CCNP 642-811 BCMSN Exam Certification Guide - Cisco press

A P P E N D I X A

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

Chapter 1

“Do I Know This Already?”

1.c

2.e

3.d

4.d

5.a

6.c

7.b

8.c

9.c

10.d

11.a

The Catalyst 2950 is not the best choice for the distribution layer because it doesn’t support Layer 3 functionality.

12.a

Q&A

1.For each layer of the OSI model, match the forwarding criteria used by a switch:

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2.What is multilayer switching (MLS)?

MLS forwards traffic using information from Layer 2, Layer 3, and Layer 4—all in hardware at wire speed.

3.Fill in the blanks in the following statement:

In the 20/80 rule of networking, 20 percent of the traffic on a segment usually stays local while 80 percent travels across the network.

4.What is a collision domain, and where does it exist in a switched LAN?

A collision domain is a network segment where shared media access is supported. Devices on the shared media must compete for access when transmitting data. In a switched network, the collision domain is restricted to a single switch port and does not extend across the switch.

5.What is a broadcast domain, and where does it exist in a switched LAN?

A broadcast domain is the extent of a network where broadcast frames propagate. Basically, a broadcast domain covers an area where Layer 2 devices are located and terminates at the boundary of a Layer 3 device. In a switched network, the broadcast domain extends to all switch ports assigned to a common VLAN. This is because a switch forwards broadcasts out all available ports in a VLAN.

6.What is a VLAN, and why is it used?

A VLAN (virtual LAN) is a group of switch ports that communicate as if they were attached to a single shared-media LAN segment. VLANs can extend across buildings or backbones, as long as the VLAN is connected end-to-end through trunking or physical connections. A VLAN is a broadcast domain. VLANs segment networks for ease of management and better performance.

7.At what OSI Layer(s) do devices in the distribution layer usually operate? Layers 2, 3, and 4

8.What is network segmentation? When is it necessary, and how is it done in a campus network design?

Segmentation is the process of dividing a LAN into smaller, discrete collision domains. If a large percentage of collisions is observed on a LAN, segmentation is appropriate. In a campus network design, segmentation occurs at each switch port. A similar form of segmentation involves reducing the size of broadcast domains. Placing Layer 3 devices in the distribution and core layers terminates the broadcast domains at those layer boundaries.

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9.Is it possible to use Layer 2 switches in the distribution layer, rather than Layer 3 switches? If so, what are the limitations?

It is generally best practice to use Layer 3 or multilayer switches in the distribution layer, as outlined in this chapter. However, in some environments, this might not be possible due to cost or implementation logistics. Layer 2 switches will work fine in the distribution layer but will not provide a VLAN or broadcast domain boundary in that layer. As a result, broadcasts will propagate on into the core layer, using unnecessary bandwidth.

10.Which of the following Cisco switch products should be used in a campus network’s distribution layer? (Check all that apply.)

a.Catalyst 2950

b.Catalyst 3550 (SMI)

c.Catalyst 3550 (EMI)

d.Catalyst 4000/4500

e.Catalyst 6500

Answer: c, d, e

11.When might you select a Catalyst 4000 to use in a wiring closet? What attributes make it a good choice?

A Catalyst 4000 might be a good choice for an access layer switch if a high port density exists in that location. A single switch could be used, whereas multiple Catalyst 2950 or 3550 switches would be needed to equal the same port density. This would allow a small number of highspeed uplinks to be used into the distribution layer. With multiple smaller switches, you would either require a number of uplinks, or the access layer switches would have to be daisy-chained or “stacked.” Note that a single Catalyst 4000 does introduce a single point of failure into the network; if that unit failed, many users would lose network service. In that case, a Catalyst 4500, with its redundant features, would make a good choice.

12.Which Cisco switch family has the most scalable performance?

The Catalyst 6500’s large chassis and variety of modules makes it possible to support a high density of connections and increase switching capacity with new MLS hardware.

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

Chapter 2

“Do I Know This Already?”

1.c

2.c, d, e

3.a, b

4.a, c

5.c

6.d

7.a, c

8.c, d, e

9.b

10.b, c

11.c, d

12.a, b, c

Q&A

1.Where is the most appropriate place to connect a block of enterprise (internal) servers? Why?

You should connect a block of enterprise servers into the core, just as you would switch blocks. The server farm building block should have a layer of access and distribution switches, just as in any switch block. This maximizes connectivity from the servers to all other devices in the network. In effect, all users will see the same number of switch “hops” to access a server. Connecting into the core also provides maximum scalability because you can add more server blocks in the future.

2.How can you provide redundancy at the switch and core block layers? (Consider physical means, as well as functional methods using protocols, algorithms, and so on.)

In a switch block, you can provide redundancy through two distribution switches. Each access switch can be linked to both distribution switches for fault tolerance. The Layer 3 distribution layer allows both uplinks to be used at the same time, with little or no failover time required. In the core layer, a dual core can be used with two core switches. Each distribution switch has dual uplinks, with one link to each core switch. Here, the redundant links can stay active for load sharing and redundancy, thanks to the Layer 3 routing protocols running in the distribution and core layers.

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3.What factors should you consider when sizing a switch block?

Consider traffic types, flows, and patterns, as well as the size and number of common workgroups. Additionally, the Layer 3 switching capacity in the distribution layer should be sized according to the amount of traffic crossing from one VLAN to another.

4.What are the signs of an oversized switch block?

The distribution switches begin to become bottlenecks in handling the interVLAN traffic volume. Access list processing in the distribution layer can also become a rate-limiting factor. Broadcast and multicast traffic forwarding can slow down the Layer 2 and Layer 3 switches in the block.

5.What are the attributes and issues of having a collapsed core block?

Attributes: Cost savings (no separate high-end core switches) and design simplicity. Issues: Scalability becomes limited.

6.How many switches are sufficient in a core block design?

Two switches are usually sufficient in a core block, offering load sharing and redundancy. However, you can add more core switches as the size of the network and core traffic flow dictates.

7.What building blocks are used to build a scalable campus network?

The switch block is the template used to group access layer switches and their respective distribution layer switches. Switch blocks are then connected into the core block to build a scalable network. Depending on the other resources that are present in a campus network, other building blocks can include a server farm block, network management block, and enterprise edge block. The service provider edge block exists outside the campus network; although, it does interface with the enterprise edge block.

8.What are two types of core, or backbone, designs? Collapsed core and dual core.

9.Why should links and services provided to remote sites be grouped in a distinct building block?

Remote sites and roaming VPN users should be considered corporate users, as if they were connected directly inside the enterprise network. These users should enjoy the same efficient access to any enterprise resource that internal users have. Additionally, VPN tunnels should terminate in a secure area of the enterprise network.

Chapter 3 523

2.A TCAM lookup involves which values? Mask

Value Result

3.How many table lookups are required to find a MAC address in the CAM table? 1

4.How many table lookups are required to match a packet against an access list that has been compiled into 10 TCAM entries?

1

5.How many value patterns can a TCAM store for each mask? 8

6.Can all packets be switched in hardware by a multilayer switch? No; some must be flagged for process switching by the switch CPU.

7.Multilayer switches must rewrite which portions of an Ethernet frame? Source and destination MAC addresses

IP time-to-live Checksums

8.If a station only receives Ethernet frames and doesn’t transmit anything, how will a switch learn of its location?

You must configure a static CAM entry with the station’s MAC address and the switch port where it is located. Otherwise, the switch must flood each and every frame destined for that host out every switch port in an effort to find it.

9.What is a TCAM’s main purpose?

To process access lists as a single table lookup

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10.Why do the TCAM mask and pattern fields consist of so many bits?

So that a combination of several address fields in a frame can be inspected at once

11.In a multilayer switch with a TCAM, a longer access list (more ACEs or statements) takes longer to process for each frame. True or false?

Answer: False

12.A multilayer switch receives a packet with a certain destination IP address. Suppose the switch has that IP address in its Layer 3 forwarding table, but no corresponding Layer 2 address. What happens to the packet next?

The switch CPU sends an ARP request.

13.If a multilayer switch can’t support a protocol with CEF, it relies on fallback bridging. Can the switch still route that traffic?

No. That traffic is transparently bridged. An external multilayer switch or router is required to take the bridged packets and route them.

14.To configure a static CAM table entry, the mac address-table static mac-address command is used. Which two other parameters must also be given?

vlan vlan-id

interface type mod/num

15.As a network administrator, what aspects of a switch TCAM should you be concerned with? The size of the TCAM resources

16.What portion of the TCAM is used to evaluate port number comparisons in an access list? LOU

17.Someone has asked you where the host with MAC address 00-10-20-30-40-50 is located. Assuming you already know the switch it is connected to, what command can you use to find it?

show mac address-table dynamic address 0010.2030.4050

show mac address-table count

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19.What protocol is used to advertise CAM table entries among neighboring switches? None; the entries are not advertised.

20.Suppose a host uses one MAC address to send frames and another to receive them. In other words, one address will always be the source address sent in frames, and the other is only used as a destination address in incoming frames. Is it possible for that host to communicate with others through a Layer 2 switch? If so, how?

Yes, but not very efficiently. The Layer 2 switch will learn one of the host’s MAC addresses as the host sends frames. That address will be seen as the source address on frames arriving at the switch. However, because the second MAC address is never used to send frames, the switch will never be able to learn of its existence. When frames are sent to the host using that second MAC address, the switch is forced to flood the frames out all relevant switch ports. The host receives those frames only because it is connected to a port where the frames were flooded. All other hosts on the same VLAN also receive the flooded frames, even though they have no interest in that traffic.

Chapter 4

“Do I Know This Already?”

1.b

2.b

3.b

4.c

5.d

6.c

7.b

8.c

9.d

10.a

The erase flash: command erases all files contained in the Flash file system. This means that all Cisco IOS Software images that were present in Flash will now be erased. Therefore, the switch will not have an IOS image file to use when it reloads. The next logical step would be to copy another IOS image file into the Flash memory.