dot1q - Configuration on Cisco/HP switches

Have you ever mixed up Cisco and HP switches in a networking environment?
It's funny how similar features can be named differently - and vice versa (e.g. meaning of 'trunk').

Now, assuming you're planning to connect HP switches (4200 series) to Cisco switches (3500 series). The first steps for configuring such a 'trunk' on Cisco switches were:

interface GigabitEthernet0/1
 switchport trunk encapsulation dot1q
 switchport mode trunk
exit 
interface GigabitEthernet0/2
 switchport trunk encapsulation dot1q
 switchport mode trunk
 switchport nonegotiate
exit

It's recommended to disable Cisco's DTP signaling (using the nonegotiate option) when connecting to HP ProCurve switches.

By default, all VLANs were allowed to pass the Cisco trunk.
To manually specifiy the allowed VLANs (e.g. 1, 10 & 11), add the following command:
 switchport trunk allowed vlan 1,10,11

On HP switches, ensure that the "Native VLAN" (usually VLAN1) is set as untagged on the uplink port (A2):
 vlan 1
 untagged A2
 exit

Checking the port status on Cisco:

show interface GigabitEthernet0/1 switchport


For more information, visit the following sites:

"Trunking" on Cisco switches
http://www.ciscopress.com/articles/article.asp?p=29803&seqNum=3

ProCurve / Cisco Interoperability Guide
http://www.tecnocael.it/ftp/docs/ProCurve_Cisco.pdf