Vlan flow through switch

Hello All,

I have a switch in which all ports are in default vlan i.e. 1 Now I attach a client to port#2 and a tcpdump on port#3 Client can generate packets of vlan (say with tag 1). I expect those packets to be received by sniffer. Both the ports are access ports and ingress rule is configured to accept tagged and untagged.

Still when I run my packet generator(vlan packets with id 1) on client it is not received by sniffer. When I send IPV4 packets they are received by sniffer.

Now I convert both ports to trunk and run packet generator(vlan packets with id 1). Sniffer receives the packet now.

I dont know whether this is the problem of packet generator or switch. Could anyone suggest something on this?

Here is my packet description :

00:57:15.233007 12:12:12:12:12:12 (oui Unknown) > Broadcast, ethertype 802.1Q (0x8100), length 60: vlan 1, p 0, CFI, LLC, dsap Null (0x00), ssap 802.1B I (0x02), cmd 0x03: Unnumbered, ui, Flags [Command], length 42 0x0000: ffff ffff ffff 1212 1212 1212 8100 1001 ................ 0x0010: 020f 0102 0304 0506 0708 090a 0b0c 0b0c ................ 0x0020: 0d00 0000 0000 0000 0000 0000 0000 ..............


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If your frame generator is explicitly transmitting frames to VLAN 1, it must be transmitting frames with the extended IEEE 802.1Q header. Which means that the switch port to which it is connected must be configured to understand these extra overhead fields. If the switch port to which that frame generator is connected is set to its default configuration, it would not normally process frames with the VLAN header. It would just drop those frames.

On the other hand, when sending standard IPv4 over Ethernet frames, you're just using Protocol ID 0x0800, and the switch is familiar with that.

So to make a switch port "VLAN-aware," you need to configure it that way. "Trunk port" sounds like a good name for it. The VLAN frames arrive with Protocol ID 0x8100. (Clause 9 of IEEE 802.1Q, which you can download for free from

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And configured as a "trunk port," the switch knows what to do with this odd format.

I don't think there's any problem. Once you are aware of the fact that VLANs are an afterthought to Ethernet, and that typical Ethernet hosts are oblivious to the additional header fields.


Reply to
Albert Manfredi

The port interface counters on the switch should tell you whether it is transmitting packets out the port that you expect. Assuming it's not transmitting frames to the sniffer port, but it is receiving frames from the client, it's probably a problem with the switch (based on what you say about the way you've configured it).

What happens if you send untagged packets from the client in the case where the ports are configured to be "access" ports?


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Just an observation ... the packet you've shown is a broiadcast packet ... which you'd expect all ports in the vlan to receive. You have set up port monitoring on port 3 to monitor port 2 haven't you?


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