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VLAN Questions

July 6th, 2017 in SWITCH 300-115 Go to comments

Question 1


The Port Fast feature is automatically enabled when voice VLAN is configured. When you disable voice VLAN, the Port Fast feature is not automatically disabled.

Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3550/software/release/12-1_19_ea1/configuration/guide/3550scg/swvoip.html

Question 2


802.1Q VLAN frames are distinguished from ordinary Ethernet frames by the insertion of a 4-byte VLAN tag into the Ethernet header.


Question 3


Because the 802.1Q tunneling feature increases the frame size by 4 bytes when the metro tag is added, you must configure all switches in the service-provider network to be able to process maximum frames by increasing the switch system MTU size to at least 1504 bytes.

Question 4


The VLAN ID field inside an 802.1q frame consists of 12 bits. Therefore we have 212 = 4096 VLAN IDs, theoretically.


Question 5


Each access port can be only assigned to one VLAN via the “switchport access vlan ” command.

Question 6


This command is used to enable tagging of native VLAN frames on all 802.1Q trunk ports.

Answer A is not correct because even when the native VLAN is set to 1, all of the frames of the native VLAN are tagged.

Answer B is not correct because the control traffic still passes via the default VLAN (VLAN 1).

Answer C is not correct because all the frames are tagged with 4-byte dot1q tag.

Only answer D is best choice because control traffic (like CDP, VTP, STP, DTP…) uses VLAN 1 for communication. When the native VLAN is tagged (VLAN 1 by default) all control traffic is tagged too. If the native VLAN is not VLAN 1 then all the control traffic on VLAN 1 is still tagged by default (without using above command).

Question 7


When you delete a VLAN, any LAN ports configured as access ports assigned to that VLAN become inactive. The ports remain associated with the VLAN (and inactive) until you assign them to a new VLAN.

Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/12-2SX/configuration/guide/book/vlans.html

Question 8


The PortFast feature is automatically enabled when voice VLAN is configured. When you disable voice VLAN, the PortFast feature is not automatically disabled.

Question 9

Question 10

Question 11


First let’s review main characteristics of three layers in a campus network:

* Access layer:

+ Low cost per switch port
+ High port density
+ Scalable uplinks to higher layers
+ User access functions such as VLAN membership, traffic and protocol filtering, and quality of service (QoS)
+ Resiliency through multiple uplinks

* Distribution Layer:

+ Aggregation of multiple access-layer devices
+ High Layer 3 throughput for packet handling
+ Security and policy-based connectivity functions through access lists or packet filters
+ QoS features
+ Scalable and resilient high-speed links to the core and access layers

* Core layer:

+ Very high throughput at Layer 3
+ No costly or unnecessary packet manipulations (access lists, packet filtering)
+ Redundancy and resilience for high availability
+ Advanced QoS functions

We can see at Distribution and Core layers, Layer 3 throughput (routing) is very high -> B is correct.

Nowadays, end-to-end VLANs are not recommended in an enterprise network, unless there is a good reason. In an end-to-end VLAN, broadcast traffic is carried over from one end of the network to the other, creating the possibility for a broadcast storm or Layer 2 bridging
loop to spread across the whole extent of a VLAN. This can exhaust the bandwidth of distribution and core-layer links, as well as switch CPU resources. Now the storm or loop has disrupted users on the end-to-end VLAN, in addition to users on other VLANs that might
be crossing the core.

When such a problem occurs, troubleshooting becomes more difficult. In other words, the risks of end-to-end VLANs outweigh the convenience and benefits.

From that we can infer VLAN traffic should be local to the switch -> D is correct.

(Reference: CCNP SWITCH 642-813 Official Certification Guide)

Question 12

Comment pages
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  1. mikeSWE
    July 20th, 2017

    Can someone please confirm q12, different dump says A but AFAIK its called Native Vlan and not default Vlan so please confirm

  2. Anonymous
    July 28th, 2017


  3. TT
    July 29th, 2017

    Please advise where is the latest questions dump please?

    Thank You!

  4. TypicalStudent
    August 4th, 2017

    Can someome please check if the correct answer is B? The 358q dump says it is D but I think that is wrong. Can someone please confirm?
    Question: A swtich has been configured wtih the “vlan dot1q tag native” command. Which statement describes what the switch does with untagged frames that it receives on a trunked interface?
    A. Untagged frames are forwarded via the default VLAN.
    B. It drops the untagged frames.
    C. The trunked port is put in err-disabled state.
    D. Untagged frames are forwarded via the native VLAN.


  5. TypicalStudent
    August 4th, 2017

    Never mind I see this same question was posted under the Vlan trunking 2 section of this webpage.

  6. BL
    August 8th, 2017

    This question come out today.

    A question about the behaviour of VLAN 1 BPDUs in a situation where the Native VLAN configured as VLAN 99 and the Native VLAN is Tagged
    A. VLAN 1 STP BPDU tagged through VLAN 1
    B. VLAN 1 STP BPDU untagged through VLAN 1
    C. VLAN 1 STP BPDU tagged through VLAN 99
    D. VLAN 1 STP BPDU untagged through VLAN 99
    E. VLAN 1 STP BPDU tagged through VLAN 1 and 99

    Answer: B and D

  7. NJ
    August 13th, 2017

    @ BL

    For the above question, it clearly says Native VLAN (99) is tagged. So how can option D be correct?

  8. Shk
    August 18th, 2017

    @ NJ – or anyone – i think the above answer is B E

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