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Spanning tree and VOIP

Spanning tree question:

I have heard various opinions regarding using spanning tree on a switched network where voice traffic will be utilized and wanted to get some more thoughts on the subject.

I can see certain situations in which spanning tree may not be a good fit when VOIP is involved such as constantly changing topologies and the like, but I still believe that spanning tree can be a good fit in topologies where its function would be for disaster recovery on failed links only.

What are some thoughts on the subject?

Hall of Fame Super Gold

Re: Spanning tree and VOIP


spanning tree has nothing to do with voip, and voip is neither a pro nor a contrary factor for using STP.

Basically, everytime your L2 network has multiple links between switches, or a potential to have them, you should have STP running. That is also the default.


Re: Spanning tree and VOIP

"spanning tree has nothing to do with voip, and voip is neither a pro nor a contrary factor for using STP. "

You are right absolutely. However when you attached a Cisco IP phone to a L2 switched network with spanning tree enabled, it matters a great deal, does it not?. So once again you are correct in saying that STP has nothing to do with VoIP as indeed VoIP has nothing to do with STP. Nevertheless, they are intertwined so there's no escaping from that.

I hope this helps jegan537. Just ignore us if we go off babbling along like this sometimes..., we do that in an effort to be totally clear about anything and everything : )

Do yourself a favour and read the URL's posted by Rob!. If you have any questions after that then please do not hesitate, even a bit, to come back to us.

all the best


Hall of Fame Super Red

Re: Spanning tree and VOIP

Hi Joe,

From the SRND Guide for Callmanager;

Spanning Tree Protocol (STP)

To minimize convergence times and maximize fault tolerance at Layer 2, enable the following STP features:


Enable PortFast on all access ports. The phones, PCs, or servers connected to these ports do not forward bridge protocol data units (BPDUs) that could affect STP operation. PortFast ensures that the phone or PC, when connected to the port, is able to begin receiving and transmitting traffic immediately without having to wait for STP to converge.

Root guard or BPDU guard

Enable root guard or BPDU guard on all access ports to prevent the introduction of a rogue switch that might attempt to become the Spanning Tree root, thereby causing STP re-convergence events and potentially interrupting network traffic flows. Ports that are set to errdisable state by BPDU guard must either be re-enabled manually or the switch must be configured to re-enable ports automatically from the errdisable state after a configured period of time.

UplinkFast and BackboneFast

Enable these features where appropriate to ensure that, when changes occur on the Layer 2 network, STP converges as rapidly as possible to provide high availability. When using stackable switches such as the Catalyst 2950, 3550, or 3750, enable Cross-Stack UplinkFast (CSUF) to provide fast failover and convergence if a switch in the stack fails.

UniDirectional Link Detection (UDLD)

Enable this feature to reduce convergence and downtime on the network when link failures or misbehaviors occur, thus ensuring minimal interruption of network service. UDLD detects, and takes out of service, links where traffic is flowing in only one direction. This feature prevents defective links from being mistakenly considered as part of the network topology by the Spanning Tree and routing protocols.


Note With the introduction of RSTP 802.1w, features such as PortFast and UplinkFast are not required because these mechanisms are built in to this standard. If RSTP has been enabled on the Catalyst switch, these commands are not necessary.

Hope this helps!