Technically, both of these have been removed from the CCNA topics list, but there might still be questions on them.
DTP
- DTP = Dynamic Trunking Protocol
- Old, Cisco-Proprietary automatic trunking protocol. Not commonly used, and should probably be disabled for security purposes.
- Allows interfaces to dynamically determine their status (access or trunk) without manual configuration.
- Enabled with
switchport mode dynamic- Two options:
autoanddesirabledesirable: will actively try to form a trunk with other Cisco switches. It will form a trunk if connected to another switchport in the following modes:switchport mode trunkswitchport mode dynamic desirableswitchport mode dynamic auto- If the connected interface is in access mode, then a trunk cannot be formed and the dynamic desirable interface will act in access mode as well
auto: will not actively try to form a trunk, but will form a trunk if the other interface initiates. Will form a trunk if the connected interface is in the following modes:switchport mode trunkswitchport mode dynamic desirable
- Two options:
- DTP will not form a trunk with routers, PCs, etc.; Only with other switches! (And only Cisco switches, at that!)
- On older switches,
switchport mode dynamic desirablewas the default administrative mode.- On newer switches,
switchport mode dynamic autois the default.
- On newer switches,
- You can disable DTP negotiation on an interface with
switchport nonegotiate- configuring the interface with
switchport mode accesswill also disable DTP negotiation. This obviously only works if you need it to be an access port, though.
- configuring the interface with
- Again, it is generally recommended to disable DTP on all interfaces for security, and manually configure them as access or trunk ports.
| Admin. Mode | Trunk | Dynamic Desirable | Access | Dynamic Auto |
|---|---|---|---|---|
| Trunk | Trunk | Trunk | X | Trunk |
| Dynamic Desirable | Trunk | Trunk | Access | Trunk |
| Access | X | Access | Access | Access |
| Dynamic Auto | Trunk | Trunk | Access | Access |
DTP and Encapsulation
- Switches that support both 802.1Q and ISL can use DTP to negotiate which they will use.
- negotiation is enabled by default as the default trunk encapsulation mode is
switchport trunk encapsulation negotiate - ISL is given preference. If both are present on both switches, ISL will be selected.
- negotiation is enabled by default as the default trunk encapsulation mode is
VTP
- VTP = VLAN Trunking Protocol
- Allows you to configure VLANs on a central VTP server switch, and other switches (VTP clients) will synchronize their FLAN database to the server.
- Intended for large networks with many VLANs. Saves time configuring VLANs on every individual switch; you change them on the VTP server switch, and all other switches update from that.
- Like DTP, it is rarely used and it’s recommended that you do not use it.
- 3 versions available: 1, 2, and 3
- 3 modes: server, client, and transparent
- Cisco switches operate in VTP Server mode by default
- VTP Servers:
- Can add/modify/delete VLANs
- Store database in non-volatile RAM (NVRAM)
- Increase the revision number every time a VLAN is added/modified/deleted
- Advertise the latest version of the VLAN database on trunk interfaces, and clients will synchronize their VLAN databases to it.
- Also function as VTP Clients
- Therefore, a VTP server will synchronize to another VTP server with a higher revision number
- VTP Clients:
- Cannot add/modify/delete VLANs.
- Do not store the VLAN database in NVRAM. (in VTPv3 they do)
- Will synchronize their VLAN database to the server with the highest revision number in their VTP domain.
- Will advertise their VLAN database, and forward VLAN advertisements to other clients over their trunk ports.
- VTP Transparent:
- Does not participate in the VTP domain (doesn’t sync its VLAN database).
- Maintains its own VLAN database in NVRAM. It can add/modify/delete VLANs, but they won’t be advertised to other switches.
- Will forward VTP advertisements that are in the same domain as it.
- If a switch with a NULL VTP domain name receives an advertisement from a VTP with a domain name it will join that VTP domain.
- View info on VTP with
show vtp status - Change current VTP version with
vtp version (version-number)- This will increment the revision number and advertise an update to all switches in the domain.
- A danger of VTP: if you connect an old switch with a higher revision number to your network (and the VTP domain name matches), all switches in the domain will sync their VLAN databases to that switch.
- Can result in loss of connection for some/all end hosts if the foreign VLAN database is incorrect.
- Reconnecting an old switch is a likely scenario, but technically any switch with the same domain and a higher revision number could cause this. This could even be done intentionally, if someone were nasty.
- This is a big reason why it’s recommended you not use VTP.