Ensurepass

CCNA Routing and Switching v3.0

 

QUESTION 71

What is one benefit of PVST+?

 

A.

PVST+ supports Layer 3 load balancing without loops.

B.

PVST+ reduces the CPU cycles for all the switches in the network.

C.

PVST+ allows the root switch location to be optimized per VLAN.

D.

PVST+ automatically selects the root bridge location, to provide optimized bandwidth usage.

 

Correct Answer: C

Explanation:

The PVST+ provides Layer 2 load-balancing for the VLAN on which it runs. You can create different logical topologies by using the VLANs on your network to ensure that all of your links are used but that no one link is oversubscribed. Each instance of PVST+ on a VLAN has a single root switch. This root switch propagates the spanning-tree information associated with that VLAN to all other switches in the network. Because each switch has the same information about the network, this process ensures that the network topology is maintained and optimized
per VLAN.

 

Reference: http://www.cisco.com/en/US/docs/switches/lan/catalyst3750x_3560x/software/release/12.2_55_se/configuration/guide/swstp.html

 

 

QUESTION 72

Which statement about VLAN operation on Cisco Catalyst switches is true?

 

A.

When a packet is received from an 802.1Q trunk, the VLAN ID can be determined from the source MAC address and the MAC address table.

B.

Unknown unicast frames are retransmitted only to the ports that belong to the same VLAN.

C.

Broadcast and multicast frames are retransmitted to ports that are configured on different VLAN.

D.

Ports between switches should be configured in access mode so that VLANs can span across the ports.

 

Correct Answer: B

Explanation:

Each VLAN resides in its own broadcast domain, so incoming frames with unknown destinations are only transmitted to ports that reside in the same VLAN as the incoming frame.

 

 

QUESTION 73

Three switches are connected to one another via trunk ports. Assuming the default switch configuration, which switch is elected as the root bridge for the spanning-tree instance of VLAN 1?

 

A.

the switch with the highest MAC address

B.

the switch with the lowest MAC address

C.

the switch with the highest IP address

D.

the switch with the lowest IP address

 

Correct Answer: B

Explanation:

Each switch in your network will have a Bridge ID Priority value, more commonly referred to as a BID. This BID is a combination of a default priority value and the switch’s MAC address, with the priority value listed first. The lowest BID will win the election process.

For example, if a Cisco switch has the default priority value of 32,768 and a MAC address of 11-22-33-44-55-66, the BID would be 32768:11-22-33-44-55-66. Therefore, if the switch priority is left at the default, the MAC address is the deciding factor in the root bridge election.

 

 

QUESTION 74

Refer to the exhibit. Switch-1 needs to send data to a host with a MAC address of 00b0.d056.efa4. What will Switch-1 do with this data?

 

clip_image002

 

A.

Switch-1 will drop the data because it does not have an entry for that MAC address.

B.

Switch-1 will flood the data out all of its ports except the port from which the data originated.

C.

Switch-1 will send an ARP request out all its ports except the port from which the data originated.

D.

Switch-1 will forward the data to its default gateway.

 

Correct Answer: B

Explanation:

This question tests the operating principles of the Layer 2 switch. Check the MAC address table of Switch1 and find that the MAC address of the host does not exist in the table. Switch1 will flood the data out all of its ports except the port from which the data originated to determine which port the host is located in.

 

Switches work as follows:

 

clip_image004Switches learn the MAC addresses of PCs or workstations that are connected to their switch ports by examining the source address of frames that are received on that port.

clip_image004[1]Machines may have been removed from a port, turned off, or moved to another port on the same switch or a different switch.

clip_image004[2]This could cause confusion in frame forwarding.

clip_image004[3]The MAC address entry is automatically discarded or aged out after 300 seconds

clip_image004[4]If there is not MAC address of destination host in MAC table, switch sends broadcast to all ports except the source to find out the destination host.

 

In output there is no MAC address of give host so switch floods to all ports except the source port.

 

 

 

 

QUESTION 75

Which command can be used from a PC to verify the connectivity between hosts that connect through a switch in the same LAN?

 

A.

ping address

B.

tracert address

C.

traceroute address

D.

arp address

 

Correct Answer: A

Explanation:

ICMP pings are used to verify connectivity between two IP hosts. Traceroute is used to verify the router hop path traffic will take but in this case since the hosts are in the same LAN there will be no router hops involved.

 

 

QUESTION 76

Refer to the exhibit. Which two statements are true about interVLAN routing in the topology that is shown in the exhibit? (Choose two.)

 

clip_image006

 

Router1 and Switch2 should be connected via a crossover cable.

A.

Host E and host F use the same IP gateway address.

B.

C.

Router1 will not play a role in communications between host A and host

D.

D.The FastEthernet 0/0 interface on Router1 must be configured with subinterfaces.

E.

Router1 needs more LAN interfaces to accommodate the VLANs that are shown in the exhibit.

F.

The FastEthernet 0/0 interface on Router1 and the FastEthernet 0/1 interface on Switch2 trunk ports must be configured using the same encapsulation type.

 

Correct Answer: DF

Explanation:

In order for multiple VLANs to connect to a single physical interface on a Cisco router, subinterfaces must be used, one for each VLAN. This is known as the router on a stick configuration. Also, for any trunk to be formed, both ends of the trunk must agree on the encapsulation type, so each one must be configured for 802.1q or ISL.

 

 

 

QUESTION 77

What value is primarily used to determine which port becomes the root port on each nonroot switch in a spanning-tree topology?

 

A.

path cost

B.

lowest port MAC address

C.

VTP revision number

D.

highest port priority number

E.

port priority number and MAC address

 

Correct Answer: A

Explanation:

The path cost to the root bridge is the most important value to determine which port will become the root port on each non-root switch. In particular, the port with lowest cost to the root bridge will become root port (on non-root switch).

 

 

QUESTION 78

Which two protocols are used by bridges and/or switches to prevent loops in a layer 2 network? (Choose two.)

 

A.

802.1d

B.

VTP

C.

802.1q

D.

STP

E.

SAP

 

Correct Answer: AD

Explanation:

This question is to examine the STP protocol.

STP (802.1d) is used to prevent Layer 2 loops.

802.1q is a Frame Relay protocol which belongs to VLAN.

SAP is a concept of the OSI model.

 

 

QUESTION 79

Which two of these are characteristics of the 802.1Q protocol? (Choose two.)

 

A.

It is used exclusively for tagging VLAN frames and does not address network reconvergence following switched network topology changes.

B.

It modifies the 802.3 frame header, and thus requires that the FCS be recomputed.

C.

It is a Layer 2 messaging protocol which maintains VLAN configurations across networks.

D.

It includes an 8-bit field which specifies the priority of a frame.

E.

It is a trunking protocol capable of carrying untagged frames.

 

Correct Answer: BE

Explanation:

802.1Q protocol, or Virtual Bridged Local Area Networks protocol, mainly stipulates the realization of the VLAN. 802.1Q is a standardized relay method that inserts 4 bytes field into the original Ethernet frame and re-calculate the FCS. 802.1Q frame relay supports two types of frame: marked and non-marked. Non-marked frame carries no VLAN identification information.

 

 

 

QUESTION 80

Which command enables RSTP on a switch?

 

A.

spanning-tree uplinkfast

B.

spanning-tree mode rapid-pvst

C.

spanning-tree backbonefast

D.

spanning-tree mode mst

 

Correct Answer: B

Explanation:

Rapid Spanning Tree Protocol (RSTP) is an enhancement of the original STP 802.1D protocol. The RSTP 802.1w protocol is an IEEE open implementation. Cisco has its own proprietary implementation of RSTP, that includes the benefits of its Per-VLAN spanning tree protocols, called Rapid-PVST+.

To activate the Rapid-PVST+ protocol:

switch(config)#spanning-tree mode rapid-pvst

 

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