CISCO 4500R+E, Sup7L-E redundancy: RPR or SSO?

Question

We are going to deploy 4500R+E chassis with two redundant supervisors Sup7L-E. All ports in chassis - switchport with trunks. L3 interfaces - SVI. Chassis wll terminate L3 domain (route between zone), no dynamic routing protocols, only static routing.

What is the best redundancy mechanism for such deployment: RPR or SSO?

Answer 1

SSO is preferred due to shorter switchover if supervisor fails. From Cisco:

With supervisor engine redundancy enabled, if the active supervisor engine fails or if a manual switchover is performed, the standby supervisor engine becomes the "new" active supervisor engine. The standby supervisor engine has been automatically initialized with the startup configuration of the active supervisor engine, shortening the switchover time (30 seconds or longer in RPR mode, depending on the configuration; subsecond in SSO mode).

Operation of RPR:

RPR is supported in Cisco IOS-XE Release 3.1.0SG and later releases. When a standby supervisor engine runs in RPR mode, it starts up in a partially-initialized state and is synchronized with the persistent configuration of the active supervisor engine.

In a supervisor engine switchover, traffic is disrupted because in the RPR mode all of the physical ports restart since there is no state maintained between supervisor engines relating to module types and statuses. Upon switchover, when the standby supervisor engine completes its initialization, it will read hardware information directly from the module and become the active supervisor engine.

So the redundant supervisor is only partially initialized. This means there will be a disruption when the supervisor fails over.

SSO on the other hand is fully initialized.

SSO is supported in Cisco IOS-XE Release 3.1.0SG and later releases. When a standby supervisor engine runs in SSO mode, the standby supervisor engine starts up in a fully-initialized state and synchronizes with the persistent configuration and the running configuration of the active supervisor engine. It subsequently maintains the state on the protocols listed below, and all changes in hardware and software states for features that support stateful switchover are kept in synchronization. Consequently, it offers zero interruption to Layer 2 sessions in a redundant supervisor engine configuration.

Because the standby supervisor engine recognizes the hardware link status of every link, ports that were active before the switchover will remain active, including the uplink ports. However, because uplink ports are physically on the supervisor engine, they will be disconnected if the supervisor engine is removed.

If the active supervisor engine fails, the standby supervisor engine become active. This newly active supervisor engine uses existing Layer 2 switching information to continue forwarding traffic. Layer 3 forwarding will be delayed until the routing tables have been repopulated in the newly active supervisor engine.

It is therefore recommended to run SSO. For full information go to Configuring Supervisor Engine Redundancy Using RPR and SSO on Supervisor Engine 7-E and Supervisor Engine 7L-E