Ethernet Basics

Question 1

What committee defines wired network standards?

Answer 1

802.3

Question 2

preamble of an ethernet frame

Answer 2

7 byte series of alternating ones and zeroes, followed by a 1 byte start frame delimiter

• it always precedes a frame
• give receiving NIC time to realzie a frame is coming and exactly when it starts
• added by sending NIC

Question 3

type field of ethernet frame

Answer 3

tells the receiving NIC what type of data the frame contains

Question 4

pad of ethernet frame

Answer 4

Minimum ethernet frame is 64 bytes

pad brings up the data to 64 bytes if it is smaller

Question 5

what does each part of “10BaseT” mean

Answer 5

10 - 10 Mbps
Base - baseband
T - Twisted Pair

Question 6

What is a 10BaseT network?

Answer 6

two or more computers connected to a central hub

Question 7

Which wires trasmit on 10BaseT?

Answer 7

1 and 2 to send data

3 and 4 to receive data

Question 8

Half duplex & Full duplex

Answer 8

Half duplex - NIC can only send or recive, not both

Full Duplex - NIC can send and recieve at same time

Question 9

Memorize Wire Standards 568a/568b

Answer 9

1 - Green/White 1 - Orange/White
2 - Green 2 - Orange
3 - Orange/White 3 - Green/White
4 - Blue 4 - Blue
5 - Blue/White 5 - Blue/White
6 - Orange 6 - Green
7 - Brown/White 7 - Brown/White
8 - Brown 8 - Brown

Question 10

10BaseT summary

Answer 10

Speed 10mbps
signal type baseband
distance 100m
node limit 1024
topology star bus (physcal star, logical bus)
Cat 3 or better

Question 11

10BaseFL summary

Answer 11

speed 10mbps
signal type baseband
distance 2000 m
node limit 1024
topology star bus (physcal star, logical bus)
used multimode fiber and SC or ST connector

Question 12

What is CSMA/CD

Answer 12

Carrier Sense Multiple Access / Collision Detection

• carrier sense means each node examines the cable before sending data
• if traffic is detected, it waits a few milliseconds then rechecks
• maps to IEEE 802.3 standard
• multiple access means all machines have equal access to the wire

Question 13

What is a bridge?

Answer 13

• acts like a repeater to connect 2 networks

- filters and forwards based on MAC addresses

Question 14

What is a collision domain?

Answer 14

a group of nodes that have the capability of sending frames at the same time as each other

Question 15

How do switches handle collisions?

Answer 15

• each port is on its own colision domain

- two nodes on a switch can send data at the same time without collisions

Question 16

How do switches send broadcast messages?

Answer 16

will broadcast to all ports on the switch except the originating port

Question 17

How do you connect switches?

Answer 17

uplink port or crossover cables

Question 18

How do uplink ports work?

Answer 18

• enable you to connect two switches using a straight-through cable
• modern switches dont have a dedicated uplink port, but instead auto-sense when another switch is plugged in

Question 19

What kind of techniques are STP, BPDU and root guard?

Answer 19

Mitigation techniques - making bad things not as destructive

Question 20

What is STP?

Answer 20

Spanning Tree Protocol

• uses BDPU that eliminates accidental bridging loops
• establishes topology and elects one switch to be the root bridge
• each switch uses root bridge as a reference point
• puts ports in blocking state to stop redundant connections
• blocked ports will hear configuration BPDUs sent by the root bridge every 2 seconds
• if link goes down, STP sends a TCN that enables switches to rework themselves around the failed device

Question 21

What is a BPDU?

Answer 21

• Bridge Protocol Data Unit

- used by switches

Question 22

What is PortFast?

Answer 22

• enables interface to come up right away, without the normal latency introduced by STP
• prevents TCN BPDUs being sent out of that switch every time a PC is powered on or off (which causes all switches to flush their source address table and relearn MAC addresses)
• these ports should never receive a BPDU or it could start a bridging loop

Question 23

What is TCN?

Answer 23

Topology Change Notification

- a BDPU that causes the network to rework itself around a broken link

Question 24

What does BPDU guard do?

Answer 24

move a port configured with PortFast into an errdisable state (error occurred, disabled) if a BPDU is received on that port
- requires an admin to bring the port back up

Question 25

What is root guard?

Answer 25

will move a port into a root-inconsistent state if BPDUs coming from a certain direction indicate another switch is trying to become the root bridge

• will automatically return to its forwarding state once these BPDUs stop
• helps define locations where the root bridge should never be located

Question 26

What is RSTP?

Answer 26

Rapid Spanning Tree Protocol

- significantly faster convergence time following a network change

Question 27

Categories of switch problems

Answer 27

• obvious physical damage

- dead ports

Question 28

Steps to troubleshooting a switch

Answer 28

1 - Recognize the problem (no network on device)
2 - Examine switch for obvious damage
3 - Look for link lights
4 - Look at cables

Question 29

How does a switch direct packets?

Answer 29

By MAC address

Question 30

MTU

Answer 30

Maximum Transmission Unit