Data Link Layer Lesson-1

Data Link Layer In Network (CCNA)

Operation of switch, broadcast and collision domains

Data Link Layer (Operation of switch, broadcast and collision domains)
Data Link Layer In Network CCNA Lesson-1


Operation of switch, broadcast and collision domains

Data Link Layer :-

controls communications on the immediate link between two devices.

Thunderbolt mean serial connection

CCNA data link layer, routers connection serial connection

This immediate connection is local data link (immediate link)

In every layer the data puts into a set of structure, the set of structure is called protocol data unit. At data link layer it's called frame.

The data stream at layer 2 need a source address from which the stream sent from and a destination address, the most common of these addresses is :-

Media access control address (MAC):-

It's 48 bits in length and written in hexadecimal, hexadecimal is (1-9, A-F)

EX :- 113A.ACEF.2233 Every number or char is 4 bits.

The first 24 bits in the mac address are the OUI field (organisationally unique identifier) put it by government and can’t be changed.

The last 24 bits is for the vendor. Ex: - Dell, Lenovo…

The 7th bit is called: - universal local bit (UL)

If the 7th bit is turned off it mean its university unique

If the 7th bit is turned on that mean this address is assigned locally.

The protocol data unit at layer 2 is a frame:-

Destination MAC

Source MAC




FCS stands for frame check sequence it has CRC (Cyclic redundancy check)

Before transmit the frame the FCS use CRC to calculate a number using algorithm

The receiving device receives the frame and performs the same calculation on the frame and comes up with its own number

If the two numbers was match then the data was not corrupted, if the number doesn’t match then the frame will be dropped (mean it’s corrupted)

Devices work at data link layer:

The most specific one is switch:

The operation of switch in network ccna

Mac address table listing all ports of the switch

Switches learned mac address by only looking on the source mac address of the frame.

Switch make forwarding based on destination in the frame

If p1 send frame for p4 then switch1 will floods the frame out to all ports excepts the port that originated the signal in the first place (because MAC Table of switch will be empty)


All 1’s

Source MAC




This called broad cast frame is frame has all F’s in destination MAC

This mac address not existing so it will send frame to all port in purpose

The operation of switch is transparent bridging, bridge between PCs and transferring to them.

Broadcast and collision domains

Broadcast domain is the area of a network where broadcast will spread out to, if sent by one device.

Routers break up broad cast domains.

By default all switches are one broad cast domain.

Broad cast can’t transfer a router.

Half duplex: - by directional communication is possible but only in one direction in a time. ex:- walk talkie

Full duplex: - by directional communication and at the same time ex:- phone

Collision domain :- are area of the network where if two device transmit at the same time and they are running half duplex Ethernet the frames that sent will Collision

Each port on switch and each port on router is its own collision domain.

Hub is one big collision domain.

Switch is one big broadcast domain itself.

Ethernet in Data link layer (layer 2) protocol runs at 10 mb per second half duplex with (esmacd) carrier send multiple accesses with collision detection

Carriers send multiple accesses with collision detection: made for half duplex Ethernet.

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