CCNA Routing and Switching v3.0

 

QUESTION 31

Which of the following are benefits of VLANs? (Choose three.)

 

A.

They increase the size of collision domains.

B.

They allow logical grouping of users by function.

C.

They can enhance network security.

D.

They increase the size of broadcast domains while decreasing the number of collision domains.

E.

They increase the number of broadcast domains while decreasing the size of the broadcast domains.

F.

They simplify switch administration.

 

Correct Answer: BCE

Explanation:

When using VLAN the number and size of collision domains remain the same ->.

VLANs allow to group users by function, not by location or geography ->.

VLANs help minimize the incorrect configuration of VLANs so it enhances the security of the network ->.

VLAN increases the size of broadcast domains but does not decrease the number of collision domains ->.

VLANs increase the number of broadcast domains while decreasing the size of the broadcast domains which increase the utilization of the links. It is also a big advantage of VLAN ->.

VLANs are useful but they are more complex and need more administration ->.

 

 

QUESTION 32

Which two benefits are provided by creating VLANs? (Choose two.)

 

A.

added security

B.

dedicated bandwidth

C.

provides segmentation

D.

allows switches to route traffic between subinterfaces

E.

contains collisions

 

Correct Answer: AC

Explanation:

A VLAN is a switched network that is logically segmented on an organizational basis, by functions, project teams, or applications rather than on a physical or geographical basis.

Security:

VLANs also improve security by isolating groups. High-security users can be grouped into a VLAN, possible on the same physical segment, and no users outside that VLAN can communicate with them.

LAN Segmentation

VLANs allow logical network topologies to overlay the physical switched infrastructure such that any arbitrary collection of LAN ports can be combined into an autonomous user group or community of interest. The technology logically segments the network into separate Layer 2 broadcast domains whereby packets are switched between ports designated to be within the same VLAN. By containing traffic originating on a particular LAN only to other LANs in the same VLAN, switched virtual networks avoid wasting bandwidth.

 

 

QUESTION 33

In a switched environment, what does the IEEE 802.1Q standard describe?

 

A.

the operation of VTP

B.

a method of VLAN trunking

C.

an approach to wireless LAN communication

D.

the process for root bridge selection

E.

VLAN pruning

 

Correct Answer: B

Explanation:

A broadcast domain must sometimes exist on more than one switch in the network. To accomplish this, one switch must send frames to another switch and indicate which VLAN a particular frame belongs to. On Cisco switches, a trunk link is created to accomplish this VLAN identification. ISL and IEEE 802.1Q are different methods of putting a VLAN identifier in a Layer 2 frame.

The IEEE 802.1Q protocol interconnects VLANs between multiple switches, routers, and servers. With 802.1Q, a network administrator can define a VLAN topology to span multiple physical devices.

Cisco switches support IEEE 802.1Q for FastEthernet and Gigabit Ethernet interfaces. An 802.1Q trunk link provides VLAN identification by adding a 4-byte tag to an Ethernet Frame as it leaves a trunk port.

 

 

QUESTION 34

Which three statements about RSTP are true? (Choose three.)

 

A.

RSTP significantly reduces topology reconverging time after a link failure.

B.

RSTP expands the STP port roles by adding the alternate and backup roles.

C.

RSTP port states are blocking, discarding, learning, or forwarding.

D.

RSTP provides a faster transition to the forwarding state on point-to-point links than STP does.

E.

RSTP also uses the STP proposal-agreement sequence.

F.

RSTP uses the same timer-based process as STP on point-to-point links.

 

Correct Answer: ABD

Explanation:

One big disadvantage of STP is the low convergence which is very important in switched network. To overcome this problem, in 2001, the IEEE with document 802.1w introduced an evolution of the Spanning Tree Protocol: Rapid Spanning Tree Protocol (RSTP), which significantly reduces the convergence time after a topology change occurs in the network. While STP can take 30 to 50 seconds to transit from a blocking state to a forwarding state, RSTP is typically able to respond less than 10 seconds of a physical link failure. RSTP works by adding an alternative port and a backup port compared to STP. These ports are allowed to immediately enter the forwarding state rather than passively wait for the network to converge.

RSTP bridge port roles:

clip_image002Root port – A forwarding port that is the closest to the root bridge in terms of path cost

clip_image002[1]Designated port – A forwarding port for every LAN segment

clip_image002[2]Alternate port – A best alternate path to the root bridge. This path is different than using the root port. The alternative port moves to the forwarding state if there is a failure
on the designated port for the segment.

clip_image002[3]Backup port – A backup/redundant path to a segment where another bridge port already connects. The backup port applies only when a single switch has two links to the same segment (collision domain). To have two links to the same collision domain, the switch must be attached to a hub.

clip_image002[4]Disabled port – Not strictly part of STP, a network administrator can manually disable a port.

 

 

QUESTION 35

Which IEEE standard protocol is initiated as a result of successful DTP completion in a switch over Fast Ethernet?

 

A.

802.3ad

B.

802.1w

C.

802.1D

D.

802.1Q

 

Correct Answer: D

Explanation:

Dynamic Trunking Protocol (DTP) is a Cisco proprietary protocol for negotiating trunking on a link between two devices and for negotiating the type of trunking encapsulation (802.1Q) to be used.

QUESTION 36

Refer to the exhibit. At the end of an RSTP election process, which access layer switch port will assume the discarding role?

 

clip_image004

 

A.

Switch3, port fa0/1

B.

Switch3, port fa0/12

C.

Switch4, port fa0/11

D.

Switch4, port fa0/2

E.

Switch3, port Gi0/1

F.

Switch3, port Gi0/2

 

Correct Answer: C

Explanation:

In this question, we only care about the Access Layer switches (Switch3 & 4). Switch 3 has a lower bridge ID than Switch 4 (because the MAC of Switch3 is smaller than that of Switch4) so both ports of Switch3 will be in forwarding state. The alternative port will surely belong to Switch4.

Switch4 will need to block one of its ports to avoid a bridging loop between the two switches. But how does Switch4 select its blocked port? Well, the answer is based on the BPDUs it receives from Switch3. A BPDU is superior to 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 specific case, all the BPDUs sent by Switch3 have the same Root Bridge ID, the same path cost to the Root and the same Sending Bridge ID. The only parameter left to select the best one is the Sending Port ID (Port ID = port priority + port index). In this case the port priorities are equal because they use the default value, so Switch4 will compare port index values, which are unique to each port on the switch, and because Fa0/12 is inferior to Fa0/1, Switch4 will select the port connected with Fa0/1 (of Switch3) as its root port and block the other port -> Port fa0/11 of Switch4 will be blocked (discarding role).

 

 

 

 

 

QUESTION 37

Assuming the default switch configuration, which VLAN range can be added, modified, and removed on a Cisco switch?

 

A.

1 through 1001

B.

2 through 1001

C.

1 through 1002

D.

2 through 1005

 

Correct Answer: B

Explanation:

VLAN 1 is the default VLAN on Cisco switch. It always exists and cannot be added, modified or removed.

VLANs 1002-1005 are default VLANs for FDDI & Token Ring and they can’t be deleted or used for Ethernet.

 

clip_image006

 

 

QUESTION 38

Refer to the exhibit. Each of these four switches has been configured with a hostname, as well as being configured to run RSTP. No other configuration changes have been made. Which three of these show the correct RSTP port roles for the indicated switches and interfaces? (Choose three.)

 

clip_image008

 

SwitchA, Fa0/1, root

A.

SwitchA, Fa0/2, designated

B.

C.

SwitchB, Gi0/2, root

D.

SwitchB, Gi0/1, designated

E.

SwitchC, Fa0/2, root

F.

SwitchD, Gi0/2, root

 

Correct Answer: ABF

Explanation:

The question says “no other configuration changes have been made” so we can understand these switches have the same bridge priority. Switch C has lowest MAC address so it will become root bridge and 2 of its ports (Fa0/1 & Fa0/2) will be designated ports.

Because SwitchC is the root bridge so the 2 ports nearest SwitchC on SwitchA (Fa0/1) and SwitchD (Gi0/2) will be root ports.

Now we come to the most difficult part of this question: SwitchB must have a root port so which port will it choose? To answer this question we need to know about STP cost and port cost.

In general, “cost” is calcu
lated based on bandwidth of the link. The higher the bandwidth on a link, the lower the value of its cost. Below are the cost values you should memorize:

 

clip_image010

 

SwitchB will choose the interface with lower cost to the root bridge as the root port so we must calculate the cost on interface Gi0/1 & Gi0/2 of SwitchB to the root bridge. This can be calculated from the “cost to the root bridge” of each switch because a switch always advertises its cost to the root bridge in its BPDU. The receiving switch will add its local port cost value to the cost in the BPDU.

One more thing to notice is that a root bridge always advertises the cost to the root bridge (itself) with an initial value of 0.

Now let’s have a look at the topology again

 

clip_image012

 

SwitchC advertises its cost to the root bridge with a value of 0. Switch D adds 4 (the cost value of 1Gbps link) and advertises this value (4) to SwitchB. SwitchB adds another 4 and learns that it can reach SwitchC via Gi0/1 port with a total cost of 8. The same process happens for SwitchA and SwitchB learns that it can reach SwitchC via Gi0/2 with a total cost of 23 -> Switch B chooses Gi0/1 as its root port ->. Now our last task is to identify the port roles of the ports between SwitchA & SwitchB. It is rather easy as the MAC address of SwitchA is lower than that of SwitchB so Fa0/2 of SwitchA will be designated port while Gi0/2 of SwitchB will be alternative port.

 

Below summaries all the port roles of these switches:

 

clip_image014

+ DP: Designated Port (forwarding state)

+ RP: Root Port (forwarding state)

+ AP: Alternative Port (blocking state)

 

 

QUESTION 39

Refer to the exhibit. A technician is troubleshooting host connectivity issues on the switches. The hosts in VLANs 10 and 15 on Sw11 are unable to communicate with hosts in the same VLANs on Sw12. Hosts in the Admin VLAN are able to communicate. The port-to-VLAN assignments are identical on the two switches. What could be the problem?

 

clip_image016

 

A.

The Fa0/1 port is not operational on one of the switches.

B.

The link connecting the switches has not been configured as a trunk.

C.

At least one port needs to be configured in VLAN 1 for VLANs 10 and 15 to be able to communicate.

D.

Port FastEthernet 0/1 needs to be configured as an access link on both switches.

E.

A router is required for hosts on SW11 in VLANs 10 and 15 to communicate with hosts in the same VLAN on Sw12.

 

Correct Answer: B

Explanation:

In order for hosts in the same VLAN to communicate with each other over multiple switches, those switches need to be configured as trunks on their connected interfaces so that they can pass traffic from multiple VLANs.

 

 

 

 

 

QUESTION 40

Based on the network shown in the graphic. Which option contains both the potential networking problem and the protocol or setting that should be used to prevent the problem?

 

clip_image018

 

A.

routing loops, hold down timers

B.

switching loops, split horizon

C.

routing loops, split horizon

D.

switching loops, VTP

E.

routing loops, STP

F.

switching loops, STP

 

Correct Answer: F

Explanation:

The Spanning-Tree Protocol (STP) prevents loops from being formed when switches or bridges are interconnected via multiple paths. Spanning-Tree Protocol implements the 802.1D IEEE algorithm by exchanging BPDU messages with other switches to detect loops, and then removes the loop by shutting down selected bridge interfaces. This algorithm guarantees that there is one and only one active path between two network devices.

 

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