Configuring STP Port Parameters
Configuring Spanning Tree Parameters
page 6-36
OmniSwitch AOS Release 7 Network Configuration Guide
June 2013
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.
Spanning Tree is automatically enabled on a port and the default port priority value is set. If the switch is
running in the per-VLAN 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
instance, even if the port is associated with multiple VLANs.
To change the port priority value for a VLAN regardless of which mode (per-VLAN or flat) is active for
the switch, use the
command with the
vlan
and
port
parameters. For example, the
following command sets the priority value as 3 for the port 10/1 association with VLAN ID 10:
-> spantree vlan 10 port 10/1 priority 3
To change the port priority value for the flat mode instance, use the
command with the
cist
and
port
parameters. Note that this command is available when the switch is running in either per-
VLAN or flat mode. An instance number is not required. For example, the following command changes
the priority value for port 1/24 for the flat mode instance to 15:
-> spantree cist port 1/24 priority 15
The port priority value is also configurable for a Multiple Spanning Tree Instance (MSTI). To configure
this value for an MSTI, use the
command with the
msti
and
port
parameters. For exam-
ple, the following command configures the priority value for port 1/12 for MSTI 10 to 5:
-> spantree msti 10 port 1/12 priority 5
Note that configuring the port priority value for a MSTI is allowed in both modes (per-VLAN and flat)
only when the Spanning Tree protocol is set to MSTP.
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.
To change the priority for a link aggregate, use the
spantree priority
command with the
cist
,
msti
, or
vlan
parameters, as described above but specify a link aggregate control number instead of a slot and port
number. For example, the following command sets the priority for the link aggregate 10 association with
VLAN 755 to 9:
-> spantree vlan 755 linkagg 10 priority 9
For more information about configuring an aggregate of ports, see