Dual-Active Detection Using Fast-Hello - Cisco Catalyst 4500 Series Software Configuration Manual

Understanding Virtual Switching Systems
If the formerly VSS Active switch is still operational, it detects the dual-active scenario because the VSS
Active ID in the PAgP messages changes. This switch initiates recovery actions as described in the
"Recovery Actions" section on page

Dual-Active Detection Using Fast-Hello

Dual-Active fast-hello employs fast-hello Layer 2 messages over a direct Ethernet connection. When the
VSL goes down, the event is communicated to the peer switch. If the switch was operating as the active
before the VSL went down, it goes into recovery mode upon receipt of a VSL down indication from the
peer switch. This method is faster than IP BFD and ePAGP and does not require a neighboring switch.
Fast-Hello Link
A fast-hello link is configured between two VSS members with the intention of detecting a dual-active
condition. Configuring dual-active fast-hello automatically removes all configurations from the
specified interfaces, and restricts the interface to dual-active configuration commands. The following
commands are allowed only in restricted mode on a fast-hello interface:
No data traffic other than fast-hello can be used by fast-hello links.
For details on how to configure fast-hello dual-active detection, see the
Dual-Active Detection" section on page
Recovery Actions
An VSS Active switch that detects a dual-active condition shuts down (by err-disabling) all of its
non-VSL interfaces to remove itself from the network, and waits in recovery mode until the VSL links
have recovered. You might need to intervene directly to fix the VSL failure. When the shut down switch
detects that VSL is operational again, the switch reloads and returns to service as the VSS Standby
switch.
Loopback interfaces are also shut down in recovery mode. The loopback interfaces are operationally
down and not err-disabled.
Note If the running configuration of the switch in recovery mode has been changed without saving, the switch
will not automatically reload. In this situation, you must write the configuration to memory and then
reload manually using the reload command. Only configuration changes applied to VSL ports on the
switch can be saved. All other configuration changes are discarded as the node reboots as VSS standby.
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
5-24
default—Sets a command to its defaults
description—Describes the interface
dual-active—Specifies a virtual switch dual-active config
exit—Exits from the fast hello interface configuration mode
load-interval—Specifies the interval for load calculation on an interface
logging—Configures logging for interface
no—Negates a command or set its defaults
shutdown—Shuts down the selected interface
Chapter 5
5-24.
5-53.
Configuring Virtual Switching Systems
"Configuring Fast-Hello