In Catalyst 4500 and 6500 series switches, Cisco Non-Stop Forwarding (NSF) works with in conjunction with SSO to minimize the amount of time a network is unavailable to its users following a switchover while continuing to forward IP packets. NSF is primarily used to ensure the continued forwarding IP packets following a Supervisor Engine switchover.
NSF is supported by BGP, OSPF, EIGRP, IS-IS, as well as CEF. NSF allows the routing protocols to detect a switchover and take the necessary action to continue forwarding network traffic. NSF allows routing protocols to recover route information from the NSF-capable peer devices instead of waiting for the FIB to be rebuilt before the switch can actually begin forwarding traffic. This allows for high availability and resiliency during Supervisor Engine switchover.
When NSF is implemented, routing protocols depend on CEF to continue forwarding packets during switchover while they build the Routing Information Base (RIB) tables. After the routing protocols have converged, CEF updates the FIB table and removes stale route entries.
CEF then updates the switch modules with the new FIB information. Cisco NSF is configured on a per-routing protocol basis. While the configuration of routing protocols is beyond the scope of the SWITCH exam requirements, Examples 8-73 to 8-76 show the commands required to enable NSF for OSPF, IS-IS, EIGRP, and BGP in Cisco IOS software.
Example 8-73: Enabling NSF for OSPF
| Catalyst-6500-Switch(config)#router ospf [process id] Catalyst-6500-Switch(config-router)#nsf |
Example 8-74: Enabling NSF for OSPF
| Catalyst-6500-Switch(config)#router isis [tag] Catalyst-6500-Switch(config-router)#nsf [cisco|ietf] |
NOTE: IS-IS supports both Cisco NSF and IETF NSF. The IETF NSF implementation is based on a proposed standard while the Cisco NSF implementation is based on Cisco-proprietary operation. You are not expected to go into detail on the differences between the two standards.
Example 8-75: Enabling NSF for EIGRP
| Catalyst-6500-Switch(config)#router eigrp [autonomous system number] Catalyst-6500-Switch(config-router)#nsf |
Example 8-76: Enabling NSF for BGP
| Catalyst-6500-Switch(config)#router bgp [autonomous system number] Catalyst-6500-Switch(config-router)#bgp graceful-restart |
NOTE: By default, CEF NSF is enabled when SSO redundancy mode is enabled and no further configuration is necessary. NSF operation can be verified in the output of the show ip protocols command for OSPF, IS-IS and EIGRP. For BGP, the show ip bgp neighbors [address] command can be used to verify NSF configuration as illustrated in Example 8-77.
Example 8-77: Verifying NSF for BGP
| Catalyst-6500-Switch#show ip bgp neighbors 150.1.1.1 BGP neighbor is 150.1.1.1, remote AS 1, external link BGP version 4, remote router ID 1.1.1.1 BGP state = Established, up for 00:00:21 Last read 00:00:21, last write 00:00:21, hold time is 180, keepalive interval is 60 seconds Neighbor capabilities: Route refresh: advertised and received(old & new) Address family IPv4 Unicast: advertised and received Graceful Restart Capability: advertised and received Remote Restart timer is 120 seconds Address families preserved by peer: none Message statistics: InQ depth is 0 OutQ depth is 0 Sent Rcvd Opens: 1 1 Notifications: 0 0 Updates: 2 2 Keepalives: 3 3 Route Refresh: 0 0 Total: 6 6 Default minimum time between advertisement runs is 30 seconds … [Truncated Output] … |