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CCNP Enterprise Core (350-401) Cisco Packet Forwarding

Layer 2 Forwarding

The data link link layer of the OSI midel handles the physical addressing underneath the network layer for communication between two hosts.

Network packets include Layer 2 addressing with their unique source and destination addresses known as media access control or MAC addresses for ethernet.

A MAC address is a 48-bit address split across six octets and notated in hexadecimal.

The first three octets are assigned to the hardware manufacturer of the device, known as the organizationally unquie identifier or OUI number.

XX:XX:XX:YY:YY:YY

The manufacturer are responsbile for making sure that the last three octets of the MAC address are unique.

At layer 2 each device listens for network traffic that contains its MAC address as the destination address on recieved network traffic before moving it up the OSI layer for processing.

Other traffic recieved that does not have the MAC address of the destination device is normally discarded except for broadcast traffic under the destination address of FF:FF:FF:FF:FF which is processed by all devices recieved in that broadcast domain.

An ARP request is an example of a Layer 2 broadcast

There are technologies which do not use MAC addresses such as Frame Relay, using an entirely different Layer 2 data link protocol altogether.

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