DTP (Dynamic Trunking Protocol)
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Protocol that allows SWITCHES to negotiate the status of their SWITCHPORTS, without manual configuration, to be:
- ACCESS PORTS
- TRUNK PORTS
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DTP is ENABLED by default on all Cisco SWITCH interfaces
We’ve been manually configuring SWITCHPORTS using :
- “switchport mode access”
- “switchport mode trunk”
💡 'show interfaces <interface-id> switchport' will show you a switchport’s settings.
For security purposes, manual configuration is recommended. DTP should be disabled on ALL SWITCHPORTS
DYNAMIC DESIRABLE:
- This MODE will actively try to form a TRUNK with other Cisco SWITCHES.
- Will form a TRUNK if connected to another SWITCHPORT in the following modes:
- “switchport mode trunk”
- “switchport mode dynamic desirable”
- “switchport mode dynamic auto”
HOWEVER … if the other interface is set to “static access” (ACCESS mode), it will NOT form a TRUNK, it will be an ACCESS PORT
DYNAMIC AUTO:
- This MODE will NOT actively try to form a TRUNK with other Cisco SWITCHES
- Will form a TRUNK if connected SWTICH is actively trying to form a TRUNK.
- It will form a TRUNK with a SWITCHPORT in the following modes:
- “switchport mode trunk”
- “switchport mode dynamic desirable”
TRUNK to ACCESS connection will operate in a Mismatched Mode.
This configuration does NOT work and SHOULD result in an error. Traffic will NOT work.
TABLE SHOWING THE DIFFERENT MODES AND COMPATIBILITY IN FORMING A TRUNK
DTP will NOT form a TRUNK with:
a ROUTER
a PC
etcetera …
The SWITCHPORT will be in ACCESS Mode only!
OLD SWITCHES:
- “switchport mode dynamic desirable” = Default administrative mode.
NEWER SWITCHES:
- “switchport mode dynamic auto” = Default administrative mode.
HOW TO DISABLE DTP NEGOTIATION ON AN INTERFACE:
- “switchport nonegotiate”
- “switchport mode access”
It is a security recommendation to disable DTP on all SWITCHPORTS and manually configure them as ACCESS or TRUNK ports.
ENCAPSULATION:
SWITCHES that support both:
- 802.1Q
- ISL
TRUNK encapsulation can use DTP to negotiate the encapsulation they will use.
- Negotiation is Enabled by default
💡 'switchport trunk encapsulation negotiate'
- ISL is favored over 802.1Q
- If BOTH SWITCHES support ISL, ISL will be selected.
- DTP frames are sent in:
- VLAN1 when using ISL
- Native VLAN when using 802.1Q (the default native VLAN is VLAN1, however)
VTP (VLAN Trunking Protocol)
In Privileged EXEC mode:
💡 #show vtp status
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Protocol for configuring VLANs on a Central SWITCH
- A SERVER that other SWITCHES synch. to (auto configuring by connection)
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Other switches (VTP CLIENTS) will synchronize their VLAN database to the SERVER
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Designed for large networks with many VLANs (reduces manual configuration)
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RARELY used. Recommended you DO NOT USE it
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There are THREE VTP Versions :
- v1
- Does NOT supports Extended VLAN Range 1006-4094
- v2
- Does NOT supports Extended VLAN Range 1006-4094
- Supports Token Ring VLANs ; otherwise similar to V1
- v3
- Supports Extended VLAN Range 1006-4094
- CLIENTS store VLAN dBase in NVRAM
- v1
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There are THREE VTP modes:
- SERVER
- CLIENT
- TRANSPARENT
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Cisco SWITCHES operate in VTP SERVER mode, by default
VTP SERVERS:
- Can ADD / MODIFY / DELETE VLANs
- Store the VLAN dBase in NVRAM
- Increase Revision Number every time VLAN is Added / Modified / Deleted
- Advertises Latest Version of VLAN dBase on TRUNK interfaces.
- VTP CLIENTS synchronize their VLAN dBase to it
- VTP SERVERS also function as VTP CLIENTS
- THEREFORE, a VTP SERVER will synchronize to another VTP SERVER with a higher Revision Number
VTP CLIENTS:
💡 (config)# vtp mode client
- Cannot Add / Modify / Delete VLANs
- Does NOT store the VLAN database in NVRAM
- VTP v3 CLIENTS DO
- Will synchronize their VLAN dBase to the SERVER with the highest version number in their VTP Domain
- Advertise their VLAN dBase and forward VTP Advertisements to other CLIENTS over TRUNK Ports
VTP TRANSPARENT MODE:
💡 (config)# vtp mode transparent
- Does NOT participate in VTP Domain (does NOT sync VLAN database)
- Maintains own VLAN dBase in NVRAM.
- Can Add / Modify / Delete VLANs
- Won’t Advertise to other SWITCHES
- Will forward VTP advertisements to SWITCHES in the same Domain as it.
VTP DOMAINS
If a SWITCH with no VTP Domain (Domain NULL) receives a VTP advertisement with a VTP Domain name, it will automatically join that VTP Domain
If a SWITCH receives a VTP advertisement in the same VTP domain with a higher revision number, it will update it’s VLAN database to match
REVISION NUMBERS:
There are TWO ways to RESET a REVISION NUMBER to 0:
- Change VTP Domain to an unused Domain
- Change VTP mode to TRANSPARENT
VTP VERSION NUMBER
💡 (config)#vtp version <version number>
Changing the Version # will force sync/update all connected SWITCHES to the latest Version #