VTP CLIENT ERASE VLAN INFO

Answered Question
Mar 3rd, 2007

Could you pls explain how can a new switch added as vtp client (with higher rev no.)erase vlan info.In some materials it says it can erase it,but some says it cant.Pls clarify

I have this problem too.
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Correct Answer by CSCO10892433 about 9 years 6 months ago

Incorrect.

SW-11 will ignore the VTP updates from other switches because of their lower configuration revision number. At the same time, other switches will receive VTP updates from SW-11 ,finding that the received updates has a higher revison number and thus replace their own vlan info with the received updates. At that time, all switches will have identical vlan information as SW-11.

A typical wrong concept is that switches in VTP client mode cannot overcome switch in server mode. There is no such a definition about VTP client mode. The true definition of VTP client mode , according to "Cisco Lan Switch" by Kenndy Clark, page 545, is that switch in client mode CAN source and listen to VTP messages, but CANNOT create and remember VLAN. ("source" means send out VTP message, "remember" means store VLAN info on the NVRAM)

According to this definition, switches in server mode and client mode will both send out VTP messages, and there is no rule that messages from server mode should beat the one from client mode. The rule is the revision number determines who will win. However, it is rarely possible to make switch in client mode to have an higher revision number than one in server mode. I have designed a lab to make this happen, described as follows.

I have 3 swithces and have them connect as below:

SW1---SW3---SW2

SW1 and SW2 are VTP servers, while SW3 is a VTP client. The connections among them are trunks.

First I create 10 VLANs on SW1. All switches have 10 VLANs and VTP revision number 10.

Then I shutdown the connection between SW3 and SW2. SW2 is isolated from others, but VLAN info among them is still the same

Then I delete 5 VLAN on SW1. Now SW1 and SW3 have only 5 VLANs and VTP revision number 15. SW2 has 10 VLANs and VTP revision number 10.

Then I shutdown the connection between SW1 and SW3. Now we have 3 isolated switches. SW3 have 5 VLANs and VTP revision number 15. SW2 has 10 VLANs and VTP revision number 10.

Notice that SW3 is in VTP client mode and have an higher revision number than SW2 which is a VTP server. Now I restore the connection between SW3 and SW2 so they can start to exchange VTP message, and the result is -- Client prevails Server. HOORAY!

Option A commit a wrong concept and hope someone fall into the trap. Unfortunately , this happens all the time.

HTH

SSLIN

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CSCO10892433 Sat, 03/03/2007 - 16:07

Hi, gopalanrjkumar

A swith who is a VTP clent might erase the other swithes' vlan infomation if it meets all the following situations.

1. This switch was removed form other places and now is going to be installed to your netwok.

2. It has different vlan information from the other switches in your netwok.

3. It has a higher revision number than those of the other switches in your netwok.

3. It happens to be in the same VTP domain as other switches in your network.

4. It happens to match the VTP password ,if one is set, to the other switches in your netwok.

At the time this switch is installed in your network, it start sending VTP message with its own vlan information to other switches, just like other swithes are sending VTP message to it. Every swithes in your network will receive thhis message and check the VTP domain name and password. If those are match, they will then check the revision number. Since the revision number in received VTP message is higher than their own, they will replace their own vlan info with the recevied one. On the other hand, the newly installed swith will receive other switch's VTP message too. It simply ignore it because of the lower revision number.

A swith who is a VTP clent can't erase the other swithes' vlan infomation if it doesn't meet all the above situations.

HTH

SSLIN

gopalanrajkumar Sat, 03/03/2007 - 21:54

Thx for ur message.Could u pls answer for this Question.Answer given in the book is option A.

Is it correct?

An existing switched network has ten switches, with Switch1 and Switch2 being the only

VTP servers in the network. The other switches are all VTP clients and have successfully

learned about the VLANs from the VTP servers. The only configured VTP parameter on all

switches is the VTP domain name (Larry). The VTP revision number is 201. What happens

when a new, already-running VTP client switch, named Switch11, with domain name Larry

and revision number 301, connects via a trunk to any of the other ten switches?

a. No VLAN information changes; Switch11 ignores the VTP updates sent from the two

existing VTP servers until the revision number reaches 302.

b. The original ten switches replace their old VLAN configuration with the configuration

in Switch11.

c. Switch11 replaces its own VLAN configuration with the configuration sent to it by one

of the original VTP servers.

d. Switch11 merges its existing VLAN database with the database learned from the VTP

servers, because Switch11 had a higher revision number.

Correct Answer
CSCO10892433 Sun, 03/04/2007 - 05:59

Incorrect.

SW-11 will ignore the VTP updates from other switches because of their lower configuration revision number. At the same time, other switches will receive VTP updates from SW-11 ,finding that the received updates has a higher revison number and thus replace their own vlan info with the received updates. At that time, all switches will have identical vlan information as SW-11.

A typical wrong concept is that switches in VTP client mode cannot overcome switch in server mode. There is no such a definition about VTP client mode. The true definition of VTP client mode , according to "Cisco Lan Switch" by Kenndy Clark, page 545, is that switch in client mode CAN source and listen to VTP messages, but CANNOT create and remember VLAN. ("source" means send out VTP message, "remember" means store VLAN info on the NVRAM)

According to this definition, switches in server mode and client mode will both send out VTP messages, and there is no rule that messages from server mode should beat the one from client mode. The rule is the revision number determines who will win. However, it is rarely possible to make switch in client mode to have an higher revision number than one in server mode. I have designed a lab to make this happen, described as follows.

I have 3 swithces and have them connect as below:

SW1---SW3---SW2

SW1 and SW2 are VTP servers, while SW3 is a VTP client. The connections among them are trunks.

First I create 10 VLANs on SW1. All switches have 10 VLANs and VTP revision number 10.

Then I shutdown the connection between SW3 and SW2. SW2 is isolated from others, but VLAN info among them is still the same

Then I delete 5 VLAN on SW1. Now SW1 and SW3 have only 5 VLANs and VTP revision number 15. SW2 has 10 VLANs and VTP revision number 10.

Then I shutdown the connection between SW1 and SW3. Now we have 3 isolated switches. SW3 have 5 VLANs and VTP revision number 15. SW2 has 10 VLANs and VTP revision number 10.

Notice that SW3 is in VTP client mode and have an higher revision number than SW2 which is a VTP server. Now I restore the connection between SW3 and SW2 so they can start to exchange VTP message, and the result is -- Client prevails Server. HOORAY!

Option A commit a wrong concept and hope someone fall into the trap. Unfortunately , this happens all the time.

HTH

SSLIN

gopalanrajkumar Sun, 03/04/2007 - 11:02

Thanx a lot my friend.Now my concept is clear..This Q was in CCIE R&S official certification guide 2nd edition,thats why i got confused...

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