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I'm referring to the following article, where it mentions "The MAC layer header can be maintained if the router is bridging the network layer protocol. However, regular bridging still terminates the VLAN header."

I understand the router terminating the vlan header, but i don't understand why a bridge would do the same..? What is the possible reason for a bridge to terminate a vlan header, after all, bridging/switching is to contain the packets to a single broadcast domain (vlan domain). can someone shed the light on what exactly is meant by " However, regular bridging still terminates the VLAN".

Understanding and Configuring VLAN Routing and Bridging on a Router Using the IRB Feature

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  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 15 '17 at 0:54
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Historically, bridging on a router assumed no VLAN trunks, so VLAN tags were not read/processed.

IRB was an improvement over "regular" bridging, and added the capability of understanding VLAN tags.

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End devices as PCs, tablets, servers don't recognize VLAN headers.

The switch is the device that injects the VLAN header on the standard Ethernet frame using 802.1Q. From that moment the Ethernet frame is not standard.

When the frame moves between switches there are no problems because switches can identify VLAN headers.

When the frame is going out of the network to the destination device, then the switch must eliminate the VLAN header, to re-establish the standardness of the frame. That is what the author of the article was trying to explain.

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A bridge doesn't necessarily remove VLAN tags.

  1. When both ingress and egress interfaces are VLAN trunks the tag should keep the same VID. You could also say that the tag is removed on ingress and a new one inserted on egress however.
  2. Some dumb (802.1Q non-compliant) switches ignore 802.1Q tags and leave them on a frame. This can lead to various (more or less) strange effects and may cause bridge loops. You should never trunk to an unmanaged switch.

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