Ospf Nonstop Routing - Cisco Catalyst 6500-E Series Manual

Switch as the backbone of a unified access campus architecture

Hide thumbs Also See for Catalyst 6500-E Series:

Table of Contents

Figure 25.

OSPF NSF Communication Example

Notice that the first communication between the NSF-capable device and NSF-aware device is a "grace Link State

Advertisement (LSA)" (NSF is referred to as graceful restart in IETF standards). If the NSF-aware device did not

have NSF awareness configured, then it would not understand the grace LSA and would undergo a routing flap,

which could result in routing instability within the entire OSPF topology. After the standby control plane on the

NSF-capable device comes online, the link state database and FIB are rebuilt. A comparison of the new FIB

contents is made to determine if any of the last-known-good entries need to be updated. If so, then just those

entries requiring an update are changed, while the rest are left unchanged.

OSPF Nonstop Routing

Starting with Cisco IOS Software Release 15.1(1)SY and newer, the Cisco Catalyst 6500-E with Supervisor

Engine 2T supports the OSPFv2 Nonstop Routing (NSR) feature, which increases the availability of any

infrastructure running OSPFv2 (OSPFv3 NSR will be added in a later code release). Although OSPF NSR serves

a function similar to that of OSPF NSF, it works differently. With NSF, OSPF on the newly active standby control

plane initially has no state information, so it uses extensions to the OSPF protocol to recover its state from

neighboring OSPF routers. For this to work, the neighbors must support the NSF protocol extensions and be

willing to act as "helpers" to the restarting router. They must also continue forwarding data traffic to the restarting

router while this recovery is taking place.

With NSR, by contrast, the router performing the switchover preserves its state internally, and in most cases the

neighbors are unaware that anything has happened. Because no assistance is needed from neighboring routers,

NSR can be used in situations where NSF cannot - for example, in networks where not all the neighbors

implement the NSF protocol extensions.

Page 26 of 28

Table of Contents