Cisco Catalyst 4500 Series, Software Configuration Manual

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5-13
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
VSL Failure
To ensure fast recovery from VSL failures, fast link failure detection is enabled in virtual switch mode
on all VSL port channel members.
Note Fast link notification is based upon internal hardware assisted BFD sessions between the pair of physical
VSL links.
If a single VSL physical link goes down, the VSS adjusts the port group so that the failed link is not
selected.
If the VSS Standby switch detects complete VSL link failure, it initiates a stateful switchover (SSO). If
the VSS Active switch has failed (causing the VSL links to go down), the scenario is switch failure, as
described in the previous section.
If only the VSL has failed and the VSS Active switch is still operational, this is a dual-active scenario.
The VSS detects that both switches are operating in VSS Active mode and performs recovery action. See
the
“Dual-Active Detection” section on page 5-23 for additional details about the dual-active scenario.
User Actions
From the VSS Active switch command console, you can initiate a VSS switchover or a reload.
If you enter the reload command from the command console, it performs a reload on the switch
where reload is issued.
To reload only the VSS Standby switch, use the redundancy reload peer command.
To force a switchover from the VSS Active to the VSS Standby supervisor engine, use the
redundancy force-switchover command.
To reset both the VSS Active and Standby switch, use the redundancy reload shelf command.
To reset the in-chassis standby (ICS), use the hw-module module < supervisor slot number> reset
command.
Multichassis EtherChannels
These sections describe multichassis EtherChannels (MECs):
•
Overview, page 5-13
• MEC Failure Scenarios, page 5-14
Overview
A multichassis EtherChannel is an EtherChannel with ports that terminate on both switches of the VSS
(see
Figure 5-7 ). A VSS MEC can connect to any network element that supports EtherChannel (such as
a host, server, router, or switch).
At the VSS, an MEC is an EtherChannel with additional capability: the VSS balances the load across
ports in each switch independently. For example, if traffic enters the VSS Active switch, the VSS will
select an MEC link from the VSS Active switch. This MEC capability ensures that data traffic does not
unnecessarily traverse the VSL.