Configuring Spanning Tree Parameters Configuring STP Port Parameters
OmniSwitch 6800/6850/9000 Network Configuration Guide March 2008 page 7-31
Configuring Port Priority
A bridge port is identified within the Spanning Tree by its Port ID (a 16-bit or 32-bit hex number). The
first 4 bits of the Port ID contain a priority value and the remaining 12 bits contain the physical switch port
number. The port priority is used to determine which port offers the best path to the root when multiple
paths have the same path cost. The port with the highest priority (lowest numerical priority value) is
selected and the others are put into a blocking state. If the priority values are the same for all ports in the
path, then the port with the lowest physical switch port number is selected.
By default, Spanning Tree is enabled on a port and the port priority value is set to 7. If the switch is
running in the 1x1 Spanning Tree mode, then the port priority applies to the specified VLAN instance
associated with the port. If the switch is running in the flat Spanning Tree mode, then the port priority
applies across all VLANs associated with the port. The flat mode instance is specified as the port’s
instance, even if the port is associated with multiple VLANs.
To change the port priority value for a VLAN instance, specify a VLAN ID with the bridge slot/port
priority command when the switch is running in the 1x1 mode. For example, the following command sets
the priority value for port 8/1 to 3 for the VLAN 10 instance:
-> bridge 10 8/1 priority 3
The explicit bridge cist slot/port priority command configures the port priority value 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 10 8/1 priority 3
To change the port priority value for the flat mode instance, use either the bridge slot/port priority
command or the bridge cist slot/port 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 port 1/24 for the flat mode instance to 15:
-> bridge 1/24 priority 15
-> bridge cist 1/24 priority 10
As in previous releases, it is possible to configure the flat mode instance with the bridge slot/port prior-
ity command by specifying 1 as the instance number (e.g., bridge 1 1/24 priority 15). However, this is
only available when the switch is already running in the flat mode and STP or RSTP is the active protocol.
The port priority value is also configurable for a Multiple Spanning Tree Instance (MSTI). To configure
this value for an MSTI, use the explicit bridge msti slot/port priority command and specify the MSTI ID
for the instance number. For example, the following command configures the priority value for port 1/12
for MSTI 10 to 5:
-> bridge msti 10 1/12 priority 5
Note that when MSTP 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 2, “Using 802.1Q 2005 Multiple Spanning Tree,” for more
information.
Port Priority on Link Aggregate Ports
Physical ports that belong to a link aggregate do not participate in the Spanning Tree Algorithm. Instead,
the algorithm is applied to the aggregate logical link (virtual port) that represents a collection of physical
ports.
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