the BGP,
OSPF, IS-IS, and EIGRP protocols.
Note For NSF operation, the routing protocols depend on CEF to continue forwarding packets while the
routing protocols rebuild the routing information.
BGP Operation
When an NSF-capable router begins a BGP session with a BGP peer, it sends an OPEN message to the
peer. Included in the message is a statement 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 received this capability, it is aware that the device sending the message
is NSF-capable. Both the NSF-capable router and its BGP peers 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 will not be capable of a graceful restart.
If the BGP session is lost during the supervisor engine switchover, the NSF-aware BGP peer marks all
the routes associated with the NSF-capable router as stale; however, it continues to use these routes to
make forwarding decisions for a set period of time. This functionality prevents packets from being lost
while the newly active supervisor engine is waiting for convergence of the routing information with the
BGP peers.
After a supervisor engine switchover occurs, the NSF-capable router reestablishes the session with the
BGP peer. In establishing the new session, it sends a new graceful restart message that identifies the
NSF-capable router as having restarted.
At this point, the routing information is exchanged between the two BGP peers. After 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; the BGP protocol then is fully converged.
If a BGP peer does not support the graceful restart capability, it ignores the graceful restart capability in
an OPEN message but establishes 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 is not capable of a graceful restart.
Note BGP support in NSF requires that neighbor networking devices be NSF-aware; that is, the devices must
have the graceful restart capability and advertise that capability in their OPEN message during session
establishment. If an NSF-capable router discovers that a particular BGP neighbor does not have graceful
restart capability, it does not establish an NSF-capable session with that neighbor. All other neighbors
that have graceful restart capability continue to have NSF-capable sessions with this NSF-capable
networking device.