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tagged and untagged VLANs with Trucking


i have 2 6248 switches, and 2 5548.

i am starting to configure the network on the 6248 switches.

i have 3 VLANs (1,99,98) and 111 is my management VLAN.

all the ports are untagged (i see 'U' on VLAN management interface).

1 of the ports is connected to 5548 switch (with default settings), and i need it to be able to share some VLANs (1+98).

when i am moving the port to be trunk port, the connection to the PCs on the 5548 not works (when the port is on VLAN 1 the connection from the VLAN works great).

Do you have any idea why can it be?

is it possible to connect TRUNK port with untagged VLANs? i read that trunk is tagged only, and general is used for untagged VLANs, but if i will use general port, the broadcast from VLANs on 5548 will get only to PVID VLAN only, so its not really help me.. i need the port to be able to transfer and receive data from all VLANs on both sides (this is the reason i think i should use tagged trunk), but i need the machines on the VLANs to be untagged connections (i don't want to set on my server the VLANID). is it possible?

am i understood TRUNK and GENERAL correctly?


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RE: tagged and untagged VLANs with Trucking

You are correct, a Trunk connection passes tagged traffic except for the native VLAN. Ports set to General mode may be members of multiple VLANs. Each of these VLANs may be configured to be tagged or untagged. This setting applies to transmitted frames. Incoming untagged frames are classified into the VLAN whose VID is the currently configured PVID.


This means that for example, if the 5548 send untagged packets from VLAN 2,3,4 across the Trunk connection to the 6248. The 6248 receives these untagged packets from the three separate VLANs. Since the packets are untagged it does not know that the packets came from any specific VLAN and places all of them into the PVID, VLAN 1. If that is what you want to occur, you have the right idea.


In most cases it is usually desired for VLAN traffic to stay in the VLAN it originated from until it's destination is outside of the VLAN it originates from. This is why we usually tag all traffic across a trunk except for the PVID.


When using a general mode connection you can specify tagged or untagged.

console(config-if)# switchport mode general

console(config-if)# switchport general allowed vlan add 2,3,4 tagged

console(config-if)# switchport general allowed vlan add 5,6,7 untagged

console(config-if)# switchport general pvid 1


Hope this helps some.

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1 Copper

RE: tagged and untagged VLANs with Trucking

ok i will try it.

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