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Trunking Questions

May 22nd, 2017 Go to comments

Question 1

Explanation

Below is an example of the output of this command. Interface Ethernet1/0 is operating in trunking mode.

show_interfaces_switchport.jpg

Question 2

Explanation

The “show interfaces trunk” command and “show interfaces switchport” command can be used to verify the status of an interface (trunking or not). The outputs of these commands are shown below (port Ethernet 1/0 has been configured as trunk):

Switch_show_interfaces_trunk.jpg

show_interfaces_switchport.jpg

The “show ip interface brief” command only gives us information about the IP address, the status (up/down) of an interface:

show_ip_interface_brief.jpg

The “show interfaces vlan” command only gives us information about that VLAN, not about which ports are the trunk links:

show_interfaces_vlan.jpg

Question 3

Explanation

The “Operational Mode” is “static access” so this port is currently in access mode.

Question 4

Explanation

IEEE 802.1Q is the networking standard that supports Virtual LANs (VLANs) on an Ethernet network. It is a protocol that allows VLANs to communicate with one another using a router. 802.1Q trunks support tagged and untagged frames.

If a switch receives untagged frames on a trunk port, it believes that frame is a part of the native VLAN. Also, frames from a native VLAN are not tagged when exiting the switch via a trunk port.

The 802.1q frame format is same as 802.3. The only change is the addition of 4 bytes fields. That additional header includes a field with which to identify the VLAN number. Because inserting this header changes the frame, 802.1Q encapsulation forces a recalculation of the original FCS field in the Ethernet trailer.

Note: Frame Check Sequence (FCS) is a four-octet field used to verify that the frame was received without loss or error. FCS is based on the contents of the entire frame.

Question 5

Explanation

In switches that support both InterSwitch Link (ISL) and 802.1Q trunking encapsulations, we need to specify an trunking protocol so we must use the command “switchport trunk encapsulation dot1q” command first to set the trunk mode to 802.1Q. Then we configure trunking interface with the “switchport mode trunk” command. Then we explicitly allow vlan 20 with the command “switchport trunk allowed vlan 20” command. By default all VLANs are allowed to pass but after entering this command, only VLAN 20 is allowed.

Question 6

Explanation

Maybe this question wanted to ask “if the other end is configured with trunk/access/desirable mode” then which mode is compatible so that the link can work. In that case both “dynamic auto” and “dynamic desirable” mode are correct. The difference between these two modes is “dynamic auto” is passively waiting for the other end to request to form a trunk while “dynamic desirable” will actively attempt to negotiate to convert the link into a trunk.

Question 7

Explanation

The picture below shows the fields in IEEE 802.1Q frame.

802.1q_header.png

The SA field is the source address field. The field should be set to the MAC address of the switch port that transmits the frame. It is a 48-bit value (6 bytes). The receiving device may ignore the SA field of the frame.

Question 8

Explanation

Control traffic like CDP, DTP, PAgP, and VTP uses VLAN 1 to operate, even if you change the native VLAN.

Question 9

Explanation

Although some books and websites said DTP is disabled if the switch port is configured as trunk or access mode (via the command “switchport mode trunk” or “switchport mode access”) but in fact DTP is still running in these modes. Please read at http://packetlife.net/blog/2008/sep/30/disabling-dynamic-trunking-protocol-dtp/. The only way to disable DTP on a switch port is to use the “switchport nonegotiate” command. But notice this command can only be used after configuring that switch port in “trunk” or “access” mode.

Therefore this is a question with no correct answer but if we have to choose an answer, we will choose answer A. At least it is correct in theory.

Question 10

Explanation

The Dynamic Trunking Protocol (DTP) is a proprietary networking protocol developed by Cisco Systems for the purpose of negotiating trunking on a link between two VLAN-aware switches, and for negotiating the type of trunking encapsulation to be used. It is a Layer 2 protocol and it is enabled by default on Cisco switches (so the interfaces of your switches will be in “dynamic auto” or “dynamic desirable” mode). If you want to turn it off, use the “switchport nonegotiate” under interface mode.

Question 11

Question 12

Comments (28) Comments
  1. Sisi
    February 21st, 2017

    @9tut Then what is the answer of Question 6? Both work!

  2. lrando
    February 25th, 2017

    How can you disable DTP on a switch port?
    In CBTNuggets “Switching VLANs: Trunking VLANs to Other Switches” at 10:54 it says this is how you disable DTP:
    switch(config)#Int Fa2/0/1
    switch(config-if)#switchport mode trunk
    switch(config-if)#switchport nonegotiate

  3. punisher
    February 28th, 2017

    Have doubts about Q8. Shouldn’t it use always native VLAN??

  4. Shipan
    April 21st, 2017

    Question not visible. can you put with question and answer?

  5. qasi
    April 24th, 2017

    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

  6. harris
    April 25th, 2017

    wherre is questions? i can just see answer

  7. KACS
    April 27th, 2017

    Which statement about DTP is true?
    Answer: It sends data on VLAN 1

    Cisco’s Dynamic Trunking Protocol can facilitate the automatic creation of trunks between two switches.
    When two connected ports are configured in dynamic mode, and at least one of the ports is configured as
    desirable, the two switches will negotiate the formation of a trunk across the link. DTP isn’t to be confused
    with VLAN Trunking Protocol (VTP), although the VTP domain does come into play.
    DTP on the wire is pretty simple, essentially only advertising the VTP domain, the status of the interface, and
    it’s DTP type. These packets are transmitted in the native (or access) VLAN every 60 seconds both natively and with ISL encapsulation (tagged as VLAN 1) when DTP is enabled.

  8. Adam
    May 3rd, 2017

    watched the complete Cisco CCNA 200-125 Study guide with each and every detail about exam only on: https://www.youtube.com/watch?v=fg2uoWDHFeU

  9. Anonymous
    June 2nd, 2017

    any budy else networking profile interview quetion…?

  10. Mads
    June 4th, 2017

    check following for How can you disable DTP on a switch port?

    Switch(config-if)#switchport mo trunk

    Switch(config-if)#
    %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to down

    %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to up

    do sh interface fa0/1 swi

    Name: Fa0/1
    Switchport: Enabled
    Administrative Mode: trunk
    Operational Mode: trunk
    Administrative Trunking Encapsulation: dot1q
    Operational Trunking Encapsulation: dot1q
    Negotiation of Trunking: On (## DTP On and working)
    Access Mode VLAN: 1 (default)
    Trunking Native Mode VLAN: 1 (default)
    Voice VLAN: none

    Switch(config-if)#switchport mo access

    Switch(config-if)#do sh interface fa0/1 swi

    Name: Fa0/1
    Switchport: Enabled
    Administrative Mode: static access
    Operational Mode: static access
    Administrative Trunking Encapsulation: dot1q
    Operational Trunking Encapsulation: native
    Negotiation of Trunking: Off (## DTP Off and not-working)
    Access Mode VLAN: 1 (default)
    Trunking Native Mode VLAN: 1 (default)
    Voice VLAN: none

  11. Sherry
    June 5th, 2017

    Anyone can reply on this?

    Which mode is compatible with Trunk, Access, and desirable ports?

    A. Trunk Ports

    B. Access Ports

    C. Dynamic Auto

    D. Dynamic Desirable

    Which DTP switchport mode allow the port to create a trunk port if the the port is in trunk, dynamic auto and dynamic desirable mode?

    A. Dynamic Auto

    B. Dynamic Desirable

    C. Access

    D. Trunk

    How can you disable DTP on a switch port?

    A. Configure the switch port as a trunk.
    B. Add an interface on the switch to a channel group.
    C. Change the operational mode to static access
    D. Change the administrative mode to access

  12. froggen
    June 7th, 2017

    Q6. correct answer is D. Dynamic Desirable. Dynamic Auto is not active in negotiation. so when paired with and trunk and access port on the other side (both of which don’t negotiate), a trunk link will never come up.

  13. Weldon
    June 13th, 2017

    Passed with the 747q dumps from itlibraries.

  14. Mervin
    June 19th, 2017

    Confirming, the 747q dumps are 100% valid. Passed today and all question/sims were covered in the dumps.

  15. Ciic
    June 19th, 2017

    Hi,

    I wonder about the next question.
    Which three commands should you make a VLAN 20? (Choose three)
    A. Switch (config-if) #switchport auto dynamic mode
    B. Switch (config-if) #switchport trunk mode
    C. Switch (config-if) #switchport trunk allowed vlan 20
    D. Switch (config-if) #switchport mode dynamic desirable
    E. Switch (config-if) #switchport trunk encapsulation dotlq
    F. Switch (config-if) #switchport trunk native vlan 20
    Answer: B, E, F

    Can you tell me why we should choose answer F rather than C?
    I am ok for answer B and E but I do not understand why a lot of site informs the answer F.
    Can you give me an explanation?
    Thank you

  16. JD
    June 21st, 2017

    @froggen: Not true. Dynamic auto will result in trunk when paired with static trunk and in access when paired with static access.

  17. JD
    June 21st, 2017

    more @froggen: to be precise – You are right only when “switchport mode access” and “switchport mode trunk” are user along with “switchport nonegotiate” on the interfaces paired with “dynamic auto” – but in that case even “dynamic desirable” will end up in access mode. Sorry bro.

  18. Ciic
    June 23rd, 2017

    @ JD and Froggen
    for dtp disable

    If you do an int … swichport
    You will see that it is indicated administrative mode: STATIC access
    If you do before a swichport mode access on the interface.

    I want to say that the answer D is good but Cisco plays on the words, it only indicates Access in answer D and not “Static Access”.
    I end up saying that none of the answers is good except the A if we put a nonegociate after the swichport mode trunk (implicit)

  19. JD
    June 24th, 2017

    @Ciic: I’m afraid that “STATIC access” in the output of “sh int … sw” is just a word. It is always there if the port is in access mode – even if the port is dynamic and negotiated as access. Checked on two 2950 switches (S21 has just “sw mo ac” on interface and S22 has default “dynamic desirable” on interface), look what came up:

    S22#sh int f0/2 sw
    Name: Fa0/2
    Switchport: Enabled
    Administrative Mode: dynamic desirable
    Operational Mode: static access

    So the “static” is always there.

    The key is “switchport nonegotiate” command. If it is present on the interface then DTP is disabled and any device connected to this interface won’t negotiate anything. Now I configured “switchport mode trunk” and “switchport nonegotiate” on S21 and left S22 with default “dynamic desirable”. The configuration is exactly this:

    S21#sh run int f0/2

    interface FastEthernet0/2
    switchport mode trunk
    switchport nonegotiate
    end

    S22#sh run int f0/2

    interface FastEthernet0/2
    end

    And look what is negotiated:

    S21#sh int f0/2 sw
    Name: Fa0/2
    Switchport: Enabled
    Administrative Mode: trunk
    Operational Mode: trunk

    S22#sh int f0/2 sw
    Name: Fa0/2
    Switchport: Enabled
    Administrative Mode: dynamic desirable
    Operational Mode: static access

    cool huh?

  20. Ciic
    June 24th, 2017

    @JD

    Hi JD,

    I realize that this is just a word :-) But Cisco likes to play on the word which brings doubt to everyone.

    Otherwise I agree with you that the only command to totally disable dtp is the nonegociate.

    We’re going to go mad through revision.

    Thank you for your explanations.

  21. Ciic
    June 24th, 2017

    @JD

    But….
    You describe tests via the operational mode but I am talking to you about the administrative mode.

    If you do a sw mo access, the administrative mode becomes STATIC access. And the DTP does not work because an access port (administrative, sw mo access) never rises in trunk.

    What intrigues me is word play, why in question C Cisco indicates Static Access and not for D. While the D is correct because if we go in sw mode access, no trunk can Therefore, no dtp.

  22. hammy
    July 4th, 2017

    Anyone can reply on this?
    Which mode is compatible with Trunk, Access, and desirable ports?
    A. Trunk Ports
    B. Access Ports
    C. Dynamic Auto
    D. Dynamic Desirable
    Which DTP switchport mode allow the port to create a trunk port if the the port is in trunk, dynamic auto and dynamic desirable mode?
    A. Dynamic Auto
    B. Dynamic Desirable
    C. Access
    D. Trunk
    How can you disable DTP on a switch port?
    A. Configure the switch port as a trunk.
    B. Add an interface on the switch to a channel group.
    C. Change the operational mode to static access
    D. Change the administrative mode to access

    would anyone reply to these questions?

  23. @Q6
    July 8th, 2017

    […]
    switchport mode dynamic auto says: “If the other end asks me to be a trunk with DTP, then become a trunk, but I wont initiate any negotitation from this end. If no one asks me to become a trunk then I will become an access port.” […]

    https://supportforums.cisco.com/discussion/12476171/switch-port-modes

  24. @Q6
    July 8th, 2017

    switchport mode dynamic desirable says: “Ask the other end to trunk using DTP and trunk if the negotiation succeeds. If DTP negotiation fails then become an access port.”

  25. Anonymous
    July 14th, 2017

    QUESTION 9

    CORRECT ANSWERD CAN BE C OR D

    BUT WEN YOU ISSUE THE COMMAND SHOW INTERFACE FASTX/X SWITCHPORT

    IT SAY ADMINISTRATIVE MODE: STATIC ACCESS

    NEGOTIATION OF TRUNKING: OF

    SO THE WORD KEY IS ADMINISTRATIVE

    ANSWERD IS D

  26. Anonymous
    July 14th, 2017

    in the picture of the question 3 is the answerd of 9

  27. rockstepp
    July 27th, 2017

    DTP USES NATIVE VLAN IN 802.1Q TRUNKS.
    https://learningnetwork.cisco.com/thread/85047

    Place two 2960 in PT and connect them with one port in dyn auto, the other one in desirable.
    Use another vlan, different from the default 1 as the native in that trunk and start capture.
    DTP frames will be sent with 802.3 encapsulation (no VLAN ID – 802.1q).

    Configure also VTP and maybe PAgP and use CDP, u will see that they use VLAN1 (802.1q with tag 1).

    SO VTP, PAgP and CDP use VLAN1
    STP and RSTP use native VLAN – untagged
    DTP USE NATIVE VLAN in 802.1Q trunks – they use VLAN1 in ISL Trunks.

    So, like many questions, this is not a clear question…

  28. rockstepp
    July 27th, 2017

    Q9 – THE ANSWER IS D.

    IN PT SET A PORT IN TRUNK MODE WITH SWITCHPORT MODE TRUNK.
    U WILL SEE WITH SHOW INT FAX/X SWITCHPORT THAT THE NEGOTIATION OF A TRUNK REMAINS UP.
    U WILL NEED ALSO THE SWITCHPORT NONEGOTIATE COMMAND TO GET IT DOWN.

    USING SWITCHPORT MODE ACCESS THE NEGOTIATION GOES IMMEDIATELY DOWN.
    I HAVE THE DAMN EXAM TOMORROW AND STILL SO MANY CONFUSION, PLEASE DO SOME TEST WHILE CHOOSING ANSWER OR GIMME THE 10$ SUBSCRIPTION.