Cisco Catalyst 4500 Series, Software Configuration Manual

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Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Chapter 5 Configuring Virtual Switching Systems
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:
•
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.
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
Beginning in IOS XE release 3.8.0E, Quad-Supervisor VSS mode supports intra-chassis redundancy.
When the VSS Active fails, the in-chassis standby supervisor operates in RPR mode and becomes the
VSS Standby.
A VSS operates with stateful switchover (SSO) redundancy if it meets the following requirements:
• 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.
Note See the “SSO Dependencies” section on page 5-26 for additional details about the requirements for SSO
redundancy on a VSS. See Chapter 13, “Configuring Cisco NSF with SSO Supervisor Engine
Redundancy” for information about configuring SSO and NSF.
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.
The VSS runs stateful switchover (SSO) between the VSS Active and VSS Standby supervisor engines
(see
Figure 5-6 ). The VSS determines the role of each supervisor engine during initialization.
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 illustrates the switches’ roles in a VSS.