I was doing some reading in my Cisco Academy Training - Chapter 4 of the Switching Fundamentals and they are going over VTP.
It showed an example where 3 VTP Server's existed. All 3 switches connected - and that all three are VTP Servers (not clients) - -The S1 VTP Server was able to send out the VTP Domain name (Cisco) to replace "null" entries on other VTP Domain Servers.
I was just trying to understand how that is possible? I thought that only Clients could be sent information and instructions.
Why would there be 3 VTP Servers on one network? The example did not make sense....
Of course I am sure I might have overlooked something -- hope to hear from anyone who can help. Thank you in advance - your knowledge is appreciated.
So getting back to VLAN's -
Once I reach my first L3 Router - that is my boundry?
Basically outside of that router VLAN's are different.
These VLAN's are not going to be able to communicate outside the R1 boundry?
Correct. In your example R1 & R2 form a limit to vlans. So even if it was vlan1-vlan2-vlan3 on both sides they would not be the same vlans.
Bear in mind when you say they can't communicate with each other over a L3 boundary, that is they can't communicate at L2. Obviously devices in vlan 2 for example could communicate with devices in vlan 5 from your above example by routing between the sites.
You generally wouldn't split up your VTP domain. You could end up with multiple VTP domains in the same company if your company merges with another one and you have to integrate 2 VTP domains into one.
But it's important to realise that a VTP domain is only relevant at L2. So at the last place i worked we had multiple sites and all the sites were connected by L3 routed links. Each site had it's own VTP domain so we had multiple VTP domains within the network but each VTP domain was independant of any of the other and separated by a L3 link.
However there may be times when you would want separate VTP domains within the same L2 network for security reasons ie. you don't want the same vlans on all switches but if you need that you may as well use VTP transparent mode and explicitly configure each switch with the vlans you want.
As Jon said.... usually 1 VTP server per domain, however you can have two (or more) for redundancy.
1 VTP Domain has how many servers?
As a general rule in production networks you have 2 VTP servers per VTP domain.
As Federico says it is the highest revision number. In practice this means you can update the vlan info on either VTP server because once the switches have all been updated they should have the same VTP revision number. So if both VTP servers have the same revision number it doesn't matter which one you update because it will then have the higher revision number so all the other switches will synchronise to that one including the other VTP server.
If having multiple VTP servers, they will synchronize to the one that has the latest revision number (no DR/BDR concept).
Just to add to Ryan's post. In production network you will often find that at least 2 switches are assigned to be VTP servers. This does not create a problem for the reasons Ryan has covered but it does mean if one of the switches fails you still have an active VTP server to make your vlan updates on.
The VTP server is the one that makes the changes to the VTP domain. By default, all switches are VTP servers in the (null) VTP domain. Once a VTP server has been configured with a VTP domain, it starts sending out VTP advertisements with that domain name, and other switches that have a (null) domain join it.
VTP clients can't make updates to their local VLAN database. VTP Servers accept updates from other VTP servers in the same VTP domain. VTP clients accept updates from the VTP servers in the same VTP domain.
There is one gotcha with VTP that would allow a VTP Client to make changes to the VLAN database of the VTP Server and it has to do with the Revision number.
When there's a change to the VLAN information, the VTP revision number is incremented by one and an update is sent out. If a switch recieves an update with a higher revision number, it processes the update. If the recieved update is lower than its revision number, it will simply ignore the message. The gotcha is that if a VTP domain has a revision number of X, if any switch is joined to the network that is configured with the same VTP domain name and its revision is higher even if if it's a VTP client, and that VTP client starts sending VTP messages... The VTP server will see the higher revision number coming from the VTP client switch and update its VLAN database. This could result in all the "Good" VLANs being replaced with junk information. It's important to ensure there is not any "linger" VTP data on a switch that you're adding to the network.