Overview - Cisco Catalyst 4500 series Administration Manual

Understanding Virtual Switching Systems

Overview

A VSS operates stateful switchover (SSO) between the VSS Active and VSS Standby supervisor
engines. Compared to standalone mode, a VSS has the following important differences in its redundancy
model:
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If the VSS Active supervisor engine fails, the VSS Standby supervisor engine initiates a switchover and
assumes the VSS Active role.
RPR and SSO Redundancy
A VSS operates with stateful switchover (SSO) redundancy if it meets the following requirements:
•
•
•
Note See the
redundancy on a VSS. See
Redundancy"
With SSO redundancy, the VSS Standby supervisor engine is always ready to assume control following
a fault on the VSS Active supervisor engine. Configuration, forwarding, and state information are
synchronized from the VSS Active supervisor engine to the redundant supervisor engine at startup and
whenever changes to the VSS Active supervisor engine configuration occur. If a switchover occurs,
traffic disruption is minimized.
If a VSS does not meet the requirements for SSO redundancy, it will be incapable of establishing a
relationship with the peer switch. Catalyst 4500/4500-X series switches' VSS does not support route
processor redundancy (RPR) mode.
The VSS runs stateful switchover (SSO) between the VSS Active and VSS Standby supervisor engines
(see
The supervisor engine in the VSS Standby switch runs in hot standby state. The VSS uses the VSL link
to synchronize configuration data from the VSS Active to the VSS Standby supervisor engine. Also,
protocols and features that support high availability synchronize their events and state information to the
VSS Standby supervisor engine.
Switch Roles in a VSS
Figure 5-6
Software Configuration Guide—Release IOS XE 3.6.0E and IOS 15.2(2)SG
5-12
The VSS Active and VSS Standby supervisor engines are hosted in separate switches and use the
VSL to exchange information.
The VSS Active supervisor engine controls both switches of the VSS. The VSS Active supervisor
engine runs the Layer 2 and Layer 3 control protocols and manages the switching modules on both
switches.
The VSS Active and VSS Standby switches perform data traffic forwarding.
Both supervisor engines must be running the same software version, unless it is in the process of
software upgrade.
VSL-related configuration in the two switches must match.
SSO and nonstop forwarding (NSF) must be configured on each switch.
"SSO Dependencies" section on page 5-27
Chapter 12, "Configuring Cisco NSF with SSO Supervisor Engine
for information about configuring SSO and NSF.
Figure 5-6). The VSS determines the role of each supervisor engine during initialization.
illustrates the switches' roles in a VSS.
Chapter 5 Configuring Virtual Switching Systems
for additional details about the requirements for SSO
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