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VLAN Trunking

September 16th, 2017 in SWITCH 300-115 Go to comments

Question 1

Explanation

These errors are generated because the native VLAN is not matched on the two switches (the native VLAN on SW-1 is not the default native VLAN 1 while the native VLAN on the other side is VLAN 1). The errors indicate that spanning tree has detected mismatched native VLANs and has shut down VLAN 1 on the trunk.

We should verify that the configurations of the native VLAN ID is consistent on the interfaces on each end of the IEEE 802.1Q trunk connection. When the configurations are consistent, spanning tree automatically unblocks the interfaces.

Question 2

Explanation

In 802.1Q, the trunking device inserts a 4-byte tag into the original frame and recomputes the frame check sequence (FCS) before the device sends the frame over the trunk link. At the receiving end, the tag is removed and the frame is forwarded to the assigned VLAN. 802.1Q does not tag frames on the native VLAN. It tags all other frames that are transmitted and received on the trunk.

Reference: http://www.cisco.com/c/en/us/support/docs/lan-switching/8021q/17056-741-4.html

Question 3

Explanation

802.1Q is a industry standards based implementation of carrying traffic for multiple VLANs on a single trunking interface between two Ethernet switches. 802.1Q is for Ethernet networks only.

Question 4

Explanation

We can use the “switchport trunk allowed vlan ” to specify which VLANs are allowed to go through. Other VLANs will be dropped.

Question 5

Explanation

Manually configure trunking with the “switchport mode trunk” command and manually configure access interfaces with the “switchport mode access” prevent auto trunking on that interface.

Disable DTP with the “switchport nonegotiate” so that DTP messages are not advertised out of the interface is also a good way to prevent auto trunking.

Question 6

Explanation

There are two protocols that can be used for trunking: Inter-Switch Link (ISL) and 802.1Q. We can choose which protocol to run by the “switchport trunk encapsulation “. After that we can configure trunking mode with the “switchport mode trunk” command.

In fact this question is not clear and may cause confusion because Dynamic Trunking Protocol (DTP) is the protocol that can automatically negotiate for trunking.

Note: The DTP options can be dynamic auto, dynamic desirable, and trunk.

Question 7

Explanation

By default all VLANs are allowed to go through a trunk but if we apply the “switchport trunk allowed vlan ” then only these VLANs are allowed to go through, other VLANs are dropped so be careful when limiting VLANs on the trunks with this command.

Question 8

Explanation

We can use the “switchport trunk allowed vlan ” to specify which VLANs are allowed to go through. Other VLANs will be dropped.

Question 9

Explanation

First we will explain these two commands:

switchport access vlan 10
switchport mode trunk

The first command is used for an access port whist the second is used for a trunk so why are they here at the same time? In fact this interface was set as a trunk. The “switchport access vlan 10” is still there but it does not affect the operational mode of the port -> Gi1/0/1 is a trunk port so it will not appear in the “show vlan” command.

The “switchport voice vlan 11” command here only tries to confuse you. But it does have an effect on the port: Cisco uses CDP to specify a Cisco IP Phone and will automatically place that traffic into the voice VLAN. For example if we configure like this:

interface fa0/0
switchport trunk encapsulation dot1q
switchport mode trunk
switchport voice vlan 11

Then the voice traffic from a Cisco IP Phone will be placed into VLAN 11.

Cisco_IP_Phone_data_voice_VLANs.jpg

Note: In the above configuration, the data and voice use the same interface fa0/0 so it should be configured as a trunk link.

(Reference: http://www.cisco.com/en/US/products/hw/switches/ps4324/products_configuration_example09186a0080722cdb.shtml)

Question 10

Explanation

VLAN 1 is always used for CDP, VTP, PAgP traffic (except DTP uses native VLAN) even if VLAN 1 is not the native VLAN. If VLAN 1 is not the native VLAN then CDP, VTP, PAgP traffic will be tagged on the trunk.

In this question, after changing the default native VLAN to 999 while keeping the standard configuration on the other end, we cause a “native VLAN mismatched” error. Besides, CDP, VTP traffic is tagged on the local switch (as VLAN 1 is no longer the native VLAN) so the other end cannot understand them -> CDP, VTP traffic is dropped.

Question 11

Comments
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  1. elymad
    May 19th, 2017

    Need YOUR HELP
    can someone send me the dumps of CCNP switch and other material

    my email address is {email not allowed}

  2. JAckie Chan
    May 22nd, 2017

    They give the test questions, answers and simulations with additional resources if you pay the mone y to be a member. I have not found the pdf so im studying by notes and the simulations on site. took me 2 years before i paid these dudes but i do now feel its worth the money. give it a try

  3. Panther
    June 8th, 2017

    I am really confused about question 9 I manually entered the commands in packet tracer as shown in the exhibit and the outpout shows that the interface shows up in both Vlan 10 and Vlan 11:

    show vlan

    VLAN Name Status Ports
    —- ——————————– ——— ——————————-
    1 default active Fa0/2, Fa0/3, Fa0/4, Fa0/5
    Fa0/6, Fa0/7, Fa0/8, Fa0/9
    Fa0/10, Fa0/11, Fa0/12, Fa0/13
    Fa0/14, Fa0/15, Fa0/16, Fa0/17
    Fa0/18, Fa0/19, Fa0/20, Fa0/21
    Fa0/22, Fa0/23, Fa0/24, Gig0/1
    Gig0/2
    10 VLAN0010 active Fa0/1
    11 VLAN0011 active Fa0/1
    1002 fddi-default active
    1003 token-ring-default active
    1004 fddinet-default active
    1005 trnet-default active

    interface FastEthernet0/1
    switchport access vlan 10
    switchport trunk encapsulation dot1q
    switchport mode trunk
    switchport voice vlan 11
    spanning-tree portfast
    !

    How then, when it clearly shows that the interface would be placed in the vlans would this be wrong? Can someone please clarify this?

  4. Panther
    June 8th, 2017

    Ok, nevermind. I ran the same commands in GNS3 and the vlan output shows that the Interface dosn’t show up:

    vIOS-L2-01#show vlan

    VLAN Name Status Ports
    —- ——————————– ——— ——————————-
    1 default active Gi0/0, Gi0/2, Gi0/3, Gi1/0
    Gi1/1, Gi1/2, Gi1/3
    10 VLAN0010 active
    11 VLAN0011 active
    1002 fddi-default active
    1003 token-ring-default active
    1004 fddinet-default active
    1005 trnet-default active

    interface Gigabitethernet 0/1
    switchport access vlan 10
    switchport trunk encapsulation dot1q
    switchport mode trunk
    switchport voice vlan 11
    spanning-tree portfast
    !

    I am assuming that Packet Tracer has a bug in it, and that the devs got it right. Sorry about the confusion.

  5. CCNP boy
    July 7th, 2017

    Hello… Can someone confirm is studying this page ( 9tut ) is enough to pass the SW 300-115 exam ?

  6. mikeSWE
    July 19th, 2017

    Yes CCNP boy but get a fresh dump some drag and drops are not here and a few questions

  7. FREESHARE
    August 2nd, 2017
  8. Anonymous
    August 26th, 2017

    What will be the behavior of an access port when a tagged packet is received, with a VLAN ID of an access port?

    A. The tag is removed and packet is forwarded to a VLAN of access port
    B. The tag is removed and packet is forwarded of the VLAN mentioned in the tag.
    C. The tag is not removed and packet is forwarded of the VLAN mentioned in the tag.
    D. Packet is dropped.

    Can some help with this?

  9. a
    September 12th, 2017

    @certprepare

    The link for vlan trunking is showing vlan trunking 2 questions

    Can u please fix it?

  10. Anonymous
    October 11th, 2017

    If an access port receives a packet with an 802.1Q tag in the header other than the access VLAN value, that port drops the packet without learning its MAC source address.

    https://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus5000/sw/layer2/b_Cisco_Nexus_5000_Series_NX-OS_/Cisco_Nexus_5000_Series_NX-OS__chapter6.html

  11. Anonymous
    October 11th, 2017

    What will be the behavior of an access port when a tagged packet is received, with a VLAN ID of an access port?

    A. The tag is removed and packet is forwarded to a VLAN of access port
    B. The tag is removed and packet is forwarded of the VLAN mentioned in the tag.
    C. The tag is not removed and packet is forwarded of the VLAN mentioned in the tag.
    D. Packet is dropped.

    The Answer: A

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