Cisco Catalyst 6500-E Series Manual page 25

Switch as the backbone of a unified access campus architecture

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functions, including multichassis EtherChannel (MEC) and quad-supervisor SSO; and Nonstop Routing for Open

Shortest Path First Version 2 (OSPFv2), all of which are discussed in this section.

Nonstop Forwarding with Stateful Switchover (NSF/SSO)

The Cisco Catalyst 6500-E with Supervisor Engine 2T mitigates hardware malfunction by allowing a redundant

supervisor engine, either within the same chassis or in a second chassis in VSS mode, to take over if the primary

supervisor engine fails. SSO (frequently used with NSF) minimizes the time a network is unavailable to its users

following a switchover while continuing to forward IP packets.

NSF works with SSO to minimize the amount of time a network is unavailable to its users following a switchover.

The main objective of Cisco NSF is to prevent an unnecessary change in the routing topology as a result of a

control-plane failure.

Usually, when a networking device restarts, all routing peers of that device detect that the device went down and

then came back up. This transition results in what is called a 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 in SSO-enabled devices, thus reducing network

instability.

A primary element of NSF is packet forwarding. In a Cisco networking device, packet forwarding is provided by

Cisco Express Forwarding. Cisco Express Forwarding is always enabled in Cisco Catalyst 6500-E Series Switches

and cannot be disabled. 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 reduces traffic interruption during the switchover.

When working with NSF, there are two possible operational roles for each node: NSF-capable and NSF-aware.

NSF-capable devices are those that have dual control planes and are configured to perform an NSF restart should

the active control plane fail. NSF-capable devices can be one physical device with two control planes, such as a

Cisco Catalyst 6500-E with dual Supervisor Engine 2Ts, or they can be one logical device with two control planes,

such as a VSS 4T with one Supervisor Engine 2T in each chassis of the VSS.

NSF-aware devices are those devices that are running NSF-compatible routing protocols (Enhanced Interior

Gateway Routing Protocol [EIGRP], OSPF, BGP, and Intermediate System-to-Intermediate System [IS-IS]) and

are capable of assisting an NSF-capable device perform a restart of the routing process. If a device is NSF-

capable and is running a routing protocol with NSF enabled, then all of the neighbor devices running that routing

protocol must be at least NSF-aware, but they can be NSF-capable as well. Figure 25 shows the OSPF

communication between the NSF-capable device and the NSF-aware device during an NSF operation.

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