Cisco Catalyst 4500 series Administration Manual page 222

Understanding Virtual Switching Systems
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Redundancy and High Availability
In a VSS, supervisor engine redundancy operates between the VSS Active and VSS Standby switch,
using stateful switchover (SSO) and nonstop forwarding (NSF). The peer switch exchange configuration
and state information across the VSL and the VSS Standby supervisor engine runs in SSO-HOT mode.
The VSS Standby switch monitors the VSS Active switch using the VSL. If it detects failure, the VSS
Standby switch initiates a switchover and takes on the VSS Active role. When the failed switch recovers,
it takes on the VSS Standby role.
If either the VSS Active switch fails or all links that belong to the VSL port-channel fail, the VSS
Standby switch initiates a switchover and assumes the role of the VSS Active switch. If the previous VSS
Active switch has failed, it reloads and boots as the VSS Standby switch. However, if only the VSL
port-channel failure caused the switchover, the previous VSS Active switch enters recovery mode
(provided dual-active detection is configured). In this scenario, the previous VSS Active chassis (now in
recovery mode) carries no traffic and only monitors the VSL link. When one link in the VSL
port-channel is up, the recovery mode switch reloads and boots as a VSS Standby chassis. For additional
information about dual-active detection, see the
Packet Handling
The VSS Active supervisor engine runs the Layer 2 and Layer 3 protocols and features for the VSS and
manages all ports on both switches.
The VSS uses VSL to communicate protocol and system information between the peer switches and to
carry data traffic between the switches when required.
Both switches perform packet forwarding for ingress traffic on their interfaces. If possible, ingress traffic
is forwarded to an outgoing interface on the same switch to minimize data traffic that must traverse the
VSL.
System Management
The VSS Active supervisor engine acts as a single point of control for the VSS. For example, the VSS
Active supervisor engine handles OIR of switching modules on both switches. The VSS Active
supervisor engine uses VSL to send messages to and from local ports on the VSS Standby switch.
The command console on the VSS Active supervisor engine is used to control both switches. In virtual
switch mode, the command console on the VSS Standby supervisor engine blocks attempts to enter
configuration mode.
The VSS Standby switch runs a subset of system management tasks. For example, the VSS Standby
switch handles its own power management, linecard bringup, and other local hardware management.
Quad-Supervisor (In-chassis Standby Supervisor Engine) Support
Software Configuration Guide—Release IOS XE 3.6.0E and IOS 15.2(2)SG
5-6
System Management, page 5-6
Quad-Supervisor (In-chassis Standby Supervisor Engine) Support, page 5-6
Asymmetric chassis support, page 5-8
Interface Naming Convention, page 5-8
Module Number Convention, page 5-8
Key Software Features not Supported on VSS, page 5-8
Chapter 5 Configuring Virtual Switching Systems
"Dual-Active Detection" section on page 5-23.
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