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| 6.1 |  | Implementing Module Redundancy in a Multilayer Switched Network |
| 6.1.2 |  | Implementing redundant supervisor engines in Catalyst switches |

Dual supervisor engines provide hardware redundancy for the forwarding intelligence of the Catalyst 5000, 5500, 6000, and 6500 Series. These Catalyst switches can support two supervisor engines installed only in slots one and two. One supervisor serves as the running, or active, supervisor and the other serves as the standby supervisor. The active supervisor engine is the first one to come online. This can be confirmed by the "Active" LED externally on the supervisor or by typing the show module command at the console. Redundant supervisor engines must be of the same type with the same model feature card. If an active supervisor is taken offline or fails, the standby supervisor takes over control of the system.

The two supervisor engines in a redundant configuration have distinct functions. The active supervisor is responsible for controlling the system bus and all line cards. All protocols are running on the active supervisor and it performs all packet forwarding. The standby supervisor does not communicate with the line cards. The standby supervisor receives packets from the network and populates its forwarding tables with this information. However, the standby supervisor does not participate in any packet forwarding or provide any load sharing. The relevant protocols on the system are initialized, but not active, on the standby supervisor. The Catalyst 6500 series supervisor engines are hot-swappable. This means the standby supervisor engine can be installed on an active system without affecting network operation. The active supervisor provides the entire packet forwarding intelligence for the system. This is called N+1 redundancy. If the active supervisor fails, the standby supervisor is able to maintain the same system load.

The standby supervisor polls the active supervisor on a regular interval using an out-of-band channel. The active supervisor engine may go offline for the following reasons:

- Hardware failures
- System overload conditions
- Memory corruption issues
- Removal from chassis
- Operator reset

The standby supervisor detects these types of failure and becomes the active supervisor. The Catalyst IOS software on the supervisor engine is responsible for restoring the protocols, line cards, and forwarding engines to normal operation. This restoration takes place using a fast switchover or a high-availability switchover.