Understanding Vss Client - Cisco Catalyst 4500 Series Software Configuration Manual

Chapter 27 Configuring EtherChannel and Link State Tracking

Understanding VSS Client

This section describes these topics:
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Virtual Switch System
The Cisco Catalyst 6500 Series VSS 1440 allows for the combination of two Cisco Catalyst 6500 Series
switches into a single, logical network entity from the network control-plane and management
perspectives. Within the Cisco VSS, one chassis is designated as the active virtual switch, acting as the
single management point of the entire system, and the other is designated as the standby virtual switch.
There two chassis are bound together by a special link, called Virtual Switch Link (VSL), which carries
the internal signaling and control information between them.
Dual-Active Scenarios
One of the failure scenarios in a VSS is called dual-active, which occurs when the VSL fails completely.
Neither virtual switch knows of the other's status. From the perspective of the active virtual switch, the
standby chassis is lost. The standby virtual switch also views the active chassis as failed and transitions
to active state by using an SSO switchover. Two active virtual switches exist in the network with identical
configurations, causing duplicate IP addresses and bridge identifiers. This scenario has adverse effects
on the network topology and traffic if it persists.
Dual-Active Detection Using Enhanced PAgP
One method for detecting a dual-active scenario is based on enhanced PAgP (PAgP+). Specifically, the
VSS sends regularly scheduled PAgP messages with Type-Length-Values (TLVs) containing the ID of
the current active virtual switch
switch immediately sends asynchronous PAgP messages with TLVs containing its own ID on all port
channels enabled for enhanced PAgP dual-active detection
(the VSS client) connected to both VSS components by using EtherChannel links, compares every
received active ID with its stored active ID. If they match, the remote switch sends TLVs containing its
stored active ID back to the VSS in its regularly scheduled PAgP messages. If they do not match, the
remote switch stores the new active ID and immediately transmits asynchronous PAgP messages with
TLVs containing the new active ID. Upon receiving the new active ID from the remote switch, the
original active virtual switch detects the dual-active scenario and takes appropriate actions.
Virtual Switch System, page 27-21
Dual-Active Scenarios, page 27-21
Dual-Active Detection Using Enhanced PAgP, page 27-21
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
(Figure 27-1). When the VSL fails completely, the standby virtual
Displaying EtherChannel to a Virtual Switch System
(Figure 27-2). The remote switch
27-21