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3com switches and VLAN Tagging

S

Saltin

Platinum Member
I am currently working on a project where I am setting up IP telephones in an office. In order to follow best practises, I will need to divide the exisiting switched network into two VLANS, one for data (computer) traffic, and the other for voice (telephone) traffic.

The switches in question are 3com superstack 3300's. This question is really about this switch's capabilites in regard to VLAN tagging.

As it stands now, there is a default VLAN configured on the switch. I am going to configure the second VLAN (voice) and then configure VLAN tagging. In essence, all ports will be members of all VLANS. What I would like to know (and it really comes down to if you have experience with the switch and VLAN tagging), is will the switch treat untagged packets (packets from the computers) as Default VLAN packets? The voice packets will all be tagged VLAN 2.

I know that Cisco switches support this type of tagging (they call it trunking I believe). I am unable to find any info on the 3com switches though.
 
Sounds like 802.1q trunking. You'll need some kind of trunking to support multiple vlans. The VLAN tagging on a 802.1q trunk works like this..

Any frames in the default VLAN are NOT tagged with any kind of header or wrapper. This way the receiver on the other end of a trunk sees untagged traffic and assumes it is in the default VLAN. Another reason to pick a default VLAN and keep it consitent across the entire network.

Any other frames are tagged with an 802.1q header indicating the origin VLAN. Receiver picks up the frame, reads the tag and places it on the appropriate vlan.

HTH
 
As usual Spidey is right on the ball! Only speaking from Nortel experience, one thing you should remember when tagging ports, all the ports in your physical path must be tagged for that Vlan. In other words, if you go from switch to core switch, the port on the switch, the uplink port and the port on the core switch all need to be tagged for that vlan. I'm not sure if this applies to you but when you use OSPF make sure there are no loops in your paths. This will cause you major headaches and can bring your network down to a screeching halt! 🙁
 
Tru dat on trunking, Gand.

Not something you just enable willy nilly. Network must be designed and planned to support trunking.
 
so you can have one port on a switch in multiple vlans?


also, would the phone's themselves be tagging the frames as being in VLAN2, or is that up to the switch? And then how would it know, mac?

bart
 
My understanding is that I can place a port in more than one VLAN, that's the goal/benifit of tagging.
 
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