CCNP 642-811 BCMSN Exam Certification Guide - Cisco press

566Appendix A: Answers to Chapter “Do I Know This Already?” Quizzes and Q&A Sections

8.How many HSRP groups are needed to load balance traffic over two routers? Two

9.What load-balancing methods can GLBP use? Round robin, weighted, host dependent

10.What command can you use to see the status of the active and standby routers on the VLAN 171 interface?

show standby vlan 171

11.How many GLBP groups are needed to load balance traffic over four routers? One

12.When should you use SLB?

When load balancing is needed for clients to access a server farm.

13.What command defines and names an SLB server farm? ip slb serverfarm serverfarm-name

14.A virtual server has just been defined with the following commands:

ip slb vserver CISCO serverfarm CISCO-FARM virtual 192.168.199.17

Can the virtual server be used immediately? If not, what additional command is needed?

inservice

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Q&A

1.By default, what does a router or Layer 3 switch do with multicast packets? It drops them because multicast routing is not enabled by default.

2.By default, what does a Layer 2 switch do with a multicast packet?

It floods the packet out all ports on the source VLAN, as if it were a broadcast packet.

3.What high-order bit combination signals that an IP address is used for multicast? 1110

4.If the IP-to-MAC multicast address mapping is somewhat ambiguous, how can a frame be forwarded to the correct destination group?

The IP header must be examined in addition to the multicast MAC destination address.

5.What IP multicast address range is set aside for use only on the local network segment? 224.0.0.0 – 224.0.0.255

6.For the RPF check, the source IP address is looked up in the unicast routing table. To forward the packet, what should the result of the test be?

The unicast route to the source must be out the interface where the packet arrived.

7.What important difference exists between IGMPv1 and IGMPv2?

Hosts can request to leave a multicast group immediately with IGMPv2. IGMPv1 doesn’t have any mechanism for leaving a group, other than an aging-out process.

568Appendix A: Answers to Chapter “Do I Know This Already?” Quizzes and Q&A Sections

8.To join a multicast group, what type of message is sent? Where is this message sent?

An IGMP Membership Report is sent. The IGMP message is with a destination address of the multicast group itself so that the nearest multicast router will receive it.

9.What is the purpose of the IGMP Querier?

It is a router that periodically asks all hosts if any of them want to join the multicast group. Interested hosts must respond with a Membership Report.

10.For PIM Dense Mode, how is the multicast tree built?

The source is the root of the tree, and all Dense Mode routers are included by default. After all of these have joined the group, a pruning phase occurs so that routers with no member hosts are pruned off the tree.

11.Where is the root of the PIM Sparse Mode tree located? At a Rendezvous Point (RP) router

12.With PIM Sparse-Dense Mode, is the PIM mode determined per interface or per group? Per group. If the group has an RP, Sparse Mode is used; otherwise, Dense Mode is used.

13.What routing table is used for PIM?

The unicast routing table is used. PIM does not build or maintain its own multicast routing table.

14.What command is used to configure an interface for the hybrid PIM mode? ip pim sparse-dense-mode

15.When a switch performs IGMP snooping, what is it snooping for?

It listens to all IGMP membership reports so that it can learn the location of hosts joining and leaving multicast groups.

16.What else does a Layer 2 switch need when it is configured for CGMP?

A multicast router. The router relays group join and leave requests as CGMP packets that the switch can use.

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17.When should IGMP snooping and CGMP be used together on a switch? Never; they are mutually exclusive features.

18.At a trade show, several PCs and servers are connected to a single Layer 2 switch. The switch has CGMP enabled. When a server begins to send video data to a multicast address, what will happen to that traffic?

Even though CGMP is enabled, no multicast router is available to assist the switch with its multicast forwarding decisions. Therefore, the video packets will be flooded out all switch ports on the server’s VLAN.

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Q&A

1.What are some of the problems that QoS can help relieve? Packet delay, packet loss, jitter

2.Which protocol is used to reserve network resources in the IntServ QoS model? RSVP

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3.What protocol is used to maintain the DiffServ QoS model?

None; DiffServ is defined as a per-hop behavior, so no communication is needed between networking devices.

4.What range of values can the CoS field contain? 0 through 7 because CoS is three bits in length.

5.How is the CoS information passed from LAN switch to LAN switch?

Only across trunking links, where the CoS is contained in the trunk encapsulation.

6.How is the IP Precedence field related to the DSCP field?

They are both located in the same byte of the IP header. The three IP Precedence bits are actually the same as the first 3 bits of the DSCP (the Class Selector). However, the DSCP contains three additional bits for greater flexibility.

7.Put the following DSCP codepoints in order of increasing service quality:

a.EF

b.AF11

c.AF23

d.AF21

e.Default

Answer: e, b, c, d, a

8.If a packet contains the DSCP codepoint name “AF31,” what would the IP Precedence value be? 3; the Class Selector bits (“3” in the codepoint name) are the same as the IP Precedence bits.

9.If a switch port is configured as “untrusted,” what is the resulting CoS value of incoming packets?

Each packet’s CoS is overwritten with the default CoS value of the switch port itself. In the default case, the port’s CoS is zero.

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10.Should there be a trust boundary at every switch, where each overwrites QoS information? Explain why or why not.

No, although it is possible to do that. Instead, it is a best practice to define a trust boundary or domain for the bulk of the enterprise network where QoS information is known, predictable, or comes from a reliable source. Every switch at the QoS domain edge or the trust boundary should be configured to examine inbound QoS information and overwrite the values, if necessary.

11.What method is used for scheduling in Catalyst switches? Weighted Round Robin (WRR)

12.Name one method that is used for congestion avoidance. Tail drop or Weighted Random Early Detection (WRED)

13.When are packets dropped from the strict priority queue on an interface?

Packets are never dropped from the priority queue by WRED. The only condition that causes packets to be dropped is when the priority queue is full. In that case, packets are dropped by the tail drop method.

14.What is the disadvantage of using the tail drop method to avoid congestion?

Tail drop waits until the queue is full before dropping the latest arriving packets. This tends to avoid congestion after congestion occurs. The main disadvantage is with TCP connections, which begin to back off and enter the slow-start state once tail drop begins. All active TCP connections will do this simultaneously, synchronized with each other. This causes a widespread throughput issue and makes the overall network utilization oscillate.

15.If WRED drops packets at random, isn’t that bad?

Not necessarily. Packets are randomly dropped only if the queue fills above a threshold. Then, drops are more probable the farther the queue fills above that threshold. Dropped packets are retransmitted as part of the connection protocol, so the effect is seen on only a few connections rather than on all connections with tail drop.

16.A switch port has a queue type 1p1q4t. What does this mean?

The port has one strict-priority queue (1p), one standard queue (1q), and four WRED thresholds per standard queue (4t).

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17.If a switch port has a queue type 1p2q2t, what is the strict priority queue’s queue number?

The strict-priority queue is queue 3. (The low-priority standard queue is 1, the high-priority standard queue is 2.)

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WRR always services the strict-priority queue before any other; therefore, there is no way to configure any other behavior.

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Q&A

1.What two ways can QoS trust be configured on a switch? Per-interface and per-policy

2.If all QoS trust decisions will be applied as part of a QoS policy, what command should you use on an interface?

no mls qos trust; All trust must be disabled on the interface because the policy will enable any specific trust conditions that are configured.

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3.When a class map is configured, what types of commands must also be used? match commands, to match specific types of traffic

4.Name two methods that you can use to identify or classify traffic.

Matching can be based on traffic that is permitted by an access list and on protocols identified by NBAR.

5.What command can classify traffic with an extended IP access list? match access-group name access-list

6.What command can classify traffic with NBAR? match protocol protocol-name

7.What does a policy map contain?

It contains references to class maps that identify types of traffic and actions to take as a result of each class map.

8.When a policy map is applied to an interface, does it control both inbound and outbound traffic?

The policy map must be applied in one direction only, each time it is configured on the interface. It can control only inbound or outbound traffic, unless it is applied twice to the interface—once in each direction.

9.What is the first command keyword used to configure WRED congestion avoidance?

wrr-queue; This seems surprising because wrr-queue is also used to configure scheduling. However, Catalyst switches use the same components to schedule and perform WRED because both involve egress queue operations.

10.What QoS information is used to map packets into the egress queues of a switch port? CoS, through the use of a CoS-to-queue mapping function

11.What command can set the WRED thresholds of the strict-priority queue (1p2q2t) to 25 percent and 75 percent?

None; WRED can’t perform any operations on the strict-priority queue. Because of this, no packets can ever be randomly dropped.

574 Appendix A: Answers to Chapter “Do I Know This Already?” Quizzes and Q&A Sections

12.What command can display the QoS and queue information about a switch port? show queueing interface

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Q&A

1.How does a Catalyst switch detect that a connected device is capable of using inline power?

The device must first loop the Ethernet connection’s transmit and receive pairs. The switch can then hear its own test tone, confirming that the device does indeed need inline power. After the power is enabled on the port, the switch and the device can exchange CDP messages so that the amount of power can be adjusted.

2.What type of trunk can be used between a Catalyst switch port and a Cisco IP Phone? A special-case 802.1Q trunk with only two VLANs

3.When a trunk is used on an IP Phone, on which VLAN is the data from an attached PC carried?

PC data is always carried over the native (untagged) VLAN in the trunk. Voice packets can be carried over the native VLAN, too, or over a unique voice VLAN.