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Posted by anoop on July 27, 2006, 4:40 pm
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Phil Schuman wrote:
> I'm trying to find some good web reading that gives a brief
> intro/tutorial
> to VLANs and various techniques used to tag the frames.
> I thought that the frames were tagged
> by which physical port switch port they entered.
That's the default behavior. If an untagged frame arrives
at a switch, it gets tagged with the PVID of that port.
This was the behavior defined in the original 802.1Q spec
(802.1Q-1998).
> Are there different ways of tagging frames today
> basaed upon different standards, that were not around a few years ago ?
802.1Q was enhanced to support port+protocol classification.
Now the VLAN can be assigned to an untagged frame based
on a protocol table that sits on the port. If the frame doesn't
match any of the protocols specified in that table, then it gets
the PVID. Here the frame's "protocol" is what is contained
in the Ethertype field if the frame is Ethertype-encapsulated,
or the protocol type if the frame is of some other encapsulation
(e.g. a frame with a SNAP-encapsulated protocol type).
There are other non-standard ways of assigning the VID
to untagged frames; e.g. subnet based, MAC address-based, etc.
I wouldn't bother with these if you really care about
interoperability.
> Can switches from different vendors all play nice together ?
Usually they can, as long as you make sure you're using
standards-based protocols for communicating between them.
In other words, if you were using PVST (per-VLAN spanning
tree) on Cisco switches, you won't be able to talk to most
other switches unless you turned off PVST.
> What happens to tagged frames vs un-tagged frames ?
>From an 802.1Q-perspective, the first thing that the
switch will do to an incoming frame is assign it to a VLAN.
It does this based on the VID in the tag if the frame was
tagged, or if it is untagged it will assign a VID based on
either the port or port+protocol.
> What if a frame doesn't have the extra 4-bytes for VLANs,
See above.
> what about a tagged frame going thru an old legacy switch
> that doesn't know about the extra bytes ?
This is usually not a problem except for, potentially, the
frame length, as pointed out by another poster.
> How does a frame obtain the VLAN ID - where does it come from ?
See above.
> Only from physically connecting to a specific switch port ?
> I've read abotu DHCP handing out VLAN ID - How ?
DHCP cannot handle VLAN IDs! A VLAN defines a broadcast
domain and you have to know what VLAN you're in before
you can used DHCP. There may be protocols whereby
a switch identifies a device as being a phone (based on
a protocol such as LLDP or CDP) and configures that port
to automatically be on a certain VLAN.
> And lastly - and most important -
> How does a VoIP phone, with it's internal 2-port switch
> create a VLAN ID for the phone's ethernet port,
> and a different one (or just untagged ?) for the connected PC ?
>
> How does it all work when the phone is just physically moved,
> and plugged into a different wall jack - hence a different switch port ?
>
> How does DHCP enter into this picture ?
I'm not conversant with the way VOIP phones work
so I don't have answers to the rest of the questions.
Anoop
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