Cisco 6503 White Paper page 5

Redundant Supervisor Engines
As previously mentioned, the High Availability feature on the Cisco Catalyst 6500 Series provides low-impact, stateful
switchover between redundant supervisor engines. This feature was first available in Cisco Catalyst OS Software Version 5.4.
Supervisor Engine Switchover
Dual supervisor engines provide hardware redundancy for the forwarding intelligence of the Cisco Catalyst 6500 Series. The
Cisco Catalyst 6500 Series can support up to two supervisor engines in slots 1 and 2 only. One is the active supervisor engine
and the other is the standby supervisor engine. The active supervisor engine is the first one to go online. This can be confirmed
by the "Active" LED on the supervisor engine or by typing the show module command from the console. Both supervisor
engines must be the same hardware models. This means that if a Policy Feature Card (PFC) and a MSFC are on a Supervisor
1A in slot 1, then a PFC and MSFC must be also on a Supervisor Engine 1A in slot 2, or if a Supervisor Engine 2 is in slot 1,
a Supervisor Engine 2 must also be in slot 2. Supervisor engines 1A and 2 can be used in the Cisco Catalyst 6000 and 6500
series. If an active supervisor is taken offline or fails, the standby supervisor takes control of the system.
The two supervisor engines in a redundant supervisor configuration have different responsibilities. The active supervisor
engine is responsible for controlling the system bus and all line cards. All protocols are running on the active supervisor engine
and it performs all packet forwarding. The standby supervisor engine does not communicate with the line cards. It receives
packets from the network and populates its forwarding tables with this information but does not participate in any packet
forwarding. The relevant protocols on the system are initialized, but not active, on the standby supervisor engine. The Cisco
Catalyst 6500 Series supervisor engines are hot swappable and the standby supervisor engine can be installed in an active
system without affecting network operation. Also important to note is that redundant supervisor engines do not perform load
sharing. The active supervisor engine is providing the entire packet forwarding intelligence for the system (N+1 redundancy).
If the active supervisor engine fails, the standby supervisor engine can maintain the same system load.
The standby supervisor engine polls the active supervisor engine via the Ethernet out-of-band channel (EOBC) every 5–10
milliseconds to monitor the online status of the active supervisor engine. The active supervisor engine may go offline for a
variety of reasons such as hardware failures, system overload conditions, memory corruption issues, removal from chassis, or
being reset by the operator. The standby supervisor engine detects this type of failure and becomes the new active supervisor
engine. The Cisco Catalyst OS software on the supervisor engine is responsible for restoring the protocols, line cards, and
forwarding engines to normal operation. This restoration takes place via a fast switchover or a high-availability switchover.
Supervisor Fast Switchover
Because the Cisco Catalyst OS High Availability feature is disabled by default, the alternative is referred to as Fast Switchover.
The Fast Switchover feature is the predecessor to the High Availability feature and as such is the supervisor switchover
mechanism in place when high availability is disabled or not supported in the software version. This feature reduces the
switchover time by skipping some events that would typically take place should a supervisor fail. Specifically, the fast
switchover mechanism allows each line card to skip the respective software downloads and a portion of the diagnostics, which
are normally a part of system re-initialization. The switchover still includes restarting all protocols at Layer 2 and above as
well as resetting all ports. The resulting switchover performance with default settings will take approximately 28 seconds plus
the time it takes for the protocols to restart. As an example, a switch with the default time values for the Spanning-Tree
Protocol took approximately 58 seconds after the fast switchover to begin forwarding traffic again. However, the time to begin
forwarding traffic after a fast switchover can be reduced by tuning the switch from the default settings. By enabling Portfast,
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