Dual-Active Detection - Cisco Catalyst 4500 Series Software Configuration Manual

Chapter 5 Configuring Virtual Switching Systems
Transferring a Large File over VSL
Because the management plane of the VSS switches are performed through the Active, you might need
to send a large-config/image file from one switch to another (that is, sending a file transfer over VSL).
When you do this, the VSL link becomes "busy." Because data is flowing on a front panel port, it [the
data] is significantly slower than what you might see on a dual-supervisor standalone system because in
the latter, this action occurs through dedicated EOBC link.
On VSS, copying a large file from one switch to another may take several minutes. Hence, you should
do this only when needed. Consider a wait of several minutes before file transfer completes.

Dual-Active Detection

If the VSL fails, the VSS Standby switch cannot determine the state of the VSS Active switch. To ensure
that switchover occurs without delay, the VSS Standby switch assumes the VSS Active switch has failed
and initiates switchover to take over the VSS Active role.
If the original VSS Active switch is still operational, both switch are now VSS Active. This situation is
called a dual-active scenario. A dual-active scenario can have adverse effects on network stability,
because both switches use the same IP addresses, SSH keys, and STP bridge ID. The VSS must detect a
dual-active scenario and take recovery action.
The VSS supports the methods, Enhanced PAgP and Fast-Hello, for detecting a dual-active scenario.
PAgP uses messaging over the MEC links to communicate between the two switches through a neighbor
switch. Enhanced PAgP requires a neighbor switch that supports the PAgP enhancements.
The dual-active detection and recovery methods are described in the following sections:
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•
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Dual-Active Detection Using Enhanced PAgP
Port aggregation protocol (PAgP) is a Cisco-proprietary protocol for managing EtherChannels. If a VSS
MEC terminates to a Cisco switch, you can run PAgP protocol on the MEC. If PAgP is running on the
MECs between the VSS and an upstream or downstream switch, the VSS can use PAgP to detect a
dual-active scenario. The MEC must have at least one port on each switch of the VSS.
In virtual switch mode, PAgP messages include a new type length value (TLV) which contains the ID of
the VSS Active switch. Only switches in virtual switch mode send the new TLV.
For dual-active detection to operate successfully, one or more of the connected switches must be able to
process the new TLV. Catalyst 4500, Catalyst 4500-X, and Catalyst 49xx series switches have this
capability. For a list of other Cisco products that support enhanced PAgP, refer to Release Notes for
Cisco IOS Release at this URL:
http://www.cisco.com/en/US/products/ps6350/tsd_products_support_series_home.html
When the VSS Standby switch detects VSL failure, it initiates SSO and becomes VSS Active.
Subsequent PAgP messages to the connected switch from the newly VSS Active switch contain the new
VSS Active ID. The connected switch sends PAgP messages with the new VSS Active ID to both VSS
switches.
Dual-Active Detection Using Enhanced PAgP, page 5-23
Dual-Active Detection Using Fast-Hello, page 5-24
Recovery Actions, page 5-24
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Understanding Virtual Switching Systems
5-23