VLAN Trunking Protocol (VTP) – VTP updates are sent out of all active trunking interfaces (dot1Q or ISL). Each VTP advertisement includes a revision number that is incremented by a VTP server, the advertisement will only be processed by VTP servers and VTP clients in the same domain and correct password if the revision number is greater than the one currently stored on the switch. Standard range VLAN information is stored in vlan.dat file stored in the flash.

Cisco switches are VTP servers by default but will not send out VTP advertisements until a VTP domain is configured.

There are 3 main modes a VTP switch can be in Server, Client & Transparent.

Server – In VTP server mode you can create, edit and delete VLAN information on the switch and it will be propagated throughout the VTP domain. VTP servers also originate periodic VTP updates.

Client – VTP client mode is exactly the same as server mode except it is not possible to create edit or delete VLANs on the switch, you will need to edit them on a server in the VTP domain and they will update the client, clients will also originate VTP updates.

Transparent – In VTP transparent mode the switch will forward VTP advertisements but not process any of the advertisements. VLAN information can be changed on the switch but the information will not be propagated and stay local to the switch.

Standard/Extended range VLANs – VTP will only update standard range VLANs which is any VLAN with a number between 1 and 1005. If you wish to configure extended range VLANs (VLAN numbers 1024 to 4094) then the server must be in VTP transparent mode as VTP does not support these.

Note: VLANs 1006 to to 1024 were reserved for compatibility with CatOS based switches and shouldn’t not be used.

Extended range VLANS cant be stored in the vlan.dat file and will be stored in the running configuration, if the startup config and the vlan.dat have any differences only the vlan.dat information will be used.

VLAN Trunking – Interconnects between switches are trunked using either ISL or 802.1Q. ISL is Cisco propriety and encapsulates each frame with a 26 byte header and an additional trailer where as dot1Q which is an IEE standard adds a 4 byte tag after the source address field in the frame. dot1q will not tag the native VLAN on a link therefore any frames receiving on a VLAN trunk without a tag are presumed to be part of the native VLAN, ISL does not support native VLANs.

Dynamic Trunk Protocol (DTP) – DTP allows a switch port to automatically negotiate a trunk, this can be a security issue and personally I am not to keep on this and prefer to manually make each port either a trunk or access port. The DTP modes are;

on - Permanent trunk even if the neighbour cant support it

off - Permanent access port, so wont trunk even if the neighbour cant support it

desirable - Actively sends out DTP frames to attmpt become a trunk but will become a trunk or an access port.

auto - attempts to passively become a trunk, so wont send out frames but will respond if it receives them. Note that if both ends are set to auto then the port will not become a trunk

nonnegotaite - The port  will not send any DTP frames out, its recommended that this should be used when connecting the port to a non cisco switch which could react strangely to DTP frames.  Either use switchport mode trunk or switchport mode access to dictate what mode the port should be in.