Introduction to VTP (VLAN Trunking Protocol)

Lessons Discussion
21

Hi Maruti,

You can reset the revision number with these methods:

- Change the VTP domain name.
- Change the VTP mode to VTP transparent and then back to VTP server.
- Delete the VLAN database.

The VLAN database is stored as “vlan.dat” on the flash memory. Each switch will have it.

Rene

1 Like
22

Thanks Rene!

23

Hi Rene,
Thank you for your clear explanation. Also can you please create some tutorial about VTP pruning

BR
Taslim

24

Hi Taslim,

You are welcome. I’ll see if VTP pruning can be debugged, otherwise there’s not that much to tell about it :slight_smile:

Rene

25

Hi Rene,

Thank you.

May be you can explain how to use the following command

switchport trunk pruning vlan {add vlan-list | all | except vlan-list | remove vlan-list}

no switchport trunk pruning

Thank you

Taslim

26

Hi,
One question.when I delete vlan.dat every thing about vlans will delete and revision number become zero.right?
but when I delete only vlans ,revision number increase.In your example if I make vlans and delete them in lab and then bring it back to the network with higher revision number , others sync themselves with this.but there is no vlan any more.How they sync themselves and make the revision number increase?They delete all their vlans and make revision higher?
Thanks

27

And a switch with different or none domain name can make problem in network? I think no , and if we set password there is no problem any more.

28

Hi Sepideh,

Deleting the VLAN database (vlan.dat) will reset the revision number yes. Adding, changing or deleting VLANs will increase the revision number.

VLAN 1 can’t be deleted which VTP uses by default. When you introduce a switch in your network with no VLANs and a higher revision number then you’ll have a problem…the domain name has to be the same though (and a password if you use one).

If your switch without a domain name sees a VTP advertisement then it will automatically change its domain name to the one in the advertisement.

Rene

29

thanks so much

30

Hi Rene,

 

Can you not change the revision of a switch to say 0 or something that is less than what the server is running, when placing it back into the domain as a client? this would allow you to put it back without erasing the vlan info on all other switches.

 

 

31

Hi Deep,

You can’t change the revision number directly but changing the domain name will do the job.

Change the domain name of the switch to something and then change it back to the correct domain name of your production network. The revision number will be reset to 0 and it’s safe to connect the switch again.

Rene

32

Questions:

  1. You said – “Broadcast frames have to be flooded by our switches and since our trunks are carrying all VLANs, this broadcast will go everywhere. However if you look at the switch in the middle do you see any computer in VLAN 10? Nope there’s only VLAN 20 there which means this broadcast is wasted bandwidth”……… So leaving VTP out of this discussion – Broadcast traffic for let say - VLAN 222 travels trunk lines even though there’s no VLAN 222 on the other end? I thought that was the whole point of VLANs – to separate broadcast traffic. I understand broadcast traffic from VLANs doesn’t travel access lines…. But I didn’t know it travels trunk lines.
  2. Can you have more than one VTP server in a single domain?
  3. Is it safe to say… changes by one VTP server in one domain doesn’t sync with another VTP server in a separate domain?
  4. This is the first I am learning about VTP…. So a switch (Cisco) out of the box… does VTP have to be initially configured or is it in VTP server mode by default right out of the box?
33

Hi Jason,

One of the reasons to use VLANs is to decrease the size of our broadcast domains yes. Another reason is that we use them to separate L2 traffic, this is useful for security reasons.

By default a trunk will carry all VLANs (1-4094). If you want you can remove VLANs that you don’t want to see on the trunk.

Having more than one VTP server in the domain is no problem. A VTP server is also a VTP client.

When the VTP domain is different then the switch will not sync itself. The only exception to this rule is when you don’t have a domainname…in that case your switch will automatically configure itself with the domainname it sees in the VTP packets.

By default, your switch will be in VTP server mode.

Hope this helps.

Rene

1 Like
34

I understand VTP syncs VLAN information from one server-switch to all client-switches…. Bust just to confirm… trunking configuration still has to be configured between each switch….right?

35

Correct, VTP does not configure trunk links. You must manually configure the trunk protocol (802.1Q or ISL–if your switch is old enough), what VLANs are or are not allowed via the trunk, and what state of DTP (dynamic trunking protocol) you want the link to have.

36

Simply put you’re the best…so easy to comprehend. Thank you very much

37

Hi Rene, Nice Article… What is difference between VTP1 and VTP2.

38

Hi Pavithra,

Here’s a short overview, found on the Cisco support section:

VTP version 2 supports these features that are not supported in version 1:

  • Token Ring support—VTP version 2 supports Token Ring Bridge Relay Function (TrBRF) and Token Ring Concentrator Relay Function (TrCRF) VLANs. For more information about Token Ring VLANs, see the "Configuring Normal-Range VLANs" section on page 13-4.
  • Unrecognized Type-Length-Value (TLV) support—A VTP server or client propagates configuration changes to its other trunks, even for TLVs it is not able to parse. The unrecognized TLV is saved in NVRAM when the switch is operating in VTP server mode.
  • Version-Dependent Transparent Mode—In VTP version 1, a VTP transparent switch inspects VTP messages for the domain name and version and forwards a message only if the version and domain name match. Because VTP version 2 supports only one domain, it forwards VTP messages in transparent mode without inspecting the version and domain name.
  • Consistency Checks—In VTP version 2, VLAN consistency checks (such as VLAN names and values) are performed only when you enter new information through the CLI or SNMP. Consistency checks are not performed when new information is obtained from a VTP message or when information is read from NVRAM. If the MD5 digest on a received VTP message is correct, its information is accepted.

Rene

1 Like
39

19 posts were merged into an existing topic: Introduction to VTP (VLAN Trunking Protocol)

40

Rene,

Nice article. It would be nice to have some elaboration on pruning.

How does pruning work?
How is pruning controlled?
What is the default pruning behavior?
When the last switchport in a vlan is removed from said vlan, how and when is VTP triggered to prune said vlan?

I am very thankful for your site, it is very helpful in my CCNP studies.
Thank you!