What is VTP and what do its modes do?
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VTP is Cisco's protocol for syncing the VLAN database (IDs and names, never port assignments) across the switches of one domain, over trunks. A server creates, changes and deletes VLANs and advertises them. A client takes VLANs from the servers and cannot change them locally. Transparent keeps its own local VLANs and only relays advertisements. Off ignores VTP and does not relay it. Catalyst switches default to server mode with no domain name.
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Modes
| Mode | Create VLANs locally | Follows the domain | Relays adverts |
|---|---|---|---|
| Server | Yes | Yes | Yes |
| Client | No | Yes | Yes |
| Transparent | Yes, local only | No | Yes |
| Off | Yes, local only | No | No |
In server and client mode the VLANs are not in the running config. In transparent and off mode they are, and they are saved with it.
Versions
- v1: the original.
- v2: adds consistency checks, and a transparent switch relays advertisements without checking the domain name.
- v3: adds a primary server, extended VLANs, private VLANs, MST region sync and a hidden password.
Mechanics
Advertisements go to 0100.0ccc.cccc on trunks only: a summary every 5 minutes and right after every
change. Every change on a server raises the configuration revision number. VTP pruning, if enabled,
stops flooding a VLAN over a trunk toward switches with no ports in it.
Many networks run every switch transparent or off and push VLANs with automation instead.
They will ask next
Does VTP put ports into VLANs?
No. It only syncs which VLANs exist and their names. Port membership is configured on each switch.
What happens if you try to create a VLAN on a client?
The switch refuses it with a message that VLAN configuration is not allowed in client mode.
Traps
- Thinking a client cannot overwrite the domain. In versions 1 and 2, a client with a higher revision number can.