Cisco Catalyst 4500 series Administration Manual page 220

Understanding Virtual Switching Systems
Virtual Switch Link
For the two switches of the VSS to act as one network element, they need to share control information
and data traffic.
The virtual switch link (VSL) is a special link that carries control and data traffic between the two
switches of a VSS, as shown in
links. The VSL gives control and management traffic higher priority than data traffic so that control and
management messages are never discarded. Data traffic is load balanced among the VSL links by the
EtherChannel load-balancing algorithm.
Note EtherChannel load balancing method is a global configuration; VSL observes that method of load
balancing.
Figure 5-3
Chassis 1
When you configure VSL, all existing configurations are removed from the interface except for specific
allowed commands. When you configure VSL, the system puts the interface into a restricted mode. This
means that only specific configuration commands can be configured on the interface.
The following VSL configuration commands are inserted automatically on all VSL member ports:
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In VSL restricted mode, only these configuration commands are available:
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Software Configuration Guide—Release IOS XE 3.6.0E and IOS 15.2(2)SG
5-4
Virtual Switch Link
switchport mode trunk
switchport nonegotiate
no lldp transmit
no lldp receive
no cdp enable
service-policy output VSL-Queuing-Policy
channel-group
default
description
exit
load-interval
logging
no
power
Figure 5-3. The VSL is implemented as an EtherChannel with up to eight
Virtual switch
Virtual switch link
(VSL)
Chapter 5 Configuring Virtual Switching Systems
Chassis 2
OL-30933-01