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802.1q trunking between C2950 and 3Com Superstack III 4400


I have a problem trunking vlan between a 3Com superstack and Cisco 2950.

The LAN has until now not been using VLAN:s, but I now want to create a second VLAN 2 named Video to transport that traffic thru the switches. There is no Layer 3 connection between the VLAN:s.

In the 3Com I first created a vlan 2:

Select menu option (bridge/vlan): create

Select VLAN ID (2-4094)[2]:

Enter VLAN Name [VLAN 2]: video

Select menu option (bridge/vlan)

Then I added the port where the 2950-switch was connected to both vlan 1 and vlan 2, tagged:

Select menu option (bridge/vlan/modify): addpo

Select VLAN ID (1-2)[1]: 2

Select bridge ports (AL1-AL4,unit:port...,?): 3:17

Enter tag type (untagged,tagged): tagged

Select menu option (bridge/vlan/modify): addpo

Select VLAN ID (1-2)[1]: 1

Select bridge ports (AL1-AL4,unit:port...,?): 3:17

Enter tag type (untagged,tagged): tagged

Select menu option (bridge/vlan): det

Select VLAN ID (1-2)[1]:

VLAN ID: 1 Name: Default VLAN

Unit Untagged Member Ports Tagged Member Ports


1 1-24 none

2 1-24 none

3 1-16,19-24 17

Aggregated Links AL1-AL4 none

Select menu option (bridge/vlan): det

Select VLAN ID (1-2)[1]: 2

VLAN ID: 2 Name: video

Unit Untagged Member Ports Tagged Member Ports


1 none none

2 none none

3 none 17

Aggregated Links none none

On the Cisco-switch I simply made the fa0/1 port where the 3com-switch was connected a trunk:

(as c2950 doesnt support ISL-tagging I dont have to care about specifying tag-mode to be dot1q, right?)

nterface FastEthernet0/1

description Uplink to serverrum

no ip address

duplex full

speed 100

and of course I created a VLAN 2:

ya-aa01-01#sh vlan

VLAN Name Status Ports

---- -------------------------------- --------- -------------------------------

1 default active Fa0/1, Fa0/2, Fa0/3, Fa0/4, Fa0/5, Fa0/6, Fa0/7, Fa0/8, Fa0/9, Fa0/10, Fa0/11

Fa0/12, Fa0/13, Fa0/14, Fa0/15, Fa0/16, Fa0/17, Fa0/18, Fa0/19, Fa0/20, Fa0/21

Fa0/22, Fa0/23, Fa0/24

2 video active

1002 fddi-default active

1003 token-ring-default active

1004 fddinet-default active

1005 trnet-default active

VLAN Type SAID MTU Parent RingNo BridgeNo Stp BrdgMode Trans1 Trans2

---- ----- ---------- ----- ------ ------ -------- ---- -------- ------ ------

1 enet 100001 1500 - - - - - 0 0

2 enet 100002 1500 - - - - - 0 0

1002 fddi 101002 1500 - - - - - 0 0

1003 tr 101003 1500 - - - - - 0 0

1004 fdnet 101004 1500 - - - ieee - 0 0

1005 trnet 101005 1500 - - - ibm - 0 0



Primary Secondary Type Ports

------- --------- ----------------- ------------------------------------------


The problem is that I cant get traffic thru the link. The funny thing about this is that CDP works, I can from the C2950-switch see other Cisco-devices connected to the 3Com switchstack.

Am I doing anything wrong? Do I need to specify that I want to use dot1q-tagging in the 3Com-switch? What is happening?

Thanks for your help. I am clueless!


Jimmy Larsson



I'm not a 3Com expert, but I'd recommend:

1) configure VLAN1 on 3Com trunk port as untagged.

If you leave native VLAN default value on the Cisco side, VLAN1 is used as native and not tagged on 802.1q Cisco trunk.

2) I'm not sure if 3Com supports trunk negotiation (I don't think so).

So I'd

C2950(config-if)#switchport mode trunk

to stop the trunk negotiation (default mode is desirable) and force the interface to become a trunk.

To your CDP note:

Does CDP work in both directions? I.e. do you see your C2950 on the other Cisco-devices connected to the 3Com switchstack via sh CDP nei?

My understanding is it should not be seen. CDP frames are sent always in VLAN1on trunks. In your case C2950 sends CDP frame untagged (native VLAN is VLAN1 by default) to 3Com. But VLAN1 is configured as tagged on 3Com, so the CDP frame is discarded.

In the opposite direction:

Cisco device sends CDP packet untagged to 3Com on access port (I suppose VLAN1, too). 3Com doesn't recognize the special Cisco destination address and treats it as an ordinary multicast - floods it to all ports. On the trunk port it's sent tagged in VLAN1. C2950 receives a CDP frame tagged in VLAN1. But Cisco 802.1q implementation is able to receive native VLAN frames both untagged and tagged (it's used in some VoIP topologies) and so CDP frame is accepted.