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Posted by stephen on July 27, 2006, 2:48 pm
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> 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.
a lot depends on the device. 1 v common "feature" is to have access ports,
with untagged access, allocate the port to an internal VLAN, and tag "stuff"
associated with that vlan if it leaves the switch on a port set for tagging.
>
> We are looking at upgrading our infrastructure to support VoIP,
> and am trying to grasp all the new tagging info for VLAN.
> Currently have older cisco 2501 routers and HP switches.
> Moving to 2801 routers and ?? switches.
>
> Are there different ways of tagging frames today
> basaed upon different standards, that were not around a few years ago ?
the tags are standardised (ieee 802.1Q?), but how a device decides what to
tag and how is config / device dependent
>
> Can switches from different vendors all play nice together ?
yes.
the more complicated issues i have had issues with are around multiple
spanning trees / single spanning tree, and whether the "native" untagged
vlan gets treated differently.
>
> What happens to tagged frames vs un-tagged frames ?
if you have a tagged port 802.1Q mentions that untagged frames can be
associated with a sort of notional vlan and treated as such once they get to
a device that uses vlans, like a switch.
> What if a frame doesn't have the extra 4-bytes for VLANs,
> what about a tagged frame going thru an old legacy switch
> that doesn't know about the extra bytes ?
it might let it thru, but some frames are going to be over max "native"
ethernet frame length...
>
> How does a frame obtain the VLAN ID - where does it come from ?
part of the network design - you have a 12 bit field to play with. Avoid 0
and 4095, and Cisco in particular have some reserved numbers between 1000
and 1023.
usually vlan IDs are the same as tag numbers, and only high end devices
understnad translating tag numbers, so tag numbers need to be consistently
used across a campus.
Also some older cisco vlan features such as ISL dont operate with tag
numbers above 1024.
> Only from physically connecting to a specific switch port ?
> I've read abotu DHCP handing out VLAN ID - How ?
i have seen this for IP phones - it used manufacturer specific tags to set
up phones
>
> 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 ?
The PC traffic is untagged from the PC. The phone embedded switch just
passes the frames thru, and they become "members" of the native vlan on that
switch port once they get to the wiring closet.
>
> How does it all work when the phone is just physically moved,
> and plugged into a different wall jack - hence a different switch port ?
DHCP.
Some phones boot initially to use DHCP on the native VLAN, get their info
such as tag numbers, then reboot and DHCP again in the right vlan. If you
have the vlan number already in the config of the phone, then the reboot
isnt needed.
>
> How does DHCP enter into this picture ?
like it always does :)
--
Regards
stephen_hope@xyzworld.com - replace xyz with ntl
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