Juniper Networks EX2500 manual Spanning Tree Group Configuration Guidelines, Port Priority

Chapter 3: Spanning Tree Protocol

Port Priority

The port priority helps determine which bridge port becomes the root or designated port. The case for the root port is when 2 switches are connected using a minimum of two links with the same path-cost. The case for the designated port is in a network topology that has multiple bridge ports with the same path-cost connected to a single segment—the port with the lowest port priority becomes the designated port for the segment. Use the following command to configure the spanning-tree port priority (Interface Port mode):

ex2500(config-if)#spanning-tree stp 1 priority <0-240, in steps of 16>

Port Path Cost

The port path cost assigns lower values to high-bandwidth ports, such as 10 Gigabit Ethernet, to encourage their use. The objective is to use the fastest links so that the route with the lowest cost is chosen. A value of 0 indicates that the default cost will be computed for an auto-negotiated link speed.

Use the following command to modify the port path cost:

ex2500(config-if)#spanning-tree stp 1 path-cost <0-200000000>

Spanning Tree Group Configuration Guidelines

This section provides important information on configuring Spanning Tree Groups (STGs):

Changing the Spanning Tree Mode

When the spanning-tree mode is changed (for example, RSTP to MSTP):

„You must reconfigure spanning-tree parameters for each STG, including VLAN assignment.

„If an STG in RSTP mode is disabled and then re-enabled, the Spanning Tree topology does not converge rapidly.

Assigning a VLAN to a Spanning Tree Group

„If no VLANs exist beyond the default VLAN 1 see “Creating a VLAN” on page 34 for information on adding ports to VLANs.

„Assign the VLAN to the STG with the following command: ex2500(config-if)#spanning-tree stp 1 vlan <1-4094>

„If the association between the Spanning Tree Group (STG) and a VLAN is broken, the spanning-tree parameters are cleared. Reconfigure all the parameters for the STG.

Spanning Tree Overview „ 33