19. DTP / VTP (Not in Syllabus)

DTP (Dynamic Trunking Protocol)

• Protocol that allows SWITCHES to negotiate the status of their SWITCHPORTS, without manual configuration, to be:

  • ACCESS PORTS
  • TRUNK PORTS

• 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

Click to enlarge

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

Click to enlarge

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

• Protocol for configuring VLANs on a Central SWITCH

  • A SERVER that other SWITCHES synch. to (auto configuring by connection)

• Other switches (VTP CLIENTS) will synchronize their VLAN database to the SERVER

• Designed for large networks with many VLANs (reduces manual configuration)

• RARELY used. Recommended you DO NOT USE it

• 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

• There are THREE VTP modes:

  • SERVER
  • CLIENT
  • TRANSPARENT

• Cisco SWITCHES operate in VTP SERVER mode, by default

Click to enlarge

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 #