Ethernet Introduction for CCNA R&S

[lagapidis (Lazarus Agapidis)]

Hello Himanshu

Let me refer you to a previous answer in this thread:

Introduction to Ethernet

Hello Mohammad Yes, there are several types of Ethernet framing, two of which are the most common that you mention above. Ethernet II has a two byte EtherType field that identifies the upper layer protocol (e.g. IP) that encapsulates the frame. For example, a value of 0x0800 indicates an IPv4 packet while a value of 0x0806 indicates an ARP frame. The IEEE 802.3 standard changes the role of the EtherType field to a “Data Length Field”. Later on, because both types were in wide use, IEEE 802.…

Now concerning your specific question: How will an IEEE802.3 frame know what the upper layer protocol in use is if this field is used as a length field? Remember that the Data Link layer is composed of two sub-layers, the Media Access Control (MAC) layer and the Logical Link Contro (LLC) layer. The LLC layer can add an additional header extension to an Ethernet frame called a SubNetwork Access Protocol (SNAP) header. This SNAP header includes things like an organization code and an Ether Type code which tells the device what upper layer protocol is being used.

Keep in mind that this information is beyond the scope of a CCNA R&S exam.

I hope this has been helpful!

Laz