VTP Configuration

      

Now the we talked about the theory of VTP, let’s see how to set it up in this topology

                We already created a Trunk links between Sw1 and Sw2 and also between Sw1 and Sw3, now let’s configure VTP on all of the switches such that, if we were to add a Vlan let’s say the Switch Sw1, that newly created Vlan would be advertised over these Trunk links and that newly created Vlan would be learned by the other Switches in this example, let’s say that Sw1 and Sw2 are gonna be in a Server Mode and Sw3 is gonna be a Client Mode, let’s begin our Configuration on Switch Sw1, first let’s set Sw1 to Server Mode and maybe it already be in a Server mode let’s just confirm that and by being in Server Mode we gonna be able to make changes to our local Vlan Database, let’s go to Global Configuration mode an say

Ø  Sw1(config)#vtp mode server

Next let’s set the VTP Domain name, this is the Case-sensitive Domain name.

Ø  Sw1(config)#vtp domain VTPDEMO

And also, set the Password, we can do that by

Ø  Sw1(config)#vtp password S3cret

                    It might also be a good idea to turn on VTP Pruning remember what that does, it’s says if the far end Switch doesn’t have any ports belonging to a particular Vlan, there is no need to send traffic for that Vlan over the Trunk, however if we make a change to that far end Switch, maybe we do assign a port to that Vlan, VTP is gonna recognize dynamically and start sending traffic for that Vlan over the Trunk, that Vlan is no longer gonna be Pruned, it truly simple to set this up, we just say

Ø  Sw1(config)#vtp pruning

                  And let’s also hard code the Version of VTP that we want to run, although we have Versions of 1, 2 and 3 available many of our new higher end Switches, we might have some Switches on network that only support Versions 1 and 2

                Well the good news is Version 3 is backwards compatible with Version 2 but personally i like to keep things consistent and i am just gonna configure these Switches for VTP Version 2, we can do that by

Ø  Sw1(config)#vtp version 2

Let’s take a look at the VTP configuration that we now have on Switch Sw1, to do that we gonna give the command.

Ø  Sw1#show vtp status

                       This is the main command we use when we dealing Verification and Troubleshooting of VTP, it show that this Switch is capable of running VTP Versions 1 or 2 or 3 but

Ø  Currently the Version that were running is Version 2

Ø  Our Domain name is a VTPDEMO

Ø  Pruning is currently enabled

Ø   Notice that we have a Hash Digest for the password we configured it’s an MD5 Hash rather than the Plain text password, that’s probably good thing

Ø   we can also see our Current Configuration Revision Number it’s 1

Ø   And the number of Existing Vlans we have right now 7

Now let’s perform an identical configuration on Switch Sw2

Let’s go to Switch Sw2

Ø  Sw2(config)#vtp mode Server

Ø  Sw2(config)#vtp domain VTPDEMO

Ø  Sw2(config)#vtp password S3cret

Ø  Sw2(config)#vtp pruning

Will make sure that our Version is Version 2

Ø  Sw2(config)#vtp version 2

                    Let’s go our Switch Sw3 now and do a nearly identical Configuration but the difference is let’s make Switch Sw3 a Client Mode Switch, we going to go to Global Configuration Mode and let’s set the VTP mode to Client

Ø  Sw3(config)#vtp mode client

Ø  Sw3(config)#vtp domain VTPDEMO

Ø  Sw3(config)vtp password S3cret

                     Let’s go back to Switch Sw1 and create a new Vlan and see that i newly created Vlan gets advertised over to Switches Sw2 and Sw3, let’s go to Global Configuration Mode and say

Ø  Sw1(config)#vlan 300

Ø  Sw1(config-vlan)#name VTP_TEST

Ø  Sw1(config-vlan)#exit

Now take a look at our local Vlan database

Ø  Sw1#show vlan brief

                       And we can see that we have Vlan’s 100, 200 and 300 that we created, no surprise that Vlan 300 is showing up because we created on this Switch

Let’s also see for our Configuration Revision Number got incremented when we did that.

Ø  Sw1#show vtp status

                    And just a movement ago our Configuration Revision number was 1, now it’s 2 we added a Vlan and that incremented the Configuration Revision Number by 1, let’s make sure that the Switches Sw2 and Sw3 know about the this newly created Vlan 300 (VTP_TEST)

Let’s go over Switch Sw2 and do a

Ø  Sw2#show vlan brief

Yes, indeed it have learned about Vlan 300 and if we do a

Ø  Sw2#show vtp status

                         We should see a matching Configuration Revision Number of 2, we should see something very similar on Sw3, let’s do a

Ø  Sw3#show vlan brief

And we do see the newly created Vlan of 300 and if we do

Ø  Sw3#show vtp status

                        We have now confirmed that thanks to VTP, we are able to create a Vlan on One Switch and have that Vlan propagated to the other Switches and now let’s think about that Configuration Revision Number, let’s say that i disconnected Switch Sw2 from this topology when Switch Sw2 disconnected, i added some Vlans, deleting some Vlans and i modified some Vlans maybe, each time i made a change the Configuration Revision Number One up it got incremented by One, for every change i made, now what would happened if i reintroduced that Switch into the topology

Ø  it’s has matching Domain Name

Ø  it’s got matching Password which means that it’s higher Configuration Revision number would make its Vlan Database to most believable Vlan database all of the Switches in the topology

                         And in the Vlan Databases are Switches Sw1 and Sw3 they would be wiped out and replaced with this newly learned Vlan information from Switch Sw2, we might do that accidently however, we might have a malicious user who did such a thing intentionally which is the big reason that we want to have a Password assign to our VTP Domain, the moral of the story is we need to be an extremely conscious when we introducing the Switch into our topology, the safest thing to do is to set the Configuration Revision Number for Switch that you about to add to a topology, to set that Configuration Revision number to 0 however, there is a lot of confusion about how that works and i wanted to demonstrated for you.

                 Now what i am going to do is disconnecting the Switch Sw2 from Sw1, i am gonna disconnect that Trunk connection and i am gonna make some changes on it, so did it has the higher Configuration Revision Number, after i disconnected Switch Sw2, i made some changes to its Vlan Database notice that if i do a

Ø  Sw2#show vtp status

It’s Configuration Revision Number of 7 and if i take a look my Vlan database, let’s do that a

Ø  Sw2#show vlan brief

                    You will see that i didn’t have any longer a Vlan 100,200,300 but i have got couple of new Vlans that i just been adding or been playing with this on my desk let say, that’s what the Vlan Database currently looks like for Switch Sw2 but it’s disconnected so we good if i go back to Switch Sw1 and do a

Ø  Sw1#show vtp status

                You can see that it’s Configuration Revision Number is a 2, it’s much lower than the what we have on a Sw2 and if i take a look at the Vlan Database, we do a

Ø  Sw1#show vlan brief

                          We have the Vlans that we had a few movement ago, 100, 200 and 300 and let’s say that after experimenting with Switch Sw2 and trying a few things maybe upgrading the Cisco iOS on a Switch, i am ready to reintroduce this Switch Sw2 into my topology.

                        Let’s say that i know about VTP and i know that with this higher Configuration Revision Number of 7, i would blow away the existing Vlan Databases on my Switches, here is common misconception many people think, to prevent that happening i just set the Switch to Client mode and please make sure that i am saying this is a misconception setting this to Client Mode is not a solution, i am about to prove that to you but some people would mistakenly say

Ø  Sw2(config)#vtp mode client

What harm could client do, well we set the mode to a Client but if we take a look at VTP status using

Ø  Sw2#show vtp status

                       That didn’t reset my configuration Revision Number but i am client what harm could it do, let’s reconnect Switch Sw2 back into the Network. I have now reconnected Switch Sw2 to the network, let’s go over to Switch Sw1 and see if anything changed on Switch Sw1.

Ø  Sw1#show vlan brief

                    Oh no! what happened Switch Sw1 had its Vlan Database blown away, we are now missing Vlans 100 and 200 and 300, now we have these 200 Odd Vlans that we didn’t want, Vlans 400 and 500, what happened was we introduce Switch Sw2 on the network, it had a matching Domain Name, it had a matching Password and it had a higher Configuration Revision number and it wiped out the Vlan Databases on Switch Sw1 and SW3, let’s confirm that on Sw3

Ø  Sw3#show vlan brief

                         Same problem here, we have done some serious damage and i don’t want you to do that accidently in a production network, so the question is how do we fix this?

How do we safely add a Switch into the topology, well we would like to have the Configuration Revision Number set to a 0, i am gonna go over and disconnect Switch Sw2 again, and with it disconnected, i am going to repair our Vlan Databases on Switch Sw1 and Sw3, let’s go to Switch Sw1 and let’s delete Vlan 400 and 500

Ø  Sw1(config)#no vlan 400

Ø  Sw1(config)#no vlan 500

Let’s put back the Vlan we originally had, we had

Ø  Sw1(config)#vlan 100

Ø  Sw1(config-vlan)#name ACCT

Ø  Sw1(config-vlan)#exit

Ø  Sw1(config)#vlan 200

Ø  Sw1(config-vlan)#name SALES

Ø  Sw1(config-vlan)#exit

Ø  Sw1(config)#vlan 300

Ø  Sw1(config-vlan)#name VTP_TEST

Ø  Sw1(config-vlan)#exit

Ø  Sw1#show vlan brief

                    That looks better, has this been advertised over to Switch Sw3, it should have been let just confirm that, let do a

Sw3#show vlan brief 

                      Things are looking happy here as well, now back to Switch Sw2, Switch Sw2 is now been disconnected from Sw1, before introducing the Switch into the network, let’s make Configuration Revision Number 0. How do we do that, setting it to Client mode didn’t help because a Client Mode Switch can originate and did originate VTP advertisements, how do we set this to 0?

We toggle on and off Transparent Mode here what I mean, let’s go to Global Configuration Mode in Sw2

Ø  Sw2#vtp mode transparent

And in transparent mode, let’s take a look at our VTP status.

Ø  Sw2#show vtp status

               Our Configuration Revision Number is 0, interestingly is, and now i can put it back to something else, i could set it back to Server if i wanted to

Ø  Sw2#vtp mode server

                      If we look at our VTP status the Configuration Revision Number remains at 0, until we start making changes to Vlan Database then it would go up by 1 but now a Server Mode Switch and my Configuration Revision Number is 0, it’s now safe to add this back into the network.

                Some people like to take this step beyond this, beyond just setting the Configuration Revision Number to a 0 some people like to also wipe out the Vlan Database on a Switch before they added to the network, if you did wanna do that remember that Vlan Database is stored in a separate file in our Flash it’s called vlan.dat

We can just delete that, if we want, we could say

Ø  Sw2#delete flash:vlan.dat

                   After pressing the Enter it’s gonna be safe to reconnect Switch Sw2 to the network and it’s safe not because we delete the vlan.dat file it’s safe because my Configuration Revision number is 0 let’s reconnect it

                  Switch Sw2 is now been reconnected to the network and we introduced it to safely because we had Configuration Revision number of 0 on that Switch let’s see what its current VTP status is

Ø  Sw2#show vtp status

                 Look at this its Configuration Revision Number went from a 0 up to 12, it now knows about the Vlans in our topology we can prove that with a

Ø  Sw2#show vlan brief

                  It knows about Vlan 100,200 and 300, it no longer knows about 400 and 500 we introduced the Switch with the Configuration Revision Number of 0 and as a result we learned from a Switch Sw1 what the Vlan Database should look like for this topology.

That’s the look at how to Configure VTP and how we could safely add a Switch into an existing Switch topology

Join me on next session about Voice Vlan Theory

  

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