802.1Q-Tagged Frames Through Unmanaged Switch – Forwarded or Dropped?
As a follow-up to the post yesterday on native VLANs, there was a question on what would happen to 802.1Q-tagged frames traversing an unmanaged switch. Unmanaged in this case being a switch that does not support VLANs. While this might be more of a theoretical question today, it’s still interesting to dive into it to better understand how a 802.1Q-tagged frame is different from an untagged frame.
Before we can answer the question on what a VLAN-unaware switch should do, let’s refresh our memory on the Ethernet header. The Ethernet frame consists of Destination MAC, Source MAC, Ethertype, and FCS. 802.1Q adds an additional four bytes consisting of Tag Protocol Identifier (TPID) and Tag Control Information (TCI). This is shown below:
Note how the TPID in the tagged frame is in the place of EtherType for untagged frames. It’s also a 2-byte field and the TPID is set to 0x8100 for tagged frames. The EtherType field is still there and would be for example 0x0800 for IPv4 payload.
To demonstrate what this looks like on the wire, I’ve setup two routers with the following configuration:
hostname R1 ! vrf definition ETHERNET ! address-family ipv4 exit-address-family ! interface GigabitEthernet1.100 encapsulation Continue reading