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Chapter 5
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- 1Ethernet Encapsulation 5.1.1.1Operates in the data link layer and the physical layer. It is IEEE 802.2 and 802.3 standards. Ethernet standards define both the Layer 2 protocols and the Layer 1 technologies. Ethernet relies on the two separate sublayers of the data link layer to operate, the Logical Link Control (LLC) and the MAC sublayers.
- 2Logical Link Control (LLC) sublayerHandles the communication between the upper layers and the lower layers. It takes the network protocol data, which is typically an IPv4 packet, and adds control information to help deliver the packet to the destination node. LLC is implemented in software, and is independent of the hardware. In a computer, the LLC can be considered the driver software for the NIC. The specifics are listed in the IEEE 802.2 standards.
- 3Midea Access Control (MAC) sublayerMAC constitutes the lower sublayer of the data link layer. MAC is implemented by hardware, typically in the computer NIC. The specifics are listed in the IEEE 802.3 standards. It has two primary responsibilities:
*Data encapsulation= frame assembly before transmission, and frame disassembly upon reception of a frame. Data encapsulation provides three primary functions: 1) Frame delimiting, 2) Addressing, 3) Error detection.
*Media access control= is responsible for the placement of frames on the media and the removal of frames from the media. - 4Ethernet frame II StructureAt the data link layer, the frame structure is nearly identical for all speeds of Ethernet. The Ethernet frame structure adds headers and trailers around the Layer 3 PDU to encapsulate the message being sent:
Preamble=8 bytes
Destination address=6 bytes
Source Address=6 bytes
Type=2 bytes
Data= 46-1500 bytes
Frame check Sequence(FCS)= 4 bytes - 5Ethernet Frame FieldsThe minimum Ethernet frame size is 64 bytes and the maximum is 1518 bytes. This includes all bytes from the Ethernet frame II structure. The Preamble field is not included when describing the size of a frame. Any frame less than 64 bytes in length is considered a “collision fragment” or “runt frame” and is automatically discarded by receiving stations. Frames with more than 1500 bytes of data are considered “jumbo” or “baby giant frames”.
- 6Frame ProcessingThe MAC address is often referred to as a burned-in address (BIA) because, historically, this address is burned into ROM (Read-Only Memory) on the NIC. This means that the address is encoded into the ROM chip permanently. When a NIC receives an Ethernet frame, it examines the destination MAC address to see if it matches the device’s physical MAC address stored in RAM. If there is no match, the device discards the frame. If there is a match, it passes the frame up the OSI layers, where the de-encapsulation process takes place.
- 7Switch FundamentalsA Layer 2 Ethernet switch uses MAC addresses to make forwarding decisions. It is completely unaware of the protocol being carried in the data portion of the frame, such as an IPv4 packet. The switch makes its forwarding decisions based only on the Layer 2 Ethernet MAC addresses.
- 8Learning MAC AddressesThe switch dynamically builds the content addressable memory (CAM) table by examining the source MAC address of the frames received on a port. The switch forwards frames by searching for a match between the destination MAC address in the frame and an entry in the MAC address table.
*If the source MAC address does not exist, it is added to the table along with the incoming port number.
*If the source MAC address does exist, the switch updates the refresh timer for that entry. By default, most Ethernet switches keep an entry in the table for 5 minutes. - 9Filtering FramesSince a switch knows where to find a specific MAC address, it could filter frames to that port only. Filtering is not done if the destination MAC is not in the CM table.
- 10Frame Forwarding Methods on Cisco SwitchesSwitches use one of the following forwarding methods for switching data between network ports:
Store-and-forward switching
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Cut-through switching - 11Store-and-forward switchingwhen the switch receives the frame, it stores the data in buffers until the complete frame has been received. During the storage process, the switch analyzes the frame for information about its destination. In this process, the switch also performs an error check using the Cyclic Redundancy Check (CRC) trailer portion of the Ethernet frame.
- 12Cut-through switchingIn cut-through switching, the switch acts upon the data as soon as it is received, even if the transmission is not complete. The switch buffers just enough of the frame to read the destination MAC address so that it can determine to which port to forward the data. The destination MAC address is located in the first 6 bytes of the frame following the preamble. The switch looks up the destination MAC address in its switching table, determines the outgoing interface port, and forwards the frame onto its destination through the designated switch port. The switch does not perform any error checking on the frame. There are two variants of cut-through switching:
Fast-forward switching
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Fragment-free switching - 13Fast-forward switchingFast-forward switching offers the lowest level of latency. Fast-forward switching immediately forwards a packet after reading the destination address. Because fast-forward switching starts forwarding before the entire packet has been received, there may be times when packets are relayed with errors. This occurs infrequently, and the destination network adapter discards the faulty packet upon receipt. In fast-forward mode, latency is measured from the first bit received to the first bit transmitted. Fast-forward switching is the typical cut-through method of switching.
- 14Fragment-free switchingIn fragment-free switching, the switch stores the first 64 bytes of the frame before forwarding. Fragment-free switching can be viewed as a compromise between store-and-forward switching and fast-forward switching. The reason fragment-free switching stores only the first 64 bytes of the frame is that most network errors and collisions occur during the first 64 bytes. Fragment-free switching tries to enhance fast-forward switching by performing a small error check on the first 64 bytes of the frame to ensure that a collision has not occurred before forwarding the frame. Fragment-free switching is a compromise between the high latency and high integrity of store-and-forward switching, and the low latency and reduced integrity of fast-forward switching.
- 15Memory Buffering on Switches
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- Fichas sobre Chapter 5, creado por Javier Campos el 06/02/2019.
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- Javier Campos
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