Switch untagged/tagged traffic at native vlan

[Umesh Prajapati11238]

Hello all,

This discussion with reference to untagged traffic on trunk port.

I am little bit confused that How switches determine that which is untagged frame or tagged frame on trunk port.

I know following things as per the below diagram.

Step 1. lets suppose PC1 (SW1) wants to communicate p2 which is same vlan but PC2 is connected SW2.

Step 2. In normal operation  when broadcast frame receives from pc1 to sw1 on F0/1 what switch will do, it will do the broadcast on every port except on which port frame is received.

Step 3. 802.1q protocol will start tagging with vlan 10 and forward frame to sw2.

Step 4. SW2 remove 802.1q tag and forward frame to vlan 10, in which pc2 belongs, which is connected on switch 2 port F0/1

Step 5.  get the reply to PC1.

But I am unable to understand how switches identify whether it tagged traffic or untagged traffic.

Also let me know which type of traffic is tagged or untagged.

Thank you in advanced.

learner

[M02@rt37]

Hello @Umesh Prajapati11238,

 **Untagged Frames:**
Frames originating from end-user devices (like PCs) are typically untagged. When a frame from an end-user device arrives at a switch port, the switch assumes it belongs to the native VLAN for that port (configured on the switch port). The frame is forwarded within that VLAN without any VLAN tag (hence, "untagged").

**Tagged Frames:**
Frames that traverse a trunk link between switches are typically tagged. When a frame is sent out from a switch port configured as a trunk, it's tagged with the appropriate VLAN ID (e.g., using 802.1Q encapsulation). The VLAN ID in the tag helps the receiving switch understand to which VLAN the frame belongs.

In the scenario you described when PC1 on SW1 sends a frame, it is an untagged frame (normal end-user traffic). As the frame traverses the network to reach PC2, it may traverse a trunk link between switches. When crossing the trunk, the switch adds a VLAN tag (802.1Q tag) to the frame, indicating the VLAN (e.g., VLAN 10). When the frame reaches SW2, it recognizes the VLAN based on the tag, processes the frame accordingly within VLAN 10, and removes the tag before forwarding the frame to PC2.

Essentially, switches determine whether a frame is tagged or untagged based on the port configuration (access or trunk), and they handle the frames accordingly within the context of the configured VLAN for that particular port.

[Joseph W. Doherty]

From Wiki:

Tag protocol identifier (TPID)

A 16-bit field set to a value of 0x8100[b] in order to identify the frame as an IEEE 802.1Q-tagged frame. This field is located at the same position as the EtherType field in untagged frames, and is thus used to distinguish the frame from untagged frames.