Ethernet LAN Switching

Question 1

Which field of an ethernet frame provides reciever clock synchronization ?

Answer 1

Preamble (7 bytes of 1010101) (usually not considered part of the header)

Question 2

Which field of an ethernet frame is used to detect errors that occured during trasmission ?

Answer 2

FCS (Frame Check Sequence)

Question 3

How long is the physical address of a network device ?

Answer 3

48 bits / 6 bytes

Question 4

How many bytes/bits of the MAC address is the OUI ?

Answer 4

24 bits / 3 bytes

Question 5

Which field of an ethernet frame is used by a switch to populate is MAC address table ?

Answer 5

Source MAC address

Question 6

What kind of frame does a switch flood out of all interfaces except the one recieved on ?

Answer 6

Unknown unicast

Question 7

What are all the fields of an ethernet frame ?

Answer 7

Preamble (7 bytes)
Start Frame Delimeter (SFD) (1 byte) (usually not considered part of the header)

Destination MAC address (6 bytes) - Header
Source MAC address (6 bytes) - Header
Type (2 bytes) - Header

Frame Check Sequence (FCS) (4 bytes) - Trailer

Question 7

What is the minimum size of an Ethernet Frame (Header + Data + Trailer) ?

Answer 7

64 bytes

Header + Trailer = 18 bytes
Minimum Data Payload Size = 46 bytes

If data is smaller than 46 bytes then padding bytes are used

Question 8

What protocol is used to discover the layer 2 (MAC) address of a known layer 3 (IP) address ?

Answer 8

Address Resolution Protocol (ARP)
It consists of 2 messages:
ARP Request (broadcast)
ARP Reply (unicast)

ARP Type: 0x0806

Question 9

How to view ARP table ?

Answer 9

arp -a

Question 10

What are the 2 messages sent with a ping ?

Answer 10

ICMP Echo Request
ICMP Echo Reply

Question 11

What is it called when MAC addresses are removed from a switches MAC address table after 5 minutes of no traffic ?

Answer 11

Aging

Question 12

Protocol Data Units (PDUs)

Answer 12

Data
Segment - Layer 4
Packet - Layer 3
Frame - Layer 2
Bits - Layer 1

Question 13

Ethernet Frame

Answer 13

Ethernet Header:
* Preamble
* Start Frame Delimiter (SFC)
* Destination
* Source
* Type

Ethernet Trailer
* FCS

Question 14

Ethernet Header - Start Frame Delimiter (SFC)

Answer 14

1 byte
Marks the end of the Preamble
(usually not considered part of the header)

Question 15

Ethernet Header - Destination and Source

Answer 15

Destination and Source MAC Address
6 bytes each

Question 16

Ethernet Header - Type / Length

Answer 16

2 bytes
1500 or less indicated the LENGTH of the encapsulated packet
1536 or more indicates the TYPE of encapsulated packet (IPv4 or IPv6)

IPv4: 0x0800
IPv6: 0x86DD

Question 17

Ethernet Trailer - Frame Check Sequence (FCS)

Answer 17

4 bytes
Detects corrupted data by running a Cyclical Redundancy Check (CRC) algorithm

Question 18

What is the destination MAC address of an unkown device ?

Answer 18

FFFF.FFFF.FFFF

Question 19

Interface Errors ?

Answer 19

Runts: Frames that are smaller than the minimum frame size 64 bytes
Giants: Frames that are larger than the maximum frame size 1518 bytes
CRC: Frames that failed the CRC check
Frame: Frames that have an incorrect format
Input Errors: Total errors from the various counters
Output Errors: Frames that the switch tried to send but couldn’t because of error

Question 20

How to set speed and duplex on an interface ?

Answer 20

speed speed
duplex duplex (auto / half / full)