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difference between trunk and tagged vlans...

Posted on 2010-11-24
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I know the cisco concept of trunk and access ports, i wanna know the difference between tagged ports and untagged ports in other vendor implementations like foundry etc...
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Question by:tpat
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by:Istvan Kalmar
ID: 34210780
Hi,

Tagged port means = cisco trunk port
Unteagged port means = cisco access port

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Expert Comment

by:Kvistofta
ID: 34210815
you can see each vlan as a channel or frequency. In an access-port you have only one channel (vlan) and therefore each packet doesnt need to be marked individually.

In a trunk, because you send traffic from different vlans in the same carrier, each packet needs to be marked (tagged), otherwise the remote end doesnt know which vlan each received packet belongs to.

Ok. So in a trunk, each packet is marked. But what if I (the switch) receive a NOT marked packet in a trunk? A packet that is not tagged belongs to what in Cisco-terms is called native vlan. This is by default vlan1 but can be configured differently.

In Cisco-terms we talk about trunks and which vlans that belongs to a specific port. But other vendors see it differently. For example HP Procurve where you configure this tagging per vlan and not per port. If you have a trunk port in a HP-switch you can add additional vlans as tagged to that port (tagged port) but only one vlan can be untagged on that port.  

If you have an access-port (in cisco-terms) which means that you have 0 tagged vlans and 1 tagged vlan, that is how the other end needs to be configured aswell.

If you in HP (or other vendors) add a previously unused switchport as tagged to a vlan, that means that this is (in cisco terms) a trunk that contains one single vlan, and that is tagged. This is in Cisco-switches configured as follows:

int gi0/x
 switchport mode trunk
 switchport trunk allowed vlan 123
 switchport trunk native vlan 1 (or whatever, except for 123)
!

Does this make sense?

/Kvistofta
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Author Comment

by:tpat
ID: 34211007
your explanation was quite good,
but i dont understand
' If you have a trunk port in a HP-switch you can add additional vlans as tagged to that port (tagged port) but only one vlan can be untagged on that port'

What i know from cisco is a trunk port can carry traffic of multiple vlan's whereas an  access port only belongs to one vlan .
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Kvistofta earned 500 total points
ID: 34211134
Yes, you are right and there is no contradiction in what I wrote and what you are saying.

The technology is the same, no matter brand. The difference is the way to configure it. In HP you add ports to vlan, in Cisco you add vlans to ports.

What I say (that you quoted) is that in a trunk you can only have one untagged vlan. You cannot send multiple vlans as untagged. In the HP-world you configure vlan/ports by saying "add vlan x to port y as (un)tagged". If port y already was configured with an untagged vlan z, that vlan z will be removed from the port and vlan x will be the untagged (native) vlan instead.

/Kvistofta
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by:tpat
ID: 34214350
i got it! thanks for helping Kvistofta!!
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