Bitmindframes cisco study guides

DTP Dynamic Trunking Protocol

Trunk negotiation is managed by the Dynamic Trunking Protocol (DTP), which is a Cisco proprietary Point-to-Point Protocol.
These various modes are configured using the switchport mode interface command
Switch trunk interfaces support different trunking modes.

• Access
• Dynamic desirable (default mode on Catalyst 2950 and 3550)
• Dynamic auto
• Trunk
• Non-negotiate
• dotq-tunnel (Not an option on the Catalyst 2950.)

Using these different trunking modes, an interface can be set to trunking or nontrunking or even able to negotiate trunking with the neighboring interface.
To automatically negotiate trunking, the interfaces must be in the same VTP domain.  (VTP is discussed Here .)

Switches have two “non-dynamic” options:
Switch1(config-if)#switchport mode access
An interface will become an access port and not allow trunking

Switch1(config-if)#switchport mode trunk
An interface will form a trunk link.

Switchport mode access – This command puts the interface (access port) into permanent nontrunking mode. The interface will generate DTP frames, negotiating with the neighboring interface to convert the link into a nontrunk link. The interface becomes a nontrunk interface even if the neighboring interface does not agree to the change.

Switchport mode dynamic desirable – This command makes the interface actively attempt to convert the link to a trunk link. The interface becomes a trunk interface if the neighboring interface is set to trunk, desirable, or auto mode. This is the default mode for all Ethernet interfaces. If the neighboring interface is set to the access or non-negotiate mode, the link will become a non-trunking link.

Switchport mode dynamic auto – This command makes the interface willing to convert the link to a trunk link if the neighboring interface is set to trunk or desirable mode. Otherwise, the link will become a non-trunking link.

Switchport mode trunk – This command puts the interface into permanent trunking mode and negotiates to convert the link into a trunk link. The interface becomes a trunk interface even if the neighboring interface does not agree to the change.

Switchport nonegotiate – Prevents the interface from generating DTP frames. You can use this command only when the interface switchport mode is access or trunk. You must manually configure the neighboring interface as a trunk interface to establish a trunk link, otherwise the link will be a non-trunking link.

By default, interfaces on most Cisco switches are set to dynamic desirable mode. (Catalyst 2950 and 3550 switches.)
Desirable mode will create a trunk link if the neighboring interface is set to desirable, trunk, or auto mode.
Because both interfaces by default are in desirable mode, this means a link between two Cisco switches will automatically become a trunk link unless configured otherwise. It is reccomended that all switch ports be set to access mode for security. Why? Because if someone were to plug in another switch it would auto create a trunk link with the rouge switch.

To Set your switch to access mode you can use the access command
Switch(config)#interface range fa 0/23 – 24
Switch(config-if-range)#switchport mode access
Switch(config-if-range)#switchport access vlan 10

Verify Switchport Access mode:
Switch#show interface fa 0/11 switchport
Name: Fa0/23
Switchport: Enabled
Administrative Mode: static access
Operational Mode: down
Administrative Trunking Encapsulation: dot1q
Negotiation of Trunking: Off
Access Mode VLAN: 10 (bitmindframes)

Selecting the right combination on the two ends of the link is important
Here you can see the basic table for trunk links modes and the effects of each.

Verify Switchport Trunk mode:
Switch#show interface fa 0/23 switchport
Name: Fa0/11
Switchport: Enabled
Administrative Mode: dynamic desirable
Operational Mode: down
Administrative Trunking Encapsulation: dot1q
Negotiation of Trunking: On
Access Mode VLAN: 10 (bitmindframes)
Trunking Native Mode VLAN: 1 (default)
Voice VLAN: none

The only modes that actually negotiate trunking between adjacent switches are trunk, dynamic desirable and dynamic auto. Even though the switchport mode trunk will not modify its own interface to be anything other than trunking, it will send out DTP frames to attempt to negotiate the other end of the link to be a trunking interface.

Nonegotiate command

The switchport nonegotiate command prevents the interface from generating DTP frames, turning the Negotiation of Trunking off.
You can set any interface to nonegotiate with:
2950-1(config)#inter fa 0/1
2950-1(config-if)#switchport mode access
2950-1(config-if)#switchport nonegotiate
2950-1(config-if)#end

Verify your nonegotiate settings:

Switch1#show interface switchport
Name: Fa0/23
Switchport: Enabled
Administrative Mode: static access
Operational Mode: static access
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: native
Negotiation of Trunking: Off
Access Mode VLAN: 10 (default)
Trunking Native Mode VLAN: 10 (default)
Administrative private-vlan host-association: none
Administrative private-vlan mapping: none
Operational private-vlan: none
Trunking VLANs Enabled: ALL
Pruning VLANs Enabled: 2-1001

The switchport mode access and switchport nonegotiate interface configuration commands should be used when configuring a non-trunking interface to another switch that does not support DTP.

Here is a basic topology in Packet tracer, Under simulation mode you can see  the switches transmit STP , DTP frames are sent from S1 to S0 to negotiate a trunk. S0’s interface is set to access mode, so the red envelope with an X over it shows Trunk negotiation was attempted and failed.

Tags: 802.1q, interface, ip, study, guides, vtp

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