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New Member

VLAN routing with Cisco 3640 and 3COM 3900 switch.

Hello all,

I need to configure VLAN's on a 3COM 3900 switch and route between them using a Cisco 3640. Is this possible? I now both support 802.1q encapsulation but I'd like to know how do I have to configure the 3COM switch to get it working.

I've done this extensively using Cisco equipment only (ie 3640 vs Cat3548) using 802.1q instead of ISL. but never with 3com switches.

Besides, what 3COM calls Trunks is called Etherchannel on Cisco, so I'm confused. All the examples I've seen are to connect 3COM switches to Catalyst switches, but I need to connect a router not a switch.

Thanks in advance.



Re: VLAN routing with Cisco 3640 and 3COM 3900 switch.


I cannot comment on the VLAN configuration on the 3COM 3900 but here is what you need to do.

1. Configure needed VLANs on 3900 (assign access ports to them as needed)

2. Configure Interface that is going to connect to 3640 as a 802.1q trunk.

3. Configure subinterfaces on the 3640, with encapsulation dot1q to allow

desired routing to take place.

See the bottom of this link for an example:

Trunk = Tagged interface not a member of a vlan to allow all vlan traffic to pass

Etherchannel= technology to allow bundling of multiple ethernets to make appear as

bigger pipe.

It's been a while since I've worked on 3Com equipment but as I recall the menu system on the switches is pretty straight forward.

Hope this helps,


New Member

Re: VLAN routing with Cisco 3640 and 3COM 3900 switch.

Thanks for your reply, but it didn't help much. I already know how to configure 802.1q trunks with Cisco equipment. As I stated in my previous post, 3COM defines Trunk as:

"Aggregated link that works at layer 2 of the OSI model and allows you to combine multiple Fastethernet (switch 3900 only) or Gigabit Ethernet ports into a single high-speed link between two switches". I need to know how to configure the equivalent of a Cisco 802.1q trunk on a 3Com switch.

Cisco Employee

Re: VLAN routing with Cisco 3640 and 3COM 3900 switch.

Your guess is as good as ours I expect. Your best bet for finding out how to configure a 3com switch is probably to ask 3com.

They do not appear to post their docs on their web site for public use, you have to give a product serial no. etc. to access their knowledge base.


Re: VLAN routing with Cisco 3640 and 3COM 3900 switch.


It is possible to do this. I have configured 801.2q VLANs on 3Com 3300 switches with Gigabit Ethernet uplink ports for a healthcare customer. (A Cisco core network connects several hospitals together; some hospitals still using 3Com switches for edge/access.)

The port which serves as the uplink port on the 3Com switch belongs to each of the VLANs you define on that switch. On this uplink port, only one VLAN can be untagged (= native VLAN on the Cisco VLAN trunk side), the rest must be 802.1q tagged.

On the 3Com, you have to be careful that you do not tag your access VLAN ports (where your users plug in), or the users won't work. The GUI and the command line don't always work like you think they should (found this out the hard way), but once you figure out how to specify the tagging vs. not-tagged ports, it's pretty straightforward. (Tedious, too -- you must define each VLAN you want on each 3Com switch, similar to Cisco switch's "transparent" mode vs. VTP client/server modes).

In my case, the 3Com switch connected to a Cat3508G port configured as a VLAN trunk port. Traffic from the VLANs makes its way back to two Cat6509s with Layer3 capability; several "Interface Vlan xxx" commands there handle the routing between VLANs.

In your case, maybe use a 10/100 port on the 3Com 3900 as your uplink port, create the VLANs on that switch, and overlap them on the uplink port. On your Cisco 3640, assuming you have a Fast Ethernet TX interface, use subinterfaces on that interface and "encapsulation dot1q xxx" for tagged VLANs, and "encap dot1q yyy native" for the untagged VLAN on the 3Com side.

Software revs. on the 3Com switches may play a role in what you can and cannot do from the GUI vs. the command line. I have a 3Com 3300 and a couple of 3900s, and a Cisco 2620 I can use to simulate your 3640. If you run into issues configuring your 3900 with VLANs let me know more info (software rev., intended VLAN configuration), I'll test it out and get back to you.

-Larry Konig

New Member

Re: VLAN routing with Cisco 3640 and 3COM 3900 switch.

Hi Larry

I agree with your procedure for the 3Com configuration. But I do have a concern and maybe you could help me out with this. Cisco uses EtherChannel for "bundling" or aggregating the ethernets. In the 3Com world this is called Trunking (dont get confused with Cisco;s Trunking). What I would like to know is can you basicaly affregate ethernet ports between a Catalyst and a non-Cisco switch?

I have looked at but have not found any answers.