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zeyad1991 | Cisco CCNA | Module 3 - Layer 2 Data Link

By: zeyad1991 | Related Course: Cisco CCNA | Published: June 14, 2016 | Modified: July 23, 2016
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NotepadLayer 2 - Broadcast Domains

*  Switch is a whole Broadcast Domain by default.

*  Routers break-up Broadcast Domain.

Collision Domains: are areas of the network if two devices transmit at the same time and running half duplex Ethernet and both Devices transmit at the same time, the frame can collide.

– Collision Domain: area in the network where collision can accrue.

– Each port on a switch or router is it in own collision Domain.

– a Hub is one big collision Domain.

– Collision Can happens if you running Half Duplex Ethernet.

* Ethernet is Layer 2 Protocol that runs at 10Mbp at Half Duplex with something Called carrier scene multiple access with collision Detection CSMCD.

EX of Half Duplex: woki toki & Bravo Phones.

EX of Full Duplex: Phone Conversation.

CSMCD Part: when you running Half Duplex Ethernet and two devices transmitting at the same time and hear where is a collision will send jamming signal to all the device and ask the devices to stop the transmitting for randomized period of time.

* Different between Collision Domain & Broadcast Domain 8:00.


Notepadlayer 2 - DataLink

* DataLink Layer control communication on the immediate link between two devices.
* Thunder Bolt Means serial Connection (Wide Area Link).

* What is the Data in Layer 1 look Like:
– Electricity – electrical Signals – Changes in Voltage.

* In Layer 2 the Data has been put in set structure name Protocol Data Unit(PDU):
– In layer 2 The PDU Called ”Frame”

NotepadLayer 2 - MAC Address

* ”show mac address-table
– to show all MAC Addresses for connected devices to a switch.
– if ”type” shows dynamic that mean the MAC learned automatically.

* ”show interface fastEthernet 0/0
– will show details one of them is the MAC address of the switch.

– burnt in address (BIA) which mean burnt into the NIC of a device with a laser.


NotepadLayer 2 - Mac addresses

Protocol data unit: the way the data structured at layer 2.

protocol data unit @ layer 2 called Frame

the Frame contain:
* Destination MAC.
* Source MAC.
* Frame check sequence, this field houses whats called (cyclic redundancy check) ”CRC”, will calculate the frame and generate a number and the reciver will do the same and check the generated number, if they match that mean the data is not corrupted otherwise if its different that mean the frame is corrupted.

* The MAC Address is 48bit length
* The Destination Mac come first to save time when the receiver get the Frame and compare the Mac address


* Operation of a switch (6:00):
– switches make their forwarding decision based on the destination MAC Address on the Frame.
– all the devices connected to the switch get the frame, each device will compare the frame destination MAC Address and compare with its own MAC Address.
– PC-4 ”the destination PC” will reply to ”PC-1” when that frame arrives on Port-4 the switch will read the source MAC Address then add the MAC address to the MAC address Table.
– when the switch read the destination MAC Address it will not flood the frame since the MAC Address for PC-1 is already in the Table.

* Uni-cast & Broadcast:
– Uni-cast is One to One Transmission (Flood)
– Broadcast is one to Many Transmission ( have all FFFF in the destination MAC Address)

NotepadLayer 2 - VLANS

* Benefit of VLANs

– Save Money.

– Vlans can transfers multiple different switches.

– by default all ports belong to VLans 1.

– the total number of Vlans is between 1-4094 (depending on the platform)

– (1-1005) are called normal range VLans

– (1006-4094) are called extended range VLans


* VLans configuration ”Old method” – command line (not case sensitive):

switch# ”Vlan Database” > will switch to VLan database Mode

switch (VLan)# ”VLan 50 name test(will createVLan 50 with name test)

# exit

switch# ”show vlan brief”(will show Vlans)


new method:

switch (config)# ”vlan 100” >create Vlan 100

switch (config-vlan)# ”name workstation” > will name the Vlan

switch (config-vlan)# ”do show vlan brief” > will show all Vlans in the switch


NotepadLayer 2 Cisco IOS Lab (Part 1)

Cisco Router have three types of memory:

– RAM (Volatile Memory): RAM hold the running configuration or the current configuration as exist on the router or switch.

MV RAM(Non Volatile Memory): Hold startup configuration.

Flash Memory: Hold the Operating System ”IOS”


Go from user to privilege mode ”Enable”

Go From privilege to user Mode ”Disable”

NotepadLayer 3 Cisco IOS Lab "Part 2"

To enter configuration Mode ”config t” or ”configuration terminal”

to exit ”exit” or ”end” or ”Ctrl+z”


– to disable broadcast for domain name ”no ip domain-lookup”


to set password for privilege mode:

go to configuration mode ” enable password ”******”


to encrypt the password

config t Mode enable secret ”*****”

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