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Consider the following example:

H1 --- S1 --- S2 --- S3 --- H2

Hx are hosts, Sx are switches. Both hosts are connected with access port on VLAN 20 to the corresponding switches, while all the other ports are trunks. No VTP is enabled, meaning that only S1 and S3 has VLAN20 in their table.

This is how I reason the process of sending a frame from H1 to H2:

  • The frame is tagged with VLAN 20 upon leaving via the trunk port from S1
  • The frame reaches S2, since it can't recognize the VLAN, it simply drops the frame, hence H2 doesn't receive the frame.

After trying to simulate this with PacketTracer, I was surprised to see that the frame did get to H2. So my new reasoning regarding this is as follows:

  • The frame is tagged with VLAN 20 upon leaving via the trunk port from S1
  • It is untagged when entering the left trunk port on S2, but S2 remembers it's tag. Since no port matches the VLAN, and no port matches the dest MAC, S2 sends it through all possible trunks other than the original one (hence through the right one).
  • S3's trunk port removes the tag, and since it can find the VLAN in its table, it tries and successfuly associates it with the access port which connects to H2.

So I'm really not sure what's happening here, so I'd like to have a more thorough explanation about what's really going on under the hood here. Both in the VLAN aspect, and even in how it combines with the switch's lookups over the MAC table (which one comes first for example, the MAC table lookup, or the VLAN table lookup).

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Packet Tracer is not always a faithful simulation of actual switch behavior. In the real (Cisco) world, the switches would operate as you initially described.

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