Alcatel-Lucent OmniSwitch 9900 Series Network Configuration Manual page 155

Configuring Spanning Tree Parameters
When AVC is enabled, it identifies undesirable ports and automatically configures them with an infinite
path cost value. For example, in the following diagram a link exists between VLAN 2 on two different
switches. The ports that provide this link belong to default VLAN 1 but are tagged with VLAN 2. In
addition, VLAN 2 is mapped to MSTI 1 on both switches.
MSTI-1
In the above diagram, port 4/2 is the Root port and port 5/1 is a Designated port for MSTI 1. AVC is not
enabled. If another link with the same speed and lower port numbers is added to default VLAN 1 on both
switches, the new link becomes the root for MSTI 1 and the tagged link between VLAN 2 is blocked, as
shown below:
MSTI-1
If AVC was enabled in the above example, AVC would have assigned the new link an infinite path cost
value that would make this link undesirable as the root for MSTI 1.
Balancing VLANs across links according to their Multiple Spanning Tree Instance (MSTI) grouping is
highly recommended to ensure that there is not a loss of connectivity during any possible topology
changes. Enabling AVC on the switch is another way to prevent undesirable ports from becoming the root
for an MSTI.
To change the default status of the AVC on the switch and to globally enable this feature for all MSTIs,
use the spantree auto-vlan-containment
disable AVC for individual MSTIs using the same command. For example, the following commands
globally enable AVC and then disable it for MSTI 10:
-> spantree auto-vlan-containment enable
-> spantree msti 10 auto-vlan-containment disable
Note. An administratively set port path cost takes precedence and prevents AVC configuration of the path
cost. The exception to this is if the port path cost is administratively set to zero, which resets the path cost to
the default value. In addition, AVC does not have any effect on root bridges.
OmniSwitch AOS Release 8 Network Configuration Guide
VLAN 1
4/2
802.1q tag
VLAN 2
3/1
VLAN 1
||
4/2
802.1q tag
VLAN 2
command. Once AVC is globally enabled, then it is possible to
Configuring STP Bridge Parameters
VLAN 1
5/1
VLAN 2
2/1
VLAN 1
5/1
VLAN 2
December 2017
MSTI-1
MSTI-1
page 6-32