Product guide

Spanning-Tree Operation

The RSTP (802.1w) and STP (802.1D) Spanning Tree Options (5300xl, 3400/6400cl switches)

node

switch A

node

switch D switch B switch C

path cost:

100

path cost: 100

path cost:200

• Active path from node A to node B: 1—> 3

• Backup (redundant) path from node A to node B: 4 —> 2 —> 3

path cost: 100

Figure 6-2. General Example of Redundant Paths Between Two Nodes

In the factory default configuration, spanning tree operation is off. If a redun-

dant link (loop) exists between nodes in your network, you should enable the

spanning tree operation of your choice.

Note Spanning tree retains its current parameter settings when disabled. Thus, if

you disable spanning tree, then later re-enable it, the parameter settings will

be the same as before spanning tree was disabled.

RSTP and STP Operation with 802.1Q VLANs. As recommended in the

IEEE 802.1Q VLAN standard, when 802.1D or 802.1w spanning tree is enabled

on the switch, a single spanning tree is configured for all ports across the

switch, including those in separate VLANs. This means that if redundant

physical links exist in separate VLANs, spanning tree will block all but one of

those links. However, if you need to use spanning tree on the switch in a VLAN

environment with redundant physical links, you can prevent blocked redun-

dant links by using a port trunk. The following example shows how you can

use a port trunk with 802.1Q (tagged) VLANs and spanning tree without

unnecessarily blocking any links or losing any bandwidth.

6-9