03 - Ethernet Basics

Question 1

Preamble

Answer 1

• 7 byte series of alternating ones and zeros, followed by a 1 byte start frame delimiter
• gives receiving NIC time to realize a frame is coming and when it starts
• added by sending NIC

Question 2

MAC Address Length

Answer 2

48 bit or 6 byte

Question 3

Frame Pad

Answer 3

Frame is 64 bytes, but if not full, NIC will add data to bring it to 64

Question 4

What do parts of 10BaseT mean

Answer 4

10 - speed - 10 Mbps
Base - signaling type - Baseband or Broadband
T - type of cabling - twisted pair

Question 5

Half-duplex vs Full-duplex

Answer 5

Half - can send or receive at one time

Full - can send or receive at same time

Question 6

Different between 568A and 568B

Answer 6

Remember “GO”

• green/orange pairs are swapped
• blue/brown stay in the same place

Question 7

10BaseT Summary

Answer 7

• 10 Mbps
• Baseband
• 100 meter length
• 1024 node limit
• star-bus topology
• cat-3 or better with RJ-45 connections

Question 8

10BaseFL Summary

Answer 8

• 10 Mbps
• Baseband
• 2 km length
• 1024 node limit
• star-bus topology
• MMF with SC or ST connectors

Question 9

Media convertor

Answer 9

Used to connect different Ethernet types

Question 10

CSMA/CD

Answer 10

Carrier Sense Multiple Access / Collision Detection

• determines with computer should use a shared cable at a given moment
• Carrier sense - examines cable before sending data frame, if network is used, waits a few seconds and tries again
• Multiple Access - all machines have equal access to the wire
• when a collision occurs, both machines generate a random number to determine how long to wait before trying again

Question 11

Collision Domain

Answer 11

group of nodes that have the capability of sending frames at the same time

Question 12

Bridge

Answer 12

• acts as a repeater to connect two networks

- filters and forwards traffic based on MAC addresses

Question 13

How do switches handle collisions

Answer 13

• each port is on its own collision domain

- the switch can send signals at the same time without collisions

Question 14

Broadcast Domain

Answer 14

a network with a switch

Question 15

How do you connect switches

Answer 15

• uplink port - uses straight-through cable
• crossover cable
• modern switches have no uplink port, switch autodetects when another switch is plugged in. any port can be used

Question 16

Crossover Cable

Answer 16

One end is 568A and the other is 568B

Question 17

Spanning Tree Protocol

Answer 17

• connecting switches can cause redundant connections called bridging loops or switching loops
• uses special STP BDUs called Bridge Protocol Data Units (BPDU) to communicate with other switches to prevent loops
• one switch elected as root bridge
• configuration BPDU’s establish the topology
• certain ports placed in “blocking” state and only hear BPDUs, fowarded by root bridge every 2 seconds

Question 18

Mitigation Techniques

Answer 18

Making bad things not as destructive

Question 19

What happens when a switch with STP goes down?

Answer 19

TCN (Topology Change Notification) BPDU’s enable the switches to rework themselves around the failed switch

Question 20

When to use STP

Answer 20

When a port on a switch is connected to another switch, not when the port is connected to a PC`

Question 21

PortFast

Answer 21

• for ports that have PCs connected
• allows interface to come up right away without the latency from STP
• also prevents TCNs from being sent out everytime a computer is powered on or off

Question 22

Root Gaurd

Answer 22

• will move a port into a root-inconsistent state if BPDUs coming from a certain direction indicate that another switch is trying to become the root bridge
• will automatically return to its forwarding state once the BPDUs stop
• helps determine where the root bridge should never be

Question 23

Troubleshoot a switch

Answer 23

1 - device cant connect to network
2 - examine switch for obvious damage
3 - examine link lights, if not on try a different port
4 - check cables