STP with Data link switching

[chris-darling]

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

thanks

chris.darling@dwn.akzonobel.com

[pgregory]

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.

Regards,

Phil Gregory.