Tagged, Untagged, Trunks etc...

[jbcom41]

Hi All,

Can someone please tell me in I have this correct?

If a port is tagged, the device connected to that port should be able to communicate with different vlans.

If a port is untagged, the deice connected to that port can only communicate with the Vlan it belongs to.

Trunk – to combine 2 ports or more to allow for redundancy and increase bandwidth

Is this roughly correct?

Any help would be greatly appreciated.

Thanks

[Vince-Whirlwind]

TRUNK:

 - meaning1 (HP):       link aggregation

 - meaning2 (CISCO): a port carrying one untagged VLAN and one or more tagged VLANs

UNTAGGED PORT:

 Input frames: if tagged in 802.1q format, they are discarded, if untagged, they are assumed to be in the VLAN that is assigned as the ACCESS or UNTAGGED VLAN for that port

 Output frames: frames are output NOT in 802.1q format, ie, with no VLAN tagging information in them

TAGGED PORT:

 This implies that one or more VLANs are assigned to that port so that frames that are TAGGED in one of the allowed VLANs can be sent and received. Input TAGGED frames belonging to a different VLAN that is not allowed on the port are discarded.

It probably isn't correct to call it a TAGGED PORT or UNTAGGED PORT.

The VLAN is UNTAGGED or TAGGED, per port.

Any interconnection must have the same configuration on both ends for frames to be sent and received on both sides.

ie, 

Switch A, port 1 is connected to Switch B, port 48

port1 is configured as untagged in VLAN23 - port 48 should be configured as untagged in VLAN23

or

port1 is configured as untagged in VLAN23 and tagged in VLANs 41,42,43,44,55, port 48 should be the same.

*** You probably don't need to know this: You can patch two ports to each other, and assign each port to a different untagged VLAN. There are very rare situations where you might do this. As neither side is tagging frames, or expecting to receive tagged frames, both switches should accept frames from the other, with each switch making a different assumption about what VLAN to assign the received frames to. Although older Cisco switches used to err-disable the port, newer switches tend to log complaints only.

[jbcom41]

Thanks for helping me  Vince-Whirlwind, much appreciated...

[jbcom41]

Hi There

We have 2 Vlans, one is the Default_VLAN, which is used for data and the other is for Voice, Vlan 2

The Mitel VOIP phones are connected to the data points on the walls and the phones then have a second feed that connects to the PCs.

So we have 2 devices that are connected to one port on the switch... how does the data travelling from the phone and PC to one port know that it is for VLAN1 or VLAN2?

Can any one share some light onto this...

[TerjeAFK]

For our Cisco IP phones, the telephones use the VLAN that is tagged on the switch port for their own communication and then pass the untagged VLAN on to their second network interface. I don't know how Mitel phones does this.

[Vince-Whirlwind · Accepted solution]

The phone is also a switch.

So you have VLAN1 UNTAGGED, VLAN2 TAGGED on your switchports.

You patch a phone in - the phone also needs to be configured same as the switch with VLAN1 UNTAGGED VLAN2 TAGGED On its LAN port.

The phone has a PC port which must be configured as VLAN1 UNTAGGED only.

The PC patches to the phone's PC port and is therefore on VLAN1.

The phone is also configured to know that VLAN2 is the Voice VLAN and so it will use that VLAN for VOICE packets, and it will simply switch the UNTAGGED VLAN1 frames same as a switch does.

The frames coming from the PC are UNTAGGED, the phone passes them on unchanged, so they reach the switch UNTAGGED. You've told the switch that UNTAGGED frames belong to VLAN1, so when they arrive on the switch, the switch assigns them to VLAN1.

The VOICE packets generated by the phone are encapsulated by the phone into frames using 802.1q format, including the VLAN2 TAG in the header and then sent to the switch. The switch is expecting VLAN2 TAGGED frames, so it accepts these frames into VLAN2.

The phone can get its configration either by you manually configuring it, it can use LLDP or CDP to become aware of the VOICE VLAN, or it can get all its config from a DHCP option when it boots up.