Chapter 7 Flash Cards - Ethernet LAN Switching

Question 1

What are the 3 primary jobs of a LAN switch?

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 bridge.
3. Creating a (Layer 2) loop-free environment with other bridges by using Spanning Tree Protocol (STP)

Question 2

Describe the following switching method: Store-and-Forward

Answer 2

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 3

Describe the following switching method: Cut-Through

Answer 3

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 4

Describe the following switching method: Fragment-Free

Answer 4

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

Question 5

What are some of the benefits offered by a switch that were not offered by older LAN devices?

Answer 5

1. Switch ports connected to a single device microsegment 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 6

Explain the layer 2 logic that switches use to process frames.

Answer 6

1. Switches forward frames based on the destination address:
   a. If the destination address is a broadcast, multicast, or unknown destination unicast (a unicast not listed in the MAC table), the switch floods the frame.
   b. If the destination address is a known unicast address (a unicast address found in the MAC table):
   i. If the outgoing interface listed in the MAC address table is different from the interface in which the frame was received, the switch forwards the frame out the outgoing interface.
   ii. If the outgoing interface is the same as the interface in which the frame was received, the switch filters the frame, meaning that the switch simply ignores the frame and does not forward it.
2. Switches use the following logic to learn MAC address table entries:
   a. For each received frame, examine the source MAC address and note the interface from which the frame was received.
   b. If they are not already in the table, add the address and interface, setting the inactivity timer to 0.
   c. If it is already in the table, reset the inactivity timer for the entry to 0.
3. Switches use STP to prevent loops by causing some interfaces to block, meaning that they do not send or receive frames.

Question 7

Define Collision Domain

Answer 7

A collision domain is 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 8

Define Broadcast Domain

Answer 8

A broadcast domain is 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 9

Which of the following benefits are gained by using a hub/switch/router between Ethernet devices?

1. Greater cabling distances are allowed
2. Creates multiple collision domains
3. Increases bandwidth
4. Creates multiple Broadcast domains.

Answer 9

1. Hub/Switch/Router
2. Switch/Router
3. Switch/Router
4. Router

Question 10

What are some of the motivations to use VLANs?

Answer 10

1. To create more flexible designs that group users by departments, or by groups that work together, instead of by physical location.
2. To segment devices into smaller LANs (broadcast domains) to reduce overhead caused to each host in the VLAN.
3. To reduce the workload for STP by limiting a VLAN to a single access switch.
4. To enforce better security by keeping hosts that work with sensitive data on a separate VLAN.
5. To separate traffic sent by an IP phone from traffic sent by PCs connected to the phones.

Question 11

Describe the following campus switch role: Access

Answer 11

Provides a connection point (access) for end-user devices. Does not forward frames between two other access switches under normal circumstances.

Question 12

Describe the following campus switch role: Distribution

Answer 12

Provides an aggregation point for access switches, forwarding frames between switches, but not connecting directly to end-user devices.

Question 13

Describe the following campus switch role: Core

Answer 13

Aggregates distribution switches in very large campus LANs, providing very high forwarding rates.

Question 14

Define Broadcast Domain

Answer 14

A set of all devices that receive broadcast frames originating from any device with the set. Devices in the same VLAN are in the same broadcast domain.

Question 15

Define Broadcast Frame

Answer 15

An Ethernet frame sent to destination address FFFF.FFFF.FFFF, meaning that the frame should be delivered to all hosts on that LAN.

Question 16

Define Flooding

Answer 16

The result of a LAN switch forwarding process for broadcasts and unknown unicast frames. Switches forward these frames out all interfaces, except the interface in which the frame arrived. Switches also forward multicasts by default, although this behavior can be changed.

Question 17

Define Micosegmentation

Answer 17

The process in LAN design by which every switch port connects to a single device, with no hubs connected to the switch ports, creating a separate collision domain per interface. The term’s origin relates to the fact that one definition for the word “segment” is “collision domain,” with a switch separating each switch port into a separate collision domain or segment.

Question 18

Define Segmentation

Answer 18

The process of breaking a large piece of data from an application into pieces appropriate in size to be sent through the network.

Question 19

Define Spanning Tree Protocol (STP)

Answer 19

A bridge protocol that uses the Spanning Tree algorithm, allowing a switch to dynamically work around loops in a network topology by creating a spanning tree. Switches exchange bridge protocol data unit (BPDU) messages with other bridges to detect loops and then remove the loops by shutting down selected bridge interfaces.

Question 20

Define Unknown unicast frame

Answer 20

An Ethernet frame whose destination MAC address is not listed in a switch’s MAC address table, so the switch must flood the frame.

Question 21

Define Virtual LAN (VLAN)

Answer 21

A group of devices, connected to one or more switches, with the devices grouped into a single broadcast domain through switch configuration. VLANs allow switch administrators to separate the devices connected to the switches into separate VLANs without requiring separate physical switches, gaining design advantages of separating the traffic without the expense of buying additional hardware.