Chapter 6 - Building Ethernet LANs with Switches

Question 1

What are the 3 primary actions of transparent bridges?

Answer 1

1. ) Deciding when to forward a frame or when to filter (not forward) a frame, based on the destination MAC address.
2. ) Learning MAC addresses by examining the source MAC address of each frame received by the switch.
3. ) Creating a (Layer 2) loop-free environment with other switches by using Spanning Tree Protocol (STP).

Question 2

What is a collision domain?

Answer 2

A set of network interface cards for which a frame sent by one NIC could result in a collision with a frame sent by any other NIC in the same collision domain.

Question 3

What is a MAC address table?

Answer 3

Lists the location of each MAC relative to that one switch.

Question 4

Describe the store-and-forward switching method of a switch.

Answer 4

The switch fully receives all bits in the frame (store) before forwarding the frame (forward). This allows the switch to check the FCS before forwarding the frame.

Question 5

Describe the cut-through switching method of a switch.

Answer 5

The switch forwards the frame as soon as it can. This reduces latency but does not allow the switch to discard frames that fail the FCS check.

Question 6

Describe the fragment-free switching method of a switch.

Answer 6

The switch forwards the frame after receiving the first 64 bytes of the frame, thereby avoiding forwarding frames that were errored because of a collision.

Question 7

What are some benefits of LAN switches over bridges and hubs?

Answer 7

1) Switch ports connected to a single device micro segment the LAN, providing dedicated bandwidth to that single device.
2) Switches allow multiple simultaneous conversations between devices on different ports.
3) Switch ports connected to a single device support full-duplex, in effect doubling the amount of bandwidth available to the device.
4) Switches support rate adaptation, which means that devices that use different Ethernet speeds can communicate through the switch (hubs cannot).

Question 8

If the destination address in a frame is a broadcast, multicast, or unknown destination unicast, what does the switch do with the frame?

Answer 8

The switch floods the frame.

Question 9

If the destination address is a known unicast address and the outgoing interface listed in the MAC address table is different from the interface in which the frame was received, what does the switch do with the frame?

Answer 9

The switch forwards the frame out the outgoing interface.

Question 10

If the destination address is a known unicast address and the outgoing interface listed in the MAC address table is the same as the interface in which the frame was received, what does the switch do with the frame?

Answer 10

The switch filters the frame, meaning that the switch simply ignores the frame and does not forward it.

Question 11

What logic does the switch use to learn MAC address table entries? (3 different steps.)

Answer 11

A. For each received frame, examine the source MAC address and note the interface from which the frame was received.
B. If it is not already in the table, add the MAC address and interface it was learned on, setting the inactivity timer to 0.
C. if it is already in the table, reset the inactivity timer for the entry to 0.

Question 12

What is the inactivity timer?

Answer 12

This is the timer the switch keeps for each entry in the MAC address table. The switch sets the timer to 0 for new entries.

Question 13

What is STP and why do switches use it?

Answer 13

STP is spanning-tree protocol and switches use this to prevent loops by causing some interfaces to block, meaning they do not send or receive frames.

Question 14

What is a collision domain?

Answer 14

A set of network interface cards (NIC) for which a frame sent by one NIC could result in a collision with a frame sent by any other NIC in the same collision domain.

Question 15

What is a broadcast domain?

Answer 15

A set of NICs for which a broadcast frame sent by one NIC is received by all other NICs in the same broadcast domain.

Question 16

Between a hub, bridge, switch, and router which separate the LAN into multiple broadcast domains?

Answer 16

Routers

Question 17

Considering a hub, switch, and router which ones allow for greater cabling distances when used for segmenting the LAN?

Answer 17

Hubs, switches, and routers

Question 18

Considering a hub, switch, and router which ones create multiple collision domains when used for segmenting the LAN?

Answer 18

Switches and routers - not hubs.

Question 19

Considering a hub, switch, and router which ones increase bandwidth when used for segmenting the LAN?

Answer 19

Switches and routers - not hubs.

Question 20

Considering a hub, switch, and router which ones create multiple broadcast domains when used for segmenting the LAN?

Answer 20

Routers - not switches or hubs.

Question 21

A LAN consists of all devices in the same what?

Answer 21

Broadcast domain

Question 22

Instead of using 2 different switches to create 2 broadcast domains, what feature of the switch could you use?

Answer 22

VLANs

Question 23

Which term describing the role of campus switches provides a connection point for end-user devices? It does not forward frames between two other access switches under normal circumstances.

Answer 23

Access

Question 24

Which term describing the role of campus switches provides an aggregation point for access switches, forwarding frames between switches but not connecting directly to end-user devices?

Answer 24

Distribution

Question 25

Which term describing the role of campus switches aggregates distribution switches in very large campus LANs, providing very high forwarding rates?

Answer 25

Core

Question 26

If auto-negotiation fails between 2 devices, what logic does the Cisco switch use to determine speed?

Answer 26

Sense the speed (without using autonegotiation), or if that fails, use the IEEE default (slowest supported speed, often 10 Mbps).

Question 27

If auto-negotiation fails between 2 devices, what logic does the Cisco switch use to determine duplex?

Answer 27

Use the IEEE defaults: If speed = 10 or 100, use half-duplex; otherwise, use full-duplex.

Question 28

What is a duplex mismatch?

Answer 28

When one device is using half duplex and the other is using full duplex or vice versa. This can usually happen when autonegotiation is disabled and devices resort to the IEEE default.