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Trunking Questions

February 6th, 2017 Go to comments

Question 1


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


Question 2


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):



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


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


Question 3


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

Question 4


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


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


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


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


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


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

Question 9


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


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

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


Answer: B

Comments (8) 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