Trunking 6509 to 3550 EMI

[ahegler]

I am having a very strange problem when I setup an ISL trunk between my 6509 and 3550. The trunk comes up fine and I my laptop is fine on the new 3550. The problem is with the PC/s that were attached to the old 3com Superstack switches. These computers cannot obtain an ip address from the DHCP server. I have portfast enabled on on access ports, the cam table and arp cache were cleared manually. I see no errors on the ports or in the logging buffer. Could this be a NIC issue? I have never experienced this problem, and I have installed quite a few switches with isl and dot1q trunks.

[skarundi]

If the PCs are connected to the 3550, make sure you apply the "switchport host" command under the interface.

Its a macro command that applies the

"switchport mode access" command, and

"spanning-tree portfast" command.

more details on what this command does is described in URL: http://www.cisco.com/warp/customer/473/12.html#c2k

[swical]

You may have heard this already but I'll reply to be sure. Make sure you don't have "portfast" enabled on any port on the switch that connects to another switch or hub. Also make sure you trunk or have the VLANs allowed on the port connected to the 3com switches or the VLAN offering DHCP may not be available.