Cisco 6500 - Manual (Page 24)

4-24
Cisco IOS Software Configuration Guide, Release 12.2SX
OL-13013-06
Chapter 4 Configuring Virtual Switching Systems
Understanding Virtual Switching Systems
Dual-Active Detection Using IP BFD
To use the IP BFD detection method, you must provision a direct Ethernet connection between the two
switches. Regular Layer 3 ping will not function correctly on this connection, as both chassis have the
same IP address. The VSS instead uses the Bidirectional Forwarding Detection (BFD) protocol.
If the VSL fails, both chassis create BFD neighbors, and try to establish adjacency. If the original VSS
active chassis receives an adjacency message, it realizes that this is a dual-active scenario, and initiates
recovery actions as described in the
“Recovery Actions” section on page 4-24 .
Note If Flex Links are configured on the VSS, we recommend using the PAgP detection method. Do not
configure Flex Links and BFD dual-active detection on the same VSS.
Dual-Active Detection Using Dual-Active Fast Hello Packets
Cisco IOS Release 12.2(33)SXI and later releases support the dual-active fast hello method.
To use the dual-active fast hello packet detection method, you must provision a direct Ethernet
connection between the two VSS chassis. You can dedicate up to four non-VSL links for this purpose.
The two chassis periodically exchange special Layer 2 dual-active hello messages containing
information about the switch state. If the VSL fails and a dual-active scenario occurs, each switch
recognizes from the peer’s messages that there is a dual-active scenario and initiates recovery actions as
described in the
“Recovery Actions” section on page 4-24 . If a switch does not receive an expected
dual-active fast hello message from the peer before the timer expires, the switch assumes that the link is
no longer capable of dual-active detection.
Recovery Actions
An VSS active chassis that detects a dual-active condition shuts down all of its non-VSL interfaces
(except interfaces configured to be excluded from shutdown) to remove itself from the network, and
waits in recovery mode until the VSL links have recovered. You might need to physically repair the VSL
failure. When the shut down chassis detects that VSL is operational again, the chassis reloads and returns
to service as the VSS standby chassis.
Loopback interfaces are also shut down in recovery mode. Do not configure loopback interfaces while
in recovery mode, because any new loopback interfaces configured in recovery mode will not be shut
down.
Note If the running configuration of the chassis in recovery mode has been changed without saving, the
chassis will not automatically reload. In this situation, you must save the running configuration and then
reload manually.
VSS Initialization
A VSS is formed when the two chassis and the VSL link between them become operational. The peer
chassis communicate over the VSL to negotiate the chassis roles.
If only one chassis becomes operational, it assumes the VSS active role. The VSS forms when the second
chassis becomes operational and both chassis bring up their VSL interfaces.