The routing protocols run only on the MSFC of the active supervisor engine,
and they receive routing updates from their neighbor routers. Routing protocols
do not run on the MSFC of the redundant supervisor engine. Following a
switchover, the routing protocols request that the NSF-aware neighbor devices
send state information to 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. Cisco NSF supports the BGP, OSPF, IS-IS, and EIGRP protocols
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NOTE:
For NSF operation, the routing protocols depend on CEF to continue
forwarding packets while the routing protocols rebuild the routing
information.
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EIGRP Operation
When an EIGRP NSF-capable router initially comes
back up from an NSF restart, it has no neighbors and its topology table is
empty. The router is notified by the redundant (now active) supervisor engine
when it needs to bring up the interfaces, reacquire neighbors, and rebuild the
topology and routing tables. The restarting router and its peers must
accomplish these tasks without interrupting the data traffic directed toward
the restarting router. EIGRP peer routers maintain the routes learned from the
restarting router and continue forwarding traffic through the NSF restart
process.
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 the peers do not exchange the graceful restart
capability, the session will not be graceful-restart-capable.
OSPF
Operation
When an OSPF NSF-capable router performs a supervisor engine
switchover, it must perform the following tasks in order to resynchronize its
link state database with its OSPF neighbors:
- Relearn the available OSPF neighbors on the network without causing a reset
of the neighbor relationship
- Reacquire the contents of the link state database for the network
As quickly as possible after a supervisor engine switchover, the
NSF-capable router sends an OSPF NSF signal to neighboring NSF-aware devices.
Neighbor networking devices recognize this signal as an indicator that the
neighbor relationship with this router should not be reset. As the NSF-capable
router receives signals from other routers on the network, it can begin to
rebuild its neighbor list.
IS-IS Operation
When an IS-IS
NSF-capable router performs a supervisor engine switchover, it must perform the
following tasks in order to resynchronize its link state database with its
IS-IS neighbors:
- Relearn the available IS-IS neighbors on the network without causing a
reset of the neighbor relationship
- Reacquire the contents of the link state database for the network
The IS-IS NSF feature offers two options when you configure NSF:
- Internet Engineering Task Force (IETF) IS-IS
- Cisco IS-IS