Selecting The Vlan Bridge Protocol (802.1D Or 802.1W); Enabling/Disabling The Vlan Bpdu Switching Status; Configuring Vlan Bridge Priority - Alcatel OmniSwitch 6624 Network Configuration Manual

Configuring Spanning Tree Parameters

Selecting the VLAN Bridge Protocol (802.1D or 802.1w)

The switch supports two Spanning Tree Algorithms: 802.1D (standard) and 802.1w (rapid reconfigura-
tion). If the switch is running in the 1x1 Spanning Tree mode, then a protocol is selected for each VLAN.
When a VLAN is created, STP is enabled and 802.1D is selected by default. If the switch is running in the
flat Spanning Tree mode, then the protocol is selected for VLAN 1, but applies across all VLANs config-
ured on the switch.
The 802.1w standard is an amendment to the 802.1D document and describes the rapid Spanning Tree
Algorithm and Protocol (RSTP). The RSTP provides rapid reconfiguration by allowing blocked ports to
transition directly into a forwarding state, bypassing the listening and learning states.
Although it supports the 802.1D standard, the 802.1w standard recommends implementing RSTP when-
ever possible. Regardless of which standard a switch or VLAN is running, it can successfully interoperate
with other switches or VLANs.
To change the Spanning Tree Protocol for a VLAN, enter bridge followed by an existing VLAN ID (or
VLAN 1 if using a flat Spanning Tree), then protocol followed by 1d or 1w. For example, the following
command changes the protocol to 802.1w for VLAN 455:
-> bridge 455 protocol 1w
To determine the current protocol selection for the switch or an individual VLAN, use the
command. For more information about this command, see the OmniSwitch CLI Reference Guide.

Enabling/Disabling the VLAN BPDU Switching Status

By default, BPDU are not switched on ports associated with Spanning Tree disabled VLANs. This may
result in a network loop if the VLAN has redundant paths to one or more other switches. Allowing Span-
ning Tree disabled VLANs to forward BPDU to all ports in the VLAN, can help to avoid this problem.
To enable or disable BPDU switching on a VLAN, enter bridge followed by an existing VLAN ID (or
VLAN 1 if using a flat Spanning Tree instance) then bpdu-switching followed by enable or disable. For
example, the following commands enable BPDU switching on VLAN 10 and disable it on VLAN 20:
-> bridge 10 bpdu-switching enable
-> bridge 20 bpdu-switching disable
Note. Make sure that disabling BPDU switching on a Spanning Tree disabled VLAN will not cause
network loops to go undetected.

Configuring VLAN Bridge Priority

A bridge is identified within the Spanning Tree by its bridge ID (an eight byte hex number). The first two
bytes of the bridge ID contain a priority value and the remaining six bytes contain a bridge MAC address.
The bridge priority is used to determine which bridge will serve as the root of the Spanning Tree. The
lower the priority value, the higher the priority. If more than one bridge have the same priority, then the
bridge with the lowest MAC address becomes the root.
Note. Configuring a VLAN with a priority value that will cause the VLAN to become the root is recom-
mended, instead of relying on the STP comparison of switch base MAC addresses to determine the root.
OmniSwitch 6624/6648 Network Configuration Guide
Configuring STP VLAN Parameters
April 2004
show spantree
page 5-11