38-39
Catalyst 3750 SwitchSoftware Configuration Guide
OL-8550-09
Chapter38 Configuring IP Unicast Routing
Configuring EIGRP
EIGRP Nonstop Forwarding
The switch stack supports two levels of EIGRP nonstop forwarding:
•EIGRP NSF Awareness, page38-39
•EIGRP NSF Capability, page38-39

EIGRP NSF Awareness

The EIGRP NSF Awareness feature is supported for IPv4 in the IP services image. When the neighboring
router is NSF-capable, the Layer 3 switch continues to forward packets from the neighboring router
during the interval between the primary Route Processor (RP) in a router failing and the backup RP
taking over, or while the primary RP is manually reloaded for a nondisruptive software upgrade.
This feature cannot be disabled. For more information on this feature, see the EIGRP Nonstop
Forwarding (NSF) Awareness Feature Guide:
http://www.cisco.com/en/US/docs/ios/12_2t/12_2t15/feature/guide/ft_ensf.html

EIGRP NSF Capability

The IP services image supports EIGRP NSF-capable routing for IPv4 for better convergence and lower
traffic loss following a stack master change. When an EIGRP NSF-capable stack master restarts or a new
stack master starts up and NSF restarts, the switch has no neighbors, and the topology table is empty.
The switch must bring up the interfaces, reacquire neighbors, and rebuild the topology and routing tables
without interrupting the traffic directed toward the switch stack. EIGRP peer routers maintain the routes
learned from the new stack master and continue forwarding traffic through the NSF restart process.
To prevent an adjacency reset by the neighbors, the new stack master uses a new Restart (RS) bit in the
EIGRP packet header to show the restart. When the neighbor receives this, it synchronizes the stack in
its peer list and maintains the adjacency with the stack. The neighbor then sends its topology table to the
stack master with the RS bit set to show that it is NSF-aware and is aiding the new stack master.
If at least one of the stack peer neighbors is NSF-aware, the stack master receives updates and rebuilds
its database. Each NSF-aware neighbor sends an end of table (EOT) marker in the last update packet to
mark the end of the table content. The stack master recognizes the convergence when it receives the EOT
marker, and it then begins sending updates. When the stack master has received all EOT markers from
its neighbors or when the NSF converge timer expires, EIGRP notifies the routing information database
(RIB) of convergence and floods its topology table to all NSF-aware peers.
Note NSF is not supported on interfaces configured for Hot Standby Router Protocol (HSRP).
Use the nsf EIGRP routing configuration command to enable EIGRP NSF routing. Use the show ip
protocols privileged EXEC command to verify that NSF is enabled on the device. See the command
reference for this release for information about the nsf command.