CCNA: Layer 2 Ethernet Frame

CCNA: Ethernet Frame

Today let’s talk about the Ethernet Frame and its different fields. Understanding the Ethernet Frame and fields helps to understand the communication between systems and troubleshooting when things are not in place.

A typical Ethernet frame looks like the one in Figure 1.

CCNA Ethernet Frame

Figure 1: Ethernet Frame

Let’s assume PC 1 receives data from the upper layers of the OSI model, Ethernet here is working as a conveyer. The Upper layers have given Ethernet a payload and asked it to give it to PC 2. The Ethernet wraps the Payload with a header and a trailer. Fields in the headers and their size (in Bytes) are mentioned in Figure 1.

Preamble: this informs the receiving system that a frame is starting and enables the synchronization.

Start Frame Delimiter (SFD): Indicates that the destination MAC address starts with the next byte.

Destination MAC (DMAC): Identifies the NIC of the destination system. In this case PC-2 reads the MAC address and knows DMAC contains its MAC address and I should accept the frame. All other systems on the LAN read the MAC and ignore the frame as the message is not for them as the DMAC value does not match with their hardware address.

Source MAC (SMAC): Identifies the NIC of the source system. This is how a system knows where the message has come from and whom to respond to.

EtherType: this field defines which layer 3 protocol will handle the processing of the frame at the next top layer, such as IPv4 or IPv6.

Payload: It is the data that is transmitted. This data is not just the data application created, but include the header information added by other upper layers i.e. TCP headers – that

Frame Check Sequence (FCS): allows detection of the data corruption during transfer. The methods that are used to find the errors are CRC or checksum. When PC-1 creates a frame, it performs the mathematical calculations on header and payload bits. It stores the result of this calculation in the FCS field. When PC-2 receives the frame, it performs the same calculation, and the result is compared with the FCS value. If the calculated result of PC-2 matches with the FCS value then there is no error during transmission else it is assumed that the data is corrupt – and the frame is discarded.

I hope you find this brief article informative!

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