Alcatel 9000 Betriebsanweisung PDF herunterladen (Seite 136)

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Configuring STP Bridge Parameters Configuring Spanning Tree Parameters

page 6-20 OmniSwitch 6800/6850/9000 Network Configuration Guide June 2006

Using Automatic VLAN Containment

In an 802.1s 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

Containment (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.

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:

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.

By default AVC is disabled on the switch. Use the bridge auto-vlan-containment command to globally

enable this feature for all 802.1s 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:

-> 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.

5/1

4/2

VLAN 1

VLAN 2

VLAN 1

VLAN 2

MSTI-1MSTI-1

802.1q tag

2/1

3/1

VLAN 1

VLAN 2

VLAN 1

VLAN 2

MSTI-1MSTI-1

802.1q tag

4/2

5/1

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