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Configuring Spanning Tree Parameters Configuring STP Bridge Parameters

OmniSwitch 6800/6850/9000 Network Configuration Guide June 2006 page 6-15

Configuring the Bridge Priority

A bridge is identified within the Spanning Tree by its bridge ID (an eight byte hex number). The first two

bytes of the bridge ID contain a priority value and the remaining six bytes contain a bridge MAC address.

The bridge priority is used to determine which bridge will serve as the root of the Spanning Tree. The

lower the priority value, the higher the priority. If more than one bridge have the same priority, then the

bridge with the lowest MAC address becomes the root.

Note. Configuring a Spanning Tree bridge instance with a priority value that will cause the instance to

become the root is recommended, instead of relying on the comparison of switch base MAC addresses to

determine the root.

If the switch is running in the 1x1 Spanning Tree mode, then a priority value is assigned to each VLAN

instance. If the switch is running in the flat Spanning Tree mode, the priority is assigned to the flat mode

instance or an 802.1s Multiple Spanning Tree Instance (MSTI). In both cases, the default priority value

assigned is 32768. Note that priority values for an MSTI must be multiples of 4096.

To change the bridge priority value for a VLAN instance, specify a VLAN ID with the bridge priority

command when the switch is running in the 1x1 mode. For example, the following command changes the

priority for VLAN 455 to 25590:

-> bridge 455 priority 25590

The explicit bridge 1x1 priority command configures the priority for a VLAN instance when the switch

is running in either mode (1x1 or flat). For example, the following command performs the same function

as the command in the previous example:

-> bridge 1x1 455 priority 25590

To change the bridge priority value for the flat mode instance, use either the bridge priority command or

the bridge cist priority command. Note that both commands are available when the switch is running in

either mode (1x1 or flat) and an instance number is not required. For example, the following commands

change the priority value for the flat mode instance to 12288:

-> bridge priority 12288

-> bridge cist priority 12288

As in previous releases, it is possible to configure the flat mode instance with the bridge protocol

command by specifying 1 as the instance number (e.g., bridge 1 protocol rstp). However, this is only

available when the switch is already running in the flat mode and STP or RSTP is the active protocol.

The bridge priority value is also configurable for an 802.1s Multiple Spanning Tree Instance (MSTI). To

configure this value for an MSTI, use the explicit bridge msti priority command and specify the MSTI

ID for the instance number and a priority value that is a multiple of 4096. For example, the following

command configures the priority value for MSTI 10 to 61440:

-> bridge msti 10 priority 61440

Note that when MSTP (802.1s) is the active flat mode protocol, explicit Spanning Tree bridge commands

are required to configure parameter values. Implicit commands are for configuring parameters when the

STP or RSTP protocols are in use. See Chapter 3, “Using 802.1s Multiple Spanning Tree,” for more infor-

mation.

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Alcatel 9000 Betriebsanweisung Seite 131