Configuring Spanning Tree Parameters Configuring STP Bridge Parameters
OmniSwitch 6800/6850/9000 Network Configuration Guide March 2008 page 7-21
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 a 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
Note. If PVST+ mode is enabled on the switch, then the priority values can be assigned only in the multi-
ples of 4096 to be compatible with the Cisco MAC Reduction mode; any other values will result in an
error message.
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 a Multiple Spanning Tree Instance (MSTI). To config-
ure 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
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