VTP issue (strange)

Answered Question
Feb 6th, 2009

Have a strange situation I never came across. Customer has a bunch of switches and is not paying attention when he sets them up so they are all in VTP server mode (default) with no domain specified.

He plugs in another switch with a VTP domain specified. He then reports that all the switches are now in this VTP domain. I would have thought that with no domain specified they would have to specifically be told that they are servers in a VTP domain.

Any thoughts?

I have this problem too.
0 votes
Correct Answer by Edison Ortiz about 7 years 10 months ago

Working as expected:

http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/12.2_46_se/configuration/guide/swvtp.html

By default, the switch is in the VTP no-management-domain state until it receives an advertisement for a domain over a trunk link (a link that carries the traffic of multiple VLANs) or until you configure a domain name. Until the management domain name is specified or learned, you cannot create or modify VLANs on a VTP server, and VLAN information is not propagated over the network.

If the switch receives a VTP advertisement over a trunk link, it inherits the management domain name and the VTP configuration revision number. The switch then ignores advertisements with a different domain name or an earlier configuration revision number.

HTH,

__

Edison.

Correct Answer by glen.grant about 7 years 10 months ago

Nope that is normal if the vtp domain name is at the factory null domain. The first vtp advertisement it recieves it will take that vtp name and input it into the vlan.dat file. Did it assume all the the vlans too ?

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Correct Answer
glen.grant Fri, 02/06/2009 - 16:36

Nope that is normal if the vtp domain name is at the factory null domain. The first vtp advertisement it recieves it will take that vtp name and input it into the vlan.dat file. Did it assume all the the vlans too ?

Correct Answer
Edison Ortiz Fri, 02/06/2009 - 16:37

Working as expected:

http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/12.2_46_se/configuration/guide/swvtp.html

By default, the switch is in the VTP no-management-domain state until it receives an advertisement for a domain over a trunk link (a link that carries the traffic of multiple VLANs) or until you configure a domain name. Until the management domain name is specified or learned, you cannot create or modify VLANs on a VTP server, and VLAN information is not propagated over the network.

If the switch receives a VTP advertisement over a trunk link, it inherits the management domain name and the VTP configuration revision number. The switch then ignores advertisements with a different domain name or an earlier configuration revision number.

HTH,

__

Edison.

badalam_nt Tue, 02/10/2009 - 04:01

Edison, as shown by Lejoe it is possible to create/delete VLANs on a VTP server without any VTP domain name set.

Have I misunderstood your statement:

"Until the management domain name is specified or learned, you cannot create or modify VLANs on a VTP server" ?

lejoe.thomas Fri, 02/06/2009 - 16:40

Hi Chris,

A switch with a NULL VTP domain, regardless of it being server or client obtains a domain name from a neighboring switch (if there is trunk port).

HTH

Lejoe

lejoe.thomas Mon, 02/09/2009 - 15:35

Hi John,

Switches within a VTP domain will share VLAN information. The value or domain name has to be consistent on all switches that belong to the same VTP domain.

Eg: all switches in VTP domain cisco, will be updated on VLANs created or deleted on any switches (VTP server), within that VTP domain.

Some of our sites have VTP Domain names and some don't.

All switches in the same VTP domain name will share VLAN information. Other sites may not want to use VTP, really depends on your objectives.

HTH

Lejoe

badalam_nt Tue, 02/10/2009 - 01:26

Lejoe, just one question:

- if the new switch (either set to VTP Client or VTP Server) has no VTP domain set, but has a higher VTP configuration revision number (by the way, could this situation happen, i.e could revision number be different from 0 if no VTP domain is set?), after connecting that switch into the network will that switch first update its VTP domain with the info received from the original VTP Server AND afterwards, once being in the same VTP domain with the other switches and having highest revision number, it will overwrite VLAN database of all the switches ?

lejoe.thomas Tue, 02/10/2009 - 02:22

Hi Petru

By default a new switch has no VTP domain and operates in VTP mode server, with configuration revision 0. However if on the new switch you keep adding or deleting VLANs, the configuration revision goes up.

I hope the test below will demonstrate what happens when this is connected to another switch with a VTP domain name and lower configuration revision

New switch, no vtp domain name, Server mode, added vlans to increase configuration revision

2950sw1#sh vtp status

VTP Version : 2

Configuration Revision : 11

Maximum VLANs supported locally : 64

Number of existing VLANs : 8

VTP Operating Mode : Server

VTP Domain Name :

VTP Pruning Mode : Disabled

VTP V2 Mode : Disabled

VTP Traps Generation : Disabled

MD5 digest : 0x20 0xA8 0xE3 0xA8 0xB3 0x38 0xF6 0xE6

Configuration last modified by 0.0.0.0 at 3-1-93 00:06:07

Local updater ID is 0.0.0.0 (no valid interface found)

A switch with VTP domain name Cisco and lower configuration revision number

3550SW1#sh vtp status

VTP Version : running VTP1 (VTP2 capable)

Configuration Revision : 7

Maximum VLANs supported locally : 1005

Number of existing VLANs : 10

VTP Operating Mode : Server

VTP Domain Name : cisco

VTP Pruning Mode : Disabled

VTP V2 Mode : Disabled

VTP Traps Generation : Disabled

MD5 digest : 0x70 0x90 0xB8 0xA6 0xC0 0x9B 0xA1 0x9E

Configuration last modified by 0.0.0.0 at 3-1-93 03:05:40

Local updater ID is 0.0.0.0 (no valid interface found)

Now lets bring up the trunk beween 2950 and 3550.

2950sw1#sh vtp status

VTP Version : 2

Configuration Revision : 7

Maximum VLANs supported locally : 64

Number of existing VLANs : 10

VTP Operating Mode : Server

VTP Domain Name : cisco

VTP Pruning Mode : Disabled

VTP V2 Mode : Disabled

VTP Traps Generation : Disabled

MD5 digest : 0x70 0x90 0xB8 0xA6 0xC0 0x9B 0xA1 0x9E

As you can see the new switch (2950), got its domain name, as well updated its configuration revision number, number of VLANS to match the switch (3550) from which it received its domain name.

HTH

Lejoe

badalam_nt Tue, 02/10/2009 - 03:41

Thanks Lejoe.

It means that what is written in many Cisco docs is not actually fully correct, i.e that if revision number of the newly added switch is higher than the revision number of the existing switches, it will overwrite the VLAN database of the other switches.

Actually to happen this (i.e the new switch to overwrite the VLAN database of the other switches) the following conditions must all be fulfilled before bringing up its trunk interface(s) with other switch(es):

- the VTP Operating Mode of the new switch to be either Server or Client

- VTP Configuration Revision of the new switch to be higher than the revision number on existing switches.

- VTP Domain to be the same as in the existing switches

(this last condition was not explicitely mentioned that it is required as well)

PS: Edison mentioned previously that:

"Until the management domain name is specified or learned, you cannot create or modify VLANs on a VTP server.."

Is it wrong that statement, as in your example you were able to add, delete VLANs when the switch was in Server mode and without any domain name ?

lejoe.thomas Tue, 02/10/2009 - 03:53

Hi Petru,

Yes I could create VLANS on the Switch, which was on VTP server mode with no VTP domain.

Lejoe

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