Cisco Express Forwarding
A key element of NSF is packet forwarding. In Cisco networking devices, packet forwarding is provided by Cisco Express Forwarding (CEF). Cisco Express Forwarding maintains the Forwarding Information Base (FIB), and uses the FIB information that was current at the time of the switchover to continue forwarding packets during a switchover. This feature eliminates downtime during the switchover.
During the normal NSF operation, Cisco Express Forwarding on the active fabric card synchronizes its FIB and adjacency databases with the FIB and adjacency databases on the standby fabric card. On switchover of the active fabric card, the standby fabric card initially has FIB and adjacency databases that are mirror images of those on the active fabric card. The packet forwarding continues after a switchover as soon as the interfaces and a data path are available.
Nonstop Forwarding (NSF) works with the Stateful Switch Over (SSO) feature to minimize the amount of time a network is unavailable following a switchover. The main objective of the NSF is to continue forwarding IP packets after the switchover of the active fabric card.
When a networking device restarts, all the routing peers of that device usually detect that the device went down and then came back up. This down-to-up transition results in routing flap, which could spread across multiple routing domains. Routing flaps caused by routing restarts create routing instabilities, which are detrimental to the overall network performance. NSF helps to suppress routing flaps and improves the network stability.
NSF allows for the forwarding of data packets to continue along known routes while the routing protocol information is being restored following a switchover. With NSF, peer networking devices do not experience routing flaps. Data traffic is forwarded through intelligent line cards while the standby fabric card assumes control from the failed active fabric card during a switchover. The ability of line cards to remain up during the switchover and to be kept current with the FIB on the active fabric card is key to NSF operation.
CPT does not support forwarding IP packets in hardware and supports forwarding only in software.
The following figure illustrates how SSO is typically deployed in service provider networks. In this example, NSF with SSO is primarily at the access layer (edge) of the service provider network. A fault at this point can result in loss of service for enterprise customers requiring access to the service provider network.
Figure 1. NSF with SSO Network Deployment: Service Provider Networks
Additional levels of availability may be gained by deploying NSF with SSO at other points in the network where a single point of failure exists. The following figure illustrates an optional deployment strategy that applies NSF with SSO at the enterprise network access layer. In this example, each access point in the enterprise network represents another single point of failure in the network design. In the event of a switchover or a planned software upgrade, enterprise customer sessions would continue uninterrupted throughout the network.
Figure 2. NSF with SSO Network Deployment: Enterprise Networks
Benefits of NSF
The NSF feature has the following benefits:
Improved network availability—NSF continues to forward network traffic and application state information so that user session information is maintained after a switchover.
Overall network stability—Network stability may be improved with the reduction in the number of route flaps that had been created when routers in the network failed and lost their routing tables.
Prevents routing flaps—Because the NSF continues to forward network traffic in the event of a switchover, routing flaps are avoided.
No loss of user sessions—User sessions established prior to the switchover are maintained.
Each protocol depends on Cisco Express Forwarding to continue forwarding packets during switchover while the routing protocols rebuild the Routing Information Base (RIB) tables. After the routing protocols have converged, Cisco Express Forwarding updates the FIB table and removes stale route entries. Cisco Express Forwarding updates the line cards with the new FIB information.
The routing protocols run only on the active fabric card, and they receive routing updates from the neighboring routers. After a switchover, the routing protocols request that the NSF-aware neighboring devices send the state information to help rebuild the routing tables.
During the NSF operation, the routing protocols depend on Cisco Express Forwarding to continue forwarding the packets while the routing protocols rebuild the routing information.
When an OSPF NSF-capable router performs a fabric card switchover, it must perform two tasks to resynchronize its link state database with its OSPF neighbors. Firstly, it must relearn the available OSPF neighbors on the network without causing a reset of the neighbor relationship. Secondly, it must reacquire the contents of the link state database for the network.
After the fabric card switchover, the NSF-capable router sends an OSPF NSF signal to the neighboring NSF-aware devices. The neighboring network devices recognize this signal as a clue that the neighbor relationship with this router must not be reset. As the NSF-capable router receives signals from other routers on the network, it can begin to rebuild its neighbor list.
When neighbor relationships are reestablished, the NSF-capable router begins to resynchronize its database with all of its NSF-aware neighbors. At this point, the routing information is exchanged between the OSPF neighbors. After this exchange is complete, the NSF-capable device uses the routing information to remove stale routers and update the RIB and FIB with the new forwarding information. The OSPF protocols are then fully converged.
The OSPF NSF requires that all the neighboring network devices be NSF-aware. If a NSF-capable router discovers that it has non NSF-aware neighbors on a particular network segment, it will disable NSF capabilities for that segment. Other network segments composed entirely of NSF-capable or NSF-aware routers will continue to provide NSF capabilities.
NSF Device Modes
Cisco had implemented the proprietary Cisco NSF. The Graceful OSPF Restart feature supports IETF NSF for OSPF processes in multivendor networks. The NSF device modes of operation common to the Cisco and IETF NSF implementations are as follows:
Restarting mode—In this mode, the OSPF device is performing nonstop forwarding recovery because of the fabric card switchover.
Helper mode—Also known as NSF–awareness. In this mode, the neighboring device is restarting and helping in the NSF recovery.