up:: [[Networking MOC]] tags:: #note/boat #note/develop #on/computing/networking # Ethernet LAN Switching - Most of this is Layer 2 stuff - Routers are used to connect separate LANs ## Ethernet Frames - Header has 5 fields - Preamble & SFD (Start Frame Delimiter): Allows device to be prepared to receive the rest of the data in the frame - Destination: Layer 2 address where data is being sent - Source: Layer 2 address of the device that sent the frame - Type: Indicated the Layer 3 protocol used in the encapsulated packet - Sometimes this is a "Length" field, indicating the length of the encapsulated data - Trailer has 1 field - FCS (Frame Check Sequence): used by the receiving device to detect errors - Header + Trailer = 26 bytes - Unicast Frame = frame destined for a single target ### Preamble & SFD - Preamble - 7 bytes long (56 bits) - 10101010 * 7 - allows devices to synchronize their receiver clocks, to make sure they're ready to receive the rest of the frame - SFD - 1 byte - 10101011 - Marks the end of the preamble, and the beginning of the rest of the frame ### Destination and Source - Indicate the devices receiving and sending the frame - Consist of MAC (Media Access Control) addresses - 6 byte (48 bit) address of the physical device, assigned to device when made ### Type/Length - 2 bytes - Value of 1500 or less == length of the encapsulated packet - Value of 1536 or greater == type of encapsulated packet (length is determined via other methods) - IPv4 = 0x0800 (2048) - IPv6 = 0x86DD (34525) ### FCS - Frame Check Sequence - 4 bytes - Detects corrupted data by running a CRC (Cyclic Redundancy Check) algorithm over received data ## MAC Addresses - 6 byte physical address assigned to a device when it is made - Different from an IP address - Also called a BIA (Burned-In Address) - Globally unique - First 3 bytes are the OUI (Organizationally Unique Identifier), which is assigned to the company making the device - Last 3 bytes are unique to the device - Written as 12 hex characters ## Switching For this example, let's say that PC1 is sending a frame to PC2 - PC1 sends the frame to the switch - The switch looks at the source address of the frame and uses that to learn where PC1 is - Switch associates the source address with the interface it's plugged into (Dynamically learned MAC address) - Unknown unicast frame == frame for which the switch doesn't have an entry in its MAC address table - Switch doesn't know where to send the frame, so it floods the frame (forward the frame out of all its interfaces except the one it received the frame on) - If a device receives a frame with a destination address that doesn't match its own, it's just ignored - PC2 receives the frame and processes it - The switch can't update its MAC address table with PC2's information unless PC2 sends something to it - Known Unicast Frame == frame for which the switch already has the destination device in it's MAC address table - On Cisco switches, MAC addresses are removed from the table after 5 minutes of inactivity - Every switch keeps a MAC address table and dynamically fills it by looking at the source addresses of frames it receives ## References _Free CCNA | Ethernet LAN Switching (Part 1) | Day 5 | CCNA 200-301 Complete Course_. YouTube Video. Vol. 5. 63 vols. CCNA 200-301 Complete Course, 2019. [https://youtu.be/u2n762WG0Vo?si=fMqNaeyslRXk3dQN](https://youtu.be/u2n762WG0Vo?si=fMqNaeyslRXk3dQN). _Free CCNA | Ethernet LAN Switching (Part 2) | Day 6 | CCNA 200-301 Complete Course_. YouTube Video. Vol. 6. 63 vols. CCNA 200-301 Complete Course, 2019. [https://youtu.be/5q1pqdmdPjo?si=tfgJI_iHyRzg9JYa](https://youtu.be/5q1pqdmdPjo?si=tfgJI_iHyRzg9JYa).