Trunking Between Cisco and HP Switches

We recently had a customer replace their core HP 8212 switches with two Cisco 4507s. We utilized HSRP to ensure high availability between the 4507s and configured Multiple Spanning-Tree to ensure BPDUs were sent/received between the Cisco and HP switches.

In this scenario, we moved the HP 8212 down to the access layer. The customer utilized Vlan 1 for their User subnet and Vlan 90 for their Voice subnet. We noticed that users on the HP 8212 had phone access, but no data access. I examined the trunk between the switches…

Clarification is in order. A trunk in Cisco lingo is a layer 2 link that allows multiple Vlans across it. A trunk in HP lingo is a port-channel/etherchannel where multiple ports are bonded together to utilize the aggregate bandwidth of all the members. For this and other discussions, I’ll stick with the Cisco lingo for trunking. Now back to the topic at hand… we setup 802.1Q trunks between the Cisco and HP switches. In 802.1Q, each frame is tagged with the Vlan it belongs to except for Vlan 1. Vlan 1 is known as the native Vlan (it can be changed if desired) and it is not tagged. In keeping the defaults of the trunk, the Cisco side will tag all Vlans except for Vlan 1 and the HP side will tag all Vlans including Vlan 1.

Well, more clarification is in order. HP has “tagged”, “untagged”, and “no untagged” configurations for Vlans on interfaces. I gleaned the following definitions from here that helped me understand the correlation to Cisco’s terms:
For end nodes – Cisco uses “mode access”, HP uses “untagged” mode.
For VLAN dot1q trunks – Cisco uses “mode trunk”, HP uses “tagged” mode.
For no VLAN association – Cisco uses no notation at all, HP uses “no” mode in the configuration menu, or you have VLAN support turned off.

When I executed “show interface x/x trunk” on the Cisco switch, I see that Vlan 1 is the Native Vlan:

switch#show interface Gi0/1 trunk

Port Mode Encapsulation Status Native vlan
Gi0/1 on 802.1q trunking 1

Port Vlans allowed on trunk
Gi0/1 1-4094

Port Vlans allowed and active in management domain
Gi0/1 1,10-15,90

Port Vlans in spanning tree forwarding state and not pruned
Gi0/1 1,10-15,90

When I execute “show run” on the HP switch and skim down to Vlan 1, I see the following:

vlan 1
tagged B24

To allow data traffic on Vlan 1 to traverse the trunk, you can change the native Vlan on the trunk or untag Vlan 1. I went with the latter, simpler option. On the HP switch, I executed “untagged B24″ under Vlan 1. Now Vlan 1 was not tagged on both ends and data traffic in Vlan 1 was successfully passed through the trunk.

– Andrew

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