Alright, this should be a quick post. This wraps up what we left out in the last VTP post as that post got too long.
The main points of today: VTP Pruning, a little more details on how VTP actually communicates and a couple of useful debugging commands.
By default all VLANs active in the VTP domain are allowed on all trunk links.
By enabling VTP pruning switches can negotiate and limit which VLANs are allowed on a trunk to minimize broadcast traffic on the link.
VTP pruning only has to be enabled on a VTP Server for it to be enabled in the domain. You enable VTP pruning with:
You can also enable VTP only on a single link rather than on the whole domain:
switch(config-if)#switchport trunk pruning vlan (((add | except | remove) [vlan-list]) | none)
How VTP Works:
VTP has two messages: VTP Summary Advertisements and VTP Subset Advertisements.
VTP sends out VTP Summary Advertisements on all trunk links periodically (every 300 seconds) and also every time the VLAN database changes. These are VTP message type 1. VTP Summary Advertisements send out the domain, password (if enabled) and the Configuration Revision Number (CFN).
These are sent at multicast messages to multicast MAC 0100:0ccc:cccc with a SNAP type of 2003. All switches will flood these out their trunk ports, if a switch is in transparent mode it floods the message but otherwise ignores it.
Some more on VTP Version 3
One more thing to note about VTPv3, not that it’s in the exam, but VTPv3 also supports private-VLANs (PVLANS) on-top of supporting VLANs all the way to 4096.
Useful show commands
Some commands I used to debug and for the exam:
switch#show vtp status
Switch#show vtp status VTP Version : 2 Configuration Revision : 0 Maximum VLANs supported locally : 1005 Number of existing VLANs : 5 VTP Operating Mode : Client VTP Domain Name : VTPDomain VTP Pruning Mode : Disabled VTP V2 Mode : Disabled VTP Traps Generation : Disabled MD5 digest : 0xD3 0x78 0x41 0xC8 0x35 0x56 0x89 0x97
switch#show vtp counters VTP statistics: Summary advertisements received: 2 Subset advertisements received: 1 Request advertisements received: 0 Summary advertisements transmitted: 2 Subset advertisements transmitted: 1 Request advertisements transmitted: 1 Number of config revision errors: 0 Number of config digest errors: 0 Number of V1 summary errors: 0 VTP pruning statistics: Trunk Join Transmitted Join Received Summary advts received from on-pruning-capable device ---------------- ---------------- ---------------- --------------------------- Fa0/1 1 1 2
switch# show interface trunk Port Mode Encapsulation Status Native vlan Gi0/1 on 802.1q trunking 1 Port Vlans allowed on trunk Gi0/1 1-4094 Port Vlans allowed and active in management domain Gi0/1 1-100 Port Vlans in spanning tree forwarding state and not pruned Gi0/1 1,5,10
This allows you to easily determine what VLANs are allowed on the link (1-4094 in the case), what VLANs are used locally on the switch (1-100) and what VLANs are actually going out the trunk interface (1,5,10). If the last two things don’t match, this means that pruning has been enabled.
And that is hopefully all I ever need to say about VTP.