Configuring Spanning Tree Parameters Configuring STP Port Parameters
OmniSwitch AOS Release 8 Network Configuration Guide December 2017 page 6-36
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 spantree priority 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 spantree priority 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 spantree priority command with the msti and port parameters. For
example, 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 Chapter 8, “Configuring Static Link
Aggregation,” and Chapter 9, “Configuring Dynamic Link Aggregation.”
Configuring Port Path Cost
The path cost value specifies the contribution of a port to the path cost towards the root bridge that
includes the port. The root path cost is the sum of all path costs along this same path and is the value
advertised in Configuration BPDU transmitted from active Spanning Tree ports. The lower the cost value,
the closer the switch is to the root.
The type of path cost value used depends on which path cost mode is active (automatic or 32-bit). If the
path cost mode is set to automatic, a 16-bit value is used when STP or RSTP is the active protocol and a
32-bit value is used when MSTP is the active protocol. If the mode is set to 32-bit, then a 32-bit path cost
value is used regardless of which protocol is active. See “Configuring the Path Cost Mode” on page 6-31
for more information.
If a 32-bit path cost value is in use and the path_cost is set to zero, the following IEEE 802.1t
recommended default path cost values based on link speed are used:
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