up vote 3 down vote favorite

Does 802.1Q protocol will remove the VLAN tag in the received place?

Such as the VLAN 10 trough 802.1Q protocol to pass frames.

enter image description here

the 802.1Q will add 802.1q header(tag) and recalculate CRC/FCS like bellow:

enter image description here

but I have a question, whether the receive side will remove the VLAN tag and recalculate the CRC/FCS again?

if it will remove the VLAN tag and recalculate the CRC/FCS, is in the receive Switch's trunk port or in other specific location to do this?

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up vote 4 down vote accepted

Whether the receiver removes the Q tag right away or keeps it for further processing is its own decision.

On a switch, this depends on whether the VLAN ID in question is tagged on the destination port (trunk port) or untagged/native (access port).

Even on an untagged port, the Q tag may sometimes still be present to preserve PCP priority but with a zeroed VLAN ID field - depending on switch and port configuration.

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  • thanks for your response, but I do not understand the your receiver means the switch on receive side or the PC, and if the receive switch transmit the frame to destination PC always should remove the Q tag? – three-blocks Nov 8 at 10:15
  • "Remove the Q tag" is meaningless for a host - it doesn't store the frame anyway. It extracts the frame's payload and that's it. Depending on the VLAN ID and the NIC configuration, the host processes the payload differently, e.g. associated with another, logical NIC. Whether a switch forwards a frame tagged or untagged depends on the switch port configuration. – Zac67 Nov 8 at 17:46
  • If the switch untagged port(access) set to do not remove Q tag, then the destination PC will discard the frame, right? because it's not a original frame. – three-blocks Nov 9 at 4:37
  • Many NICs support Q-tagged frames - that requires proper configuration though. – Zac67 Nov 9 at 5:56
up vote 0 down vote

VLAN tags are always at interface level or 'NIC' level. What it means is, when a switch receives a tagged packet on an interface. It checks if the VLAN id on the packet is the same as the one allowed on the interface. - If the tag matches, the packet is stipped off the tag and sent for further processing. If not the packet is rejected.

Here is what happens in 'further processing'. If the switch is pure L2, it will look in the forwarding table of the particular VLAN to find the outgoing interface for the packet (say for eg the incoming packet came with VLAN tag 10), It will try to find the outgoing interface for the dest MAC in the forwarding table of VLAN 10.

Once the packet is on the outgoing interface, if that interface is a trunk port - a VLAN tag of 10 will be added on the frame. If that port is VLAN 10 access port, the packet will be sent from the interface without any tagging.

Now, if the switch is not pure L2 - i.e. uses VLAN interfaces, The incoming packet is again checked for VLAN tag, the packet is sent to the corresponding vlan interface.

Here the interface looks for L3 forwarding table - also known as the routing table to find the outgoing interface for the packet.

Again once the outgoing interface is determined if that interface is a trunk port the packet is tagged with VLAN ID, if not the packet is sent without any tagging

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