Spanning-Tree Operation

The RSTP (802.1w) and STP (802.1D) Spanning Tree Options

How STP and RSTP Operate

The switch automatically senses port identity and type, and automatically defines spanning-tree parameters for each type, as well as parameters that apply across the switch. You can use the default values for these parameters, or adjust them as needed.

While allowing only one active path through a network at any time, spanning tree retains any redundant physical path to serve as a backup (blocked) path in case the existing active path fails. Thus, if an active path fails, spanning tree automatically activates (unblocks) an available backup to serve as the new active path for as long as the original active path is down. For example:

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

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

 

1

 

 

switch A

 

3

 

 

 

 

 

 

 

 

path cost:

 

 

2

 

path cost: 100

 

100

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

path cost: 100

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

switch B

 

switch C

 

switch D

 

 

 

 

 

4

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

path cost:200

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

node

 

 

 

 

 

 

node

 

 

 

 

 

A

 

 

 

 

 

 

B

 

 

 

 

 

Figure 5-2. 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.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

N o t e

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

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