Dual-active detection using enhanced pagp – Cisco 6500 User Manual
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Cisco IOS Software Configuration Guide, Release 12.2SX
OL-13013-06
Chapter 4 Configuring Virtual Switching Systems
Understanding Virtual Switching Systems
If the original VSS active chassis is still operational, both chassis are now VSS active. This situation is
called a dual-active scenario. A dual-active scenario can have adverse affects on network stability,
because both chassis 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 these three methods for detecting a dual-active scenario:
•
Enhanced PAgP—Uses PAgP messaging over the MEC links to communicate between the two
chassis through a neighbor switch. Enhanced PAgP is faster than IP BFD, but requires a neighbor
switch that supports the PAgP enhancements.
•
IP Bidirectional Forwarding Detection (BFD)—Uses BFD messaging over a backup Ethernet
connection. IP BFD uses a direct connection between the two chassis and does not require support
from a neighbor switch.
•
dual-active fast-hello—Uses special hello messages over a backup Ethernet connection. Dual-active
fast-hello is faster than IP BFD and does not require support from a neighbor switch. This method
is available only in Cisco IOS Release 12.2(33)SXI and later releases,
You can configure all three detection methods to be VSS active at the same time.
For line redundancy, we recommend dedicating at least two ports per switch for dual-active detection.
For module redundancy, the two ports can be on different switching modules in each chassis, and should
be on different modules than the VSL links, if feasible.
The dual-active detection and recovery methods are described in the following sections:
•
Dual-Active Detection Using Enhanced PAgP, page 4-23
•
Dual-Active Detection Using IP BFD, page 4-24
•
Dual-Active Detection Using Dual-Active Fast Hello Packets, page 4-24
•
Dual-Active Detection Using Enhanced PAgP
If a VSS MEC terminates on a Cisco switch, you can run the port aggregation protocol (PAgP) on the
MEC. If enhanced PAgP is running on an MEC between the VSS and another switch running Release
12.2(33)SXH1 or a later release, the VSS can use enhanced PAgP to detect a dual-active scenario.
The MEC must have at least one port on each chassis of the VSS. In VSS mode, PAgP messages include
a new type length value (TLV) that contains the ID of the VSS active switch. Only switches in VSS mode
send the new TLV.
When the VSS standby chassis detects VSL failure, it initiates SSO and becomes VSS active. Subsequent
PAgP messages to the connected switch from the newly VSS active chassis contain the new VSS active
ID. The connected switch sends PAgP messages with the new VSS active ID to both VSS chassis.
If the formerly VSS active chassis is still operational, it detects the dual-active scenario because the VSS
active ID in the PAgP messages changes. This chassis initiates recovery actions as described in the