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Enthusiast

DTP

Which statement about DTP is true?
A. It uses the native VLAN.
B. It negotiates a trunk link after VTP has been configured.
C. It uses desirable mode by default.
D. It sends data on VLAN 1.

 

May i know the answer for this question ? 

8 REPLIES 8
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Beginner

Re: DTP

Seeing as DTP is a Cisco proprietary protocol I would say it runs on the native vlan and seeing as how the default native vlan is vlan1 I would say the answer is D

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VIP Advocate

Re: DTP

Answer D:

 

 

Regards,

Deepak Kumar

Regards,
Deepak Kumar,
Don't forget to vote and accept the solution If this comment will make help you!
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Enthusiast

Re: DTP

What about "A"

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Hall of Fame Guru

Re: DTP

You are correct the answer is DTP sends traffic on the native vlan. 

 

By default vlan 1 is the native vlan but DTP moves with the native vlan so if you change the native vlan to be another vlan DTP will use that instead.

 

Jon

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Enthusiast

Re: DTP

Does DTP uses native vlan or vlan-1

Beginner

Re: DTP

Let's walk through the logic. Usually, DTP needs to communicate with other switches to negotiate trunk and on a new switch with no other configuration, this communication will be done via available vlan which is usually VLAN-1 (The only usable vlan that came preconfigured in all CISCO switches). Hence, DTP will use VLAN-1.

 

Now, let us take it further to add some more configurations to the switch. for example, best practices will disable VLAN-1 and change the native VLAN to something else. In that case, how will DTP negotiate trunk? Does DTP stop operating as soon as VLAN is unavailable? The answer is No. DTP simply uses the newly configured native VLAN.

 

Therefore, I will go with A (DTP uses native VLAN) instead of D (DTP uses VLAN-1).

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Enthusiast

Re: DTP

option b also could be true' depending on wording. trunk uses vtp domain name to negotiate a trunk link. vtp is on by default with Null as a its name.
1. default settings: if u have Null as a name on 1 or both switches, trunk forms ok.
2. you manually set some of vtp parameters : if u have vtp name cisco and other sw has name as Cisco, trunk will not form due to vtp name mismatch. (case-sensitive)

3. Trunks forms if u have configured same vtp domain.

so, do we consider this "after vtp has been configured" true or not? if it is set up by default, is it already configured? or not?

 

Here is what I found in Ciscopress books"
My old CCNP Switch Foundation book says "you can remove vlan 1 from trunk port.... however, management traffic such as CDP, VTP, DTP will use vlan 1 regardless of existence of vlan 1 on port"

CCNP Cert guide says "vlan 1 is sometimes used for control traffic" (aka management)

Cisco LAN switching Fundamentals book (D. Barnes) mentions "control plane traffic such as vtp, dtp, cdp are tagged with vlan 1 information and forwarded on a trunk regardless if the trunk has pruned vlan 1"

 

Cisco Doc pages (documentation u should read for CCIE exam) says "

 

  • CDP, VTP, and PAgP updates are always forwarded on trunks with a VLAN 1 tag. This is the case even if VLAN 1 has been cleared from the trunks and is not the native VLAN. If you clear VLAN 1 for user data, the action has no impact on control plane traffic that is still sent with the use of VLAN 1.

  • On an ISL trunk, DTP packets are sent on VLAN1. This is the case even if VLAN 1 has been cleared from the trunk and is no longer the native VLAN. On an 802.1Q trunk, DTP packets are sent on the native VLAN. This is the case even if the native VLAN has been cleared from the trunk."

source: https://www.cisco.com/c/en/us/support/docs/switches/catalyst-6500-series-switches/24330-185.html

 

Few sources did mention the desirable mode is the default mode. this could be true if u not referring to a particular model of a switch.  Unfortunately different switches have different setting: dynamic desirable and dynamic auto are common modes. 

 

I guess the best way is to lab it and capture traffic.

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Re: DTP

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