Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

STP with Data link switching

We currently have DLSW as we are still running SNA in a couple of our remote sites.

Yesterday we connected a new Cisco 2950 to a remote site and it brought down the network at 2 other sites (the 2 other sites had cisco switches).

Looking at the problem is looks like spanning tree but I don't know where to start. I can't afford to plug the switch in for the networks to go down again. The networks in all 3 remote sites use VLAN1 and portfast is enabled on the port connected to the router at all 3 sites


  • Other Network Infrastructure Subjects
New Member

Re: STP with Data link switching

Portfast should only be configured on single host ports and NOT on ports connected to any other network device. Portfast eliminates the first stages of the Spanning Tree algorithm and places the port in forwarding state with no regard for the topology of the network.

Portfast enabled ports will not send Topology change notifications (TCN) when the state of the port changes, (which is useful for end user devices as you dont really need notifications every time someone reboots a PC) - but not much use when the topology of your bridged network changes.

I advise you remove portfast from the switch ports that connect to your routers (and to any other switches) at all sites.


Phil Gregory.

This widget could not be displayed.