NUC980的网口接收 0x01-0x0E-0xCF-0x00-0x00-0x00这类MAC地址时，为什么在MAC地址和协议类型之间插入 4字节数据 观察发现是 0x81 00 00 00 ，这个可以在哪里禁止插入？
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 ques
tion 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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What do switches do to tagged and untagged VLAN packets?
Switches strip or add tags depending on the port's settings. Your hypothetical packet tagged VLAN 10 ingressing on port 1 would have its tag stripped and when it egressed port 2 (or port 3, because both are untagged members of VLAN 10).
Similarly, when a packet ingresses port 2 it will be tagged VLAN 10 before egressing port 1, as per the second part of your question.
I know of at least one Ethernet switch implementation that tags frames on ingress on an untagged port, moves all frames around inside the switch as tagged frames. That particualr switch strips tags only when frames egress an untagged port.
I would assume that this type of behavior is probably fairly common, seeing as how the switch needs some way to keep track of the origin VLAN of frames as they move around inside the switch. Since I only know the low-level details of one (now very outdated and no longer manufactured) switch, though, I won't make sweeping generalizations that I can't back up.
Differences between switch and gateway
SWITCH GATEWAY
It works as multi-port bridge between networks. It works as a passage between two networks.
It works in data link layer. It works in all networks layers.
It performs error checking before forwarding packets. It does not perform error checking.
It does not work as protocol converter. While it works as protocol converter.
Packet filtering is provided in switch. Packet filtering is not provided in gateway.
It is less complex. It is more complex than switch.
It connects devices that work on same network model. It connects two networks working on different models.
The format of packet is not changed. Packet format is changed.
Adding and Removing VLAN Tags
