Alcatel-Lucent 6850-48 ネットワークガイド
Configuring Spanning Tree Parameters
Configuring STP Bridge Parameters
OmniSwitch AOS Release 6 Network Configuration Guide
September 2009
page 13-25
Using Automatic VLAN Containment
In a Multiple Spanning Tree (MST) configuration, it is possible for a port that belongs to a VLAN that is
not a member of an instance to become the root port for that instance. This can cause a topology change
that could lead to a loss of connectivity between VLANs/switches. Enabling Automatic VLAN Contain-
ment (AVC) helps to prevent this from happening by making such a port an undesirable choice for the
root.
not a member of an instance to become the root port for that instance. This can cause a topology change
that could lead to a loss of connectivity between VLANs/switches. Enabling Automatic VLAN Contain-
ment (AVC) helps to prevent this from happening by making such a port an undesirable choice for the
root.
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 addi-
tion, VLAN 2 is mapped to MSTI 1 on both switches.
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 addi-
tion, VLAN 2 is mapped to MSTI 1 on both switches.
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:
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:
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.
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.
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.
By default AVC is disabled on the switch. Use the
command to globally
enable this feature for all MSTIs. Once AVC is globally enabled, then it is possible to disable AVC for
individual MSTIs using the same command. For example, the following commands globally enable AVC
and then disable it for MSTI 10:
individual MSTIs using the same command. For example, the following commands globally enable AVC
and then disable it for MSTI 10:
-> bridge auto-vlan-containment enable
-> bridge msti 10 auto-vlan-containment disable
Note that 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.
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.
5/1
4/2
VLAN 1
VLAN 2
VLAN 1
VLAN 2
MSTI-1
MSTI-1
802.1q tag
2/1
3/1
VLAN 1
VLAN 2
VLAN 1
VLAN 2
MSTI-1
MSTI-1
802.1q tag
4/2
5/1
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