Catalyst 3750 and RPR+/NSF

Unanswered Question
Jul 5th, 2007

As far as I understand, there are several different mechanisms of high availability for Catalyst switches. The document

"Nonstop forwarding with statefull switchover on the Cisco Catalyst 6500" (http://www.cisco.com/application/pdf/en/us/guest/products/ps708/c1244/cdccont_0900aecd801c5cd7.pdf), describes RPR+ as follows:

"No link layer or control-plane information is synchronized between the active and the

standby supervisors. Interfaces may bounce after switchover, and the hardware contents need to be reprogrammed. The RPR+ switchover time is 30 or more seconds."

SSO as:

"SSO - SSO expands the RPR+ capabilities to provide transparent failover of Layer 2 protocols when a supervisor failure occurs. SSO is stateful for Layer 2 protocols? SSO is a requirement for SRM with SSO and NSF with SSO"

And NSF:

"It allows a router experiencing the failure of an active supervisor to continue forwarding data packets along known routes while the routing protocol information is recovered and validated. This forwarding can continue to occur even though peering arrangements with neighbor routers have been lost on the restarting router. NSF relies on the separation of the control plane and the data plane during supervisor switchover."

The above suggests that in order to continue forwarding packets in case of a switchover the switch must have FIBs synchronized between active and standby supervisors. I.e. NSF can only work with SSO and not RPR/RPR+.

However the document Cisco StackWise Technology White Paper

(http://cisco.com/en/US/products/hw/switches/ps5023/products_white_paper09186a00801b096a.shtml) in chapter "Multiple Mechanisms for High Availability", says:

"RPR+ for Layer 3 resiliency-Each switch is initialized for routing capability and is ready to be elected as master if the current master fails. Subordinate switches are not reset so that Layer 2 forwarding can continue uninterrupted. Layer 3 Nonstop Forwarding (NSF) is also supported when two or more nodes are present in a stack."

So NSF is declared to work with RPR+. In fact our laboratory tests showed that a stack of 3750s keeps forwarding at L2 and L3 without interruption in case of stack master failure. We don?t quite understand how it could be provided that RPR+ doesn?t synchronize FIB tables between routing processors.

Is it possible that the very architecture of Catalyst 3750 stack calls for FIB synchronization between all stack members regardless of high availability mechanism realization?

I have this problem too.
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gmarogi Thu, 07/12/2007 - 07:42

I think it can be possible that the very architecture of Catalyst 3750 stack calls for FIB synchronization between all stack members regardless of high availability mechanism realization.

Amit Singh Thu, 07/12/2007 - 09:20

Pavel,

The Stack master generates, maintains, and distributes the distributed Cisco Express Forwarding (dCEF) database to all stack members. The routes are programmed on all switches in the stack bases on this database.The stack members program the routes into hardware. The routes programmed by the stack members are the same that are downloaded by the stack master as part of the dCEF database. If a stack master fails, the stack detects that the stack master is down and elects one of the stack members to be the new stack master. During this period, except for a momentary interruption, the hardware continues to forward packets with no active protocols.

HTH,Please rate if it does.

-amit singh

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