5-24
Software Configuration Guide—Release IOS XE 3.6.0E and IOS 15.2(2)SG
OL-30933-01
Chapter 5 Configuring Virtual Switching Systems
Understanding Virtual Switching Systems
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.
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 5-24.
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:
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
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 “Configuring Fast-Hello
Dual-Active Detection” section on page 5-52.
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.
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