Spanning Tree Protocol 50
Port path cost
The port path cost assigns lower values to high-bandwidth ports, such as 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
port cost is computed dynamically based on link speed. This works when forcing link speed, so it does not just
apply to “auto negotiated link speed”.
By default, all switch ports have the path cost set to 4, independent of the link speed. To use dynamic path cost,
based on link speed, set the path cost to 0 (zero). For example, if the path cost is set to zero:
•
A 100 Mbps link receives a path cost of 19
•
A 10 Mbps link receives a path cost of 100
Spanning Tree Group configuration guidelines
This section provides important information on configuring Spanning Tree Groups (STGs).
Default Spanning Tree configuration
In the default configuration, a single STG with the ID of 1 includes all ports except Port 19 on the switch. It is called
the default STG. All other STGs (except the default STG) are empty, and VLANs must be added by the user.
You cannot assign ports directly to an STG. Add the ports to a VLAN, and add the VLAN to the STG. STGs 1-31
are enabled by default and assigned an ID number from 1 to 31. STG 32 is disabled by default, and contains the
management VLAN 4095.
An STG cannot be deleted, only disabled. If you disable the STG while it still contains VLAN members, Spanning
Tree will be off on all ports belonging to that VLAN.
Adding a VLAN to a Spanning Tree Group
If no VLANs exist beyond the default VLAN 1, see the “Creating a VLAN” section in this chapter for information on
adding ports to VLANs.
Add the VLAN to the STG using the command
/cfg/l2/stp <stg number>/add <vlan number>
.
Creating a VLAN
When you create a VLAN, then that VLAN automatically belongs to STG 1, the default STG. If you want the VLAN
in another STG, you must move the VLAN by assigning it to another STG.
To move a newly created VLAN to an existing STG:
1.
Create the VLAN.
2.
Add the VLAN to an existing STG.
When creating a VLAN also consider the following:
•
A VLAN cannot belong to more than one STG.
•
VLANs that span multiple switches must be mapped within the same Spanning Tree Group (have the same
STG ID) across all the switches.
Rules for VLAN tagged ports
Rules for VLAN tagged ports are listed below:
•
If a port is tagged, it can belong to multiple STGs.
•
When a tagged port belongs to more than one STG, the egress BPDUs are tagged to distinguish the BPDUs
of one STG from those of another STG.
•
An untagged port cannot span multiple STGs.
