3Com untagged vs. 802.1Q VLANs
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Posted by Jonathan Sturges on February 22, 2005, 1:44 pm
Please log in for more thread options Hi, In a SuperStack II 3300 switch (model 3C16980), firmware 2.71, there are untagged and tagged VLANs. Am I correct to assume that ports defined in an untagged VLAN are partitioned off from other ports, into their own broadcast domain? What will the switch do with packets destined for a host not in the untagged VLAN? Will it forward? I can see in the admin GUI where you can control forwarding of tagged packets but nothing specific to untagged. thanks for clarifying the behavior of 3Com untagged VLANs. -Jonathan | ||||||||||||||||||||||||||||||||||||||||||||||||||||
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Posted by Walter Roberson on February 22, 2005, 7:22 pm
Please log in for more thread options :In a SuperStack II 3300 switch (model 3C16980), firmware 2.71, there are :untagged and tagged VLANs. Am I correct to assume that ports defined in :an untagged VLAN are partitioned off from other ports, into their own :broadcast domain? Yes, but... : What will the switch do with packets destined for a :host not in the untagged VLAN? Will it forward? I can see in the admin :GUI where you can control forwarding of tagged packets but nothing :specific to untagged. Tagged or untagged is not a property of the VLAN, but rather a property of a port. Unless 3Com is using terminology a very different way than everyone else, all ports, tagged or untagged, that are given the same VLAN number will be in the same broadcast domain; the ports that are marked as tagged will actually send the tag number as part of the packet when emitting a packet on the port, whereas ports that are marked as untagged will strip the tag number before emitting a packet on the port. Tagged ports are used when mostly communicating between switches (or between switches and routers), and untagged ports are mostly used for communicating with hosts; most hosts are not able to process the tag number [but it is becoming increasingly common to be able to.] Often a tagged port will be marked as being part of several VLANs; packets for all those VLANs can be sent on the same port, with the tag number being used on the remote end to figure out what goes where. -- Scintillate, scintillate, globule vivific Fain would I fathom thy nature specific. Loftily poised on ether capacious Strongly resembling a gem carbonaceous. -- Anon | ||||||||||||||||||||||||||||||||||||||||||||||||||||
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Posted by Jonathan Sturges on February 22, 2005, 2:48 pm
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Walter Roberson wrote: Thanks for this very clear explanation. :) | ||||||||||||||||||||||||||||||||||||||||||||||||||||
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Posted by Manfred Kwiatkowski on February 22, 2005, 7:40 pm
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>Hi,
>In a SuperStack II 3300 switch (model 3C16980), firmware 2.71, there are >untagged and tagged VLANs. ... No. There are VLANs. You decide if a port will transmit and receive packets for one or more of them. To distinguish the VLAN meberbership you can use explicit tags or implicitly agree on one for untagged packets. > ... Am I correct to assume that ports defined in
>an untagged VLAN are partitioned off from other ports, into their own >broadcast domain? What will the switch do with packets destined for a >host not in the untagged VLAN? Will it forward? It will establish a VLAN correspondance for every packet and then forward the packet accordingly. -- Manfred Kwiatkowski kwiatkowski@zrz.tu-berlin.de | ||||||||||||||||||||||||||||||||||||||||||||||||||||
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Posted by T. Sean Weintz on February 22, 2005, 5:03 pm
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Manfred Kwiatkowski wrote: >
>>Hi,
>>In a SuperStack II 3300 switch (model 3C16980), firmware 2.71, there are >>untagged and tagged VLANs. ... >
> > No. There are VLANs. You decide if a port will transmit and receive > packets for one or more of them. To distinguish the VLAN meberbership > you can use explicit tags or implicitly agree on one for untagged > packets. > >> ... Am I correct to assume that ports defined in
>>an untagged VLAN are partitioned off from other ports, into their own >>broadcast domain? What will the switch do with packets destined for a >>host not in the untagged VLAN? Will it forward? >
> > It will establish a VLAN correspondance for every packet and > then forward the packet accordingly. No. It will drop pacekets intended for other VLANS. (That's sort of the whole point of VLANS) | ||||||||||||||||||||||||||||||||||||||||||||||||||||
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> :untagged and tagged VLANs. Am I correct to assume that ports defined in
> :an untagged VLAN are partitioned off from other ports, into their own
> :broadcast domain?
>
> Yes, but...
>
> : What will the switch do with packets destined for a
> :host not in the untagged VLAN? Will it forward? I can see in the admin
> :GUI where you can control forwarding of tagged packets but nothing
> :specific to untagged.
>
> Tagged or untagged is not a property of the VLAN, but rather a
> property of a port. Unless 3Com is using terminology a very different
> way than everyone else, all ports, tagged or untagged, that are
> given the same VLAN number will be in the same broadcast domain;
> the ports that are marked as tagged will actually send the tag
> number as part of the packet when emitting a packet on the port,
> whereas ports that are marked as untagged will strip the tag number
> before emitting a packet on the port.
>
> Tagged ports are used when mostly communicating between switches (or
> between switches and routers), and untagged ports are mostly used for
> communicating with hosts; most hosts are not able to process the
> tag number [but it is becoming increasingly common to be able to.]
>
> Often a tagged port will be marked as being part of several VLANs;
> packets for all those VLANs can be sent on the same port, with
> the tag number being used on the remote end to figure out what goes
> where.
>