I learned from http://www.netcontractor.pl/blog/?p=184 that control traffic from Layer 2 protocols like ( DTP, VTP , CDP , PAgP , STP, etc ) use VLAN 1.
And I make sure of that by doing this experience :
- I made a simple topology of connection two switches and making connectivity as trunk.
- I made one switch VTP server and another as VTP client.
- I also setup RSPAN to monitor the packets.
- Results, I saw VTP, CDP traffic marked with VLAN ID 1.
- Then, I made another vlan and disallowed Vlan 1 in the trunk.
- Results still the same.
- I thought it might be that traffic might be sent untagged as native vlan, so then i changed the native vlan on both switches, expecting that either VTP, CDP will fail OR it will be marked as VLAN ID of new native vlan.
- However, to my surprise it was still showing as VLAN ID 1.
- So I’m baffeled and confused on this type of behaviour where Vlan 1 is NOT allowed in trunk and also native vlan has been changed, still VTP and CDP control traffic is shown as sourced from Vlan 1.
After this experience I have only one question ?
As I know that it is work, so my question is not whether should work or not work… But how actually it is working? I mean how does these protocols use VLAN 1 when it’s not allowed in trunk port or even when it’s shutdown ? can you shed some light please and explain how the packet is send on vlan 1 despite it pruning on trunk because I’m very confused ?