Dynamic Truncking Protocol (DTP) :
The Cisco-proprietary Dynamic Trunking Protocol (DTP) actively attempts to negotiate a trunk link with the remote switch. This sounds great, but there is a cost in overhead - DTP frames are transmitted every 30 seconds. If you decide to configure a port as a non-negotiable trunk port, there's no need for the port to send DTP frames.
DTP can be turned off at the interface level with the switchport nonegotiate command, but as you see below, you cannot turn DTP off until the port is no longer in dynamic desirable trunking mode. (Dynamic desirable is the default mode for most Cisco switch ports.)
SW2 (config) #int fast 0/8
SW2 (config-if)#switchport nonegotiate
Command rejected: Conflict between 'nonegotiate' and 'dynamic' status.
SW2 (config-if) #switchport mode?
access Set trunking mode to ACCESS unconditionally
dynamic Set trunking mode to dynamically negotiate access or trunk mode
trunk Set trunking mode to TRUNK unconditionally
SW2 (config-if) #switchport mode trunk
When you're working with Cisco switches in a home lab or rack rental environment, run IOS Help regularly to see what options are available for the commands you're practicing with. Cisco switch ports have quite a few options, and the best way to find them is with one simple symbol - the question mark!
Support for VTP Pruning
The VTP Pruning service is supported by both VTP 1 and VTP 2 versions of the VTP protocol. With VTP 1, VTP pruning is possible with the use of additional VTP message types.
When a Cisco Catalyst switch has ports associated with a VLAN, it will send an advertisement to its neighboring switches informing them about the ports it has active on that VLAN. This information is then stored by the neighbors and used to decide if flooded traffic from a VLAN should be forwarded to the switch via the trunk port or not.
Note: VTP Pruning is disabled by default on all Cisco Catalyst switches and can be enabled by issuing the "set VTP pruning enable" command.If this command is issued on the VTP Server(s) of your network, then pruning is enabled for the entire management domain.
VTP Pruning configuration and commands are covered in section 11.4 as outlined in the VLAN Introduction page, however, we should inform you that you can actually enable pruning for specific VLANs in your network.
When you enable VTP Pruning on your network, all VLANs become eligible for pruning on all trunk links. This default list of pruning eligibility can thankfully be modified to suite your needs but you must first clear all VLANs from the list using the "clear vtp prune-eligible vlan-range" command and then set the VLAN range you wish to add in the prune eligible list by issuing the following command: "set vtp prune-eligible vlan-range" where the 'vlan-range' is the actual inclusive range of VLANs e.g '2-20'.
By default, VLANs 2–1000 are eligible for pruning. VLAN 1 has a special meaning because it is normally used as a management VLAN and is never eligible for pruning, while VLANs 1001–1005 are also never eligible for pruning. If the VLANs are configured as pruning-ineligible, the flooding continues as illustrated in our examples.
VTP Pruning can in fact be an administrator's best friend in any Cisco powered network, increasing available bandwidth by restricting flooded traffic to those trunk links that the traffic must use to reach the destination devices.
At this point, we have also come to the end of the first part of our VLAN presentation. As we are still working on the second and final part of the VLAN topic, we hope these pages will keep you going until it is complete.