VLAN & VTP Questions

Here you will find answers to VLAN & VTP Questions

Question 1

Which of these best describes the actions taken when a VTP message is received on a switch configured with the VTP mode “transparent”?

A. VTP updates are ignored and forwarded out all ports.
B. VTP updates are ignored and forwarded out trunks only.
C. VTP updates are made to the VLAN database and are forwarded out trunks only.
D. VTP updates are ignored and are not forwarded.

Answer: B

Question 2

Refer to the exhibit. Catalyst R is the root bridge for both VLAN 1 and VLAN 2. What is the easiest way to load-share traffic across both trunks and maintain redundancy in case a link fails, without using any type of EtherChannel link-bundling?

A. Increase the root bridge priority (increasing the numerical priority number) for VLAN 2 on Catalyst D so that port D2 becomes the root port on Catalyst D for VLAN 2.
B. Decrease the port priority on R2 for VLAN 2 on Catalyst R so that port D1 will be blocked for VLAN 2 and port D2 will remain blocked for VLAN 1.
C. Decrease the path cost on R2 on Catalyst R for VLAN 2 so that port D1 will be blocked for VLAN 2 and port D2 will remain blocked for VLAN 1.
D. Increase the root bridge priority (decreasing the numerical priority number) for VLAN 2 on Catalyst R so that R2 becomes the root port on Catalyst D for VLAN 2.

Answer: B

Explanation

First we should understand what will happen if nothing is configured (use default values). Because R is the root bridge so all of its ports will forward. D will need to block one of its ports to avoid a bridging loop between the two switches. But how does D select its blocked port? Well, the answer is based on the BPDUs it receives from R. A BPDU is superior than another if it has:

1. A lower Root Bridge ID
2. A lower path cost to the Root
3. A lower Sending Bridge ID
4. A lower Sending Port ID

These four parameters are examined in order. In this case, all the BPDUs sent by R have the same Root Bridge ID, same path cost to the Root and same Sending Bridge ID. The only parameter left to select the best one is the Sending Port ID (Port ID = port priority + port index). If using default values, the default port priority’s value is 32 or 128 (128 is much more popular today), so D will compare port index values, which are unique to each port on the switch, and because port R2 is inferior to port R1 (the port’s number of R2 is higher than that of R1, for example port Fa0/2 is inferior to port Fa0/1), D will select the port connected with port R1 as its root port and block the other port.

The problem here is port D2 is blocked for both VLAN 1 & 2 and that means we can’t use the underneath link for load-sharing. The underneath link is just used in the case the above link fails.

Now as you can guess, the easiest way to load-share traffic across both trunks is decreasing the port priority on R2 for VLAN 2 on Catalyst R so that port D1 will be blocked for VLAN 2. Notice that “decreasing” here means make that port ID superior to the other port.

Question 3

The network administrator is trying to add Switch1 to the network, but the 802.1 Q trunk is not coming up. Switch1 was previously tested in the laboratory and its trunk configuration worked fine. What are three possible causes of this problem? (Choose three)

A. The trunking configuration mode on Switch1 is set to Off.
B. The trunking configuration mode on the other end is set to On.
C. The trunking configuration mode on the other end is set to Desirable.
D. Cisco Discovery Protocol is not running on the other end.
E. There is a VTP domain name mismatch.
F. Switch1 does not support 802.1Q.

Answer: B C E

Explanation

There are 5 possible trunking modes for a switch port:
+ Auto: this is the default mode. In this mode, a port will become a trunk port if the device the port is connected to is set to the on or desirable mode.
+ Desirable: allows the port to become a trunk port if the device the port is connected to is set to the on, desirable, or auto mode
+ On: sets the port to permanent trunking mode.
+ Nonegotiate: sets the port to permanent trunking mode without sending Dynamic Trunking Protocol (DTP) frames
+ Off: sets the port to permanent non-trunking mode

In this case, we can guess the trunking mode of Switch 1 is “auto” (default mode). When in the laboratory, the trunking mode of the other end is set to “On” or “Desirable” so 2 switches can negotiate and the link becomes trunk with no problem. But when plugging to the network, other switches may have the trunking mode set to “auto” so the 802.1Q trunk is not coming up -> B C are correct.

Of course these switches need to be in the same VTP domain so that they can “talk” with each other -> E is correct.

Question 4

Refer to the exhibit. The Layer 2 network uses VTP to manage its VLAN database. A network designer created all VLANs on the VTP server (switch 1) and it has been advertised through VTP to all other VTP clients (switches 2 through 4). Due to network growth, a network operator decided to add a new switch between switch 1 and switch 3. The network operator has been instructed to use a refurbished switch and use a VTP client. Which three of these factors should the network operator consider to minimize the impact of adding a new switch? (Choose three)

A. Pay special attention to the VTP revision number, because the higher value takes the priority.
B. Configure all VLANs manually on the new switch in order to avoid connectivity issues.
C. A trunk should be established between the new switch and switches 1 and 3 as VTP only runs over trunk links.
D. Set at least the VTP domain name and password to get the new switch synchronized.
E. An ISL trunk should be established between the new switch and switches 1 and 3, because VTP only runs over ISL.
F. Pay special attention to the VTP revision number, because the lower value takes the priority.

Answer: A C D

Explanation

VTP should be used whenever we have more than 1 switch with multiple VLANs. It helps us save much time so configuring all VLANs manually is just a waste of time -> B is not correct.

VLAN Trunking Protocol (VTP) can operate over 802.1q or ISL on FastEthernet link.
+ On ISL: Switch(config-if)#switchport trunk encapsulation isl
+ On 802.1q: Switch(config-if)#switchport trunk encapsulation dot1q

-> E is not correct

Note: The 2940/2950 switches only support 802.1q encapsulation with the switchport mode trunk command. The switch will automatically use 802.1q encapsulation.

Each time a VTP updates are sent out, the revision number is increased by 1. Any time a switch sees a higher revision
number, it knows the information that it’s receiving is more current, and it will overwrite the current database with that new information.

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