which statement about DTP is true?

a. It uses the native vlan
b. It negotiates a trunk link after vtp ha been configured
c. It uses desirable mode by default
d. It sends data on vlan 1

From cisco docs, I found this
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 trun
reference link: https://www.cisco.com/c/en/us/support/docs/switches/catalyst-6500-series-switches/24330-185.html

When discussing about the dtp default mode, dtp default mode depends on the switch series. Cisco doc says " The default DTP configuration for Cisco Catalyst 2960 and 3560 switches is dynamic auto"
reference: http://www.ciscopress.com/articles/article.asp?p=2181837&seqNum=8

Another cisco doc says about catalyst 6500 series "Cisco recommends an explicit trunk mode configuration of dynamic desirable at both ends. This mode is the default mode"
reference: https://www.cisco.com/c/en/us/support/docs/switches/catalyst-6500-series-switches/24330-185.html

So which one among the options could we select as the most appropriate answer?

Best Answers

  • MartinLosikMartinLosik ✭✭✭
    Accepted Answer

    I think we had post about that. look for "switch management traffic" in topics.
    I have to check my notes from labing this,

    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. manually set some of vtp parameters : if u have vtp name cisco and other sw has name as Cisco, trunk will not form.

    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?

    do some labing

  • MartinLosikMartinLosik ✭✭✭
    Accepted Answer

    to clarify #2 trunk does not form 'cause of vtp name mismatch.
    to form trunk, use same name like cisco and cisco.

    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 prunded vlan 1"

    The best way is to lab it and capture traffic.

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