Specifications

CHAPTER 5

Implementing High Availability

Optimizing Redundancy

You should be aware that redundancy does not always equal resiliency. Too much redundancy can increase the network

complexity to a point that it becomes harder to troubleshoot and actually leads to a less-available network. There are too

many paths for the data to follow, so it becomes less deterministic. The cost is much higher, also.

NSF with SSO

Layers 2–4 convergence time is enhanced in Cisco 4500 and 6500 series switches with redundant route processors (RP)

by using NSF with SSO. When using this, only one RP is active. The standby RP synchronizes its configuration and

dynamic state information (such as CEF, MAC, and FIB tables) with the active RP. When the active RP fails, SSO

enables the standby RP to take over immediately. NSF keeps the switch forwarding traffic during the switchover, using

the existing route and CEF tables. The goal of NSF with SSO is to prevent routing adjacencies from resetting, which

prevents a routing flap. The switchover to the new RP must be completed before routing timers expire, or the router’s

neighbors will tear down their adjacency and routing will be disrupted.

When the new RP is up, the old routes are marked as stale, and the RP asks its routing peers to refresh them. When

routing is converged, it updates the routing and CEF tables on the switch and the linecards.

NSF is supported with EIGRP, OSPF, ISIS, and BGP. An NSF-capable router supports NSF; an NSF-aware router does

not support NSF but understands it and continues forwarding traffic during SSO.

Use NSF with SSO in locations where you do not have a duplicate switch for failover, such as at the user access or

Enterprise network edge. Otherwise it can actually cause longer convergence. Routing protocols timers can be tuned very

short to provide fast convergence. With SSO, the switchover to the standby RP might not occur before the tuned routing

Dead timer expires, and the adjacency would be reset.

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CCNP SWITCH 642-813 Quick Reference by Denise Donohue

9781587140112.qxd 11/23/09 11:35 AM Page 64