Ethernet Switching Summary (21.5)
The following is a summary of each topic in the chapter and some questions for your reflection.
What Did I Learn in this Module? (21.5.1)
• Ethernet—There is no official local area networking standard protocol, but over time, one technology, Ethernet, has become more common than the others. Ethernet protocols define how data is formatted and how it is transmitted over the wired network. The Ethernet standards specify protocols that operate at Layer 1 and Layer 2 of the OSI model. Ethernet has become a de facto standard, which means that it is the technology used by almost all wired local area networks.
IEEE maintains the networking standards, including Ethernet and wireless standards. Each technology standard is assigned a number that refers to the committee that is responsible for approving and maintaining the standard. The 802.3 Ethernet standard has improved over time.
Ethernet switches can send a frame out all ports (excluding the port it was received from). Each host that receives this frame examines the destination MAC address and compares it to their MAC address. It is the Ethernet NIC card that examines and compares the MAC address. If it does not match the host MAC address, the rest of the frame is ignored. When it is a match, that host receives the rest of the frame and the message it contains.
• Ethernet Frames—Ethernet is defined by data link layer 802.2 and 802.3 protocols. Ethernet supports data bandwidths from 10 Mbps up to 100 Gps. EEE 802 LAN/MAN protocols, including Ethernet, use two separate sublayers of the data link layer to operate: LLC and MAC.
• LLC Sublayer – This IEEE 802.2 sublayer communicates between the networking software at the upper layers and the device hardware at the lower layers. It places information in the frame that identifies which network layer protocol is being used for the frame. This information allows multiple Layer 3 protocols, such as IPv4 and IPv6, to use the same network interface and media.
• MAC Sublayer – This sublayer (IEEE 802.3, 802.11, or 802.15 for example) is implemented in hardware and is responsible for data encapsulation and media access control. It provides data link layer addressing and is integrated with various physical layer technologies. Data encapsulation includes the Ethernet frame, Ethernet Addressing, and Ethernet error detection.
Ethernet LANs of today use switches that operate in full-duplex. Full-duplex communications with Ethernet switches do not require access control through CSMA/CD. The minimum Ethernet frame size is 64 bytes and the expected maximum is 1518 bytes. The fields are Preamble and Start Frame Delimiter, Destination MAC address, Source MAC address, Type / Length, Data, and FCS. This includes all bytes from the destination MAC address field through the FCS field.
• Ethernet MAC Address—An Ethernet MAC address consists of a 48-bit binary value. Hexadecimal is used to identify an Ethernet address because a single hexadecimal digit represents four binary bits. Therefore, a 48-bit Ethernet MAC address can be expressed using only 12 hexadecimal values.
A unicast MAC address is the unique address that is used when a frame is sent from a single transmitting device to a single destination device. The process that a source host uses to determine the destination MAC address associated with an IPv4 address is ARP. The process that a source host uses to determine the destination MAC address associated with an IPv6 address is ND.
The features of an Ethernet broadcast are as follows:
• It has a destination MAC address of FF-FF-FF-FF-FF-FF in hexadecimal (48 ones in binary).
• It is flooded out all Ethernet switch ports except the incoming port.
• It is not forwarded by a router.
The features of an Ethernet multicast are as follows:
• There is a destination MAC address of 01-00-5E when the encapsulated data is an IPv4 multicast packet and a destination MAC address of 33-33 when the encapsulated data is an IPv6 multicast packet.
• There are other reserved multicast destination MAC addresses for when the encapsulated data is not IP, such as STP and LLDP.
• It is flooded out all Ethernet switch ports except the incoming port, unless the switch is configured for multicast snooping.
• It is not forwarded by a router, unless the router is configured to route multicast packets.
• The MAC Address Table—A Layer 2 Ethernet switch uses Layer 2 MAC addresses to make forwarding decisions. It is completely unaware of the data (protocol) being carried in the data portion of the frame. An Ethernet switch examines its MAC address table to make a forwarding decision for each frame. The MAC address table is sometimes referred to as a CAM table.
The switch dynamically builds the MAC address 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 destination MAC address is a unicast address, the switch will look for a match between the destination MAC address of the frame and an entry in its MAC address table. If the destination MAC address is in the table, it will forward the frame out the specified port. If the destination MAC address is not in the table, the switch will forward the frame out all ports except the incoming port. This is called an unknown unicast.
As a switch receives frames from different devices, it is able to populate its MAC address table by examining the source MAC address of every frame. When the MAC address table of the switch contains the destination MAC address, it is able to filter the frame and forward out a single port. A switch can have multiple MAC addresses associated with a single port. This is common when the switch is connected to another switch. The switch will have a separate MAC address table entry for each frame received with a different source MAC address. When a device has an IP address that is on a remote network, the Ethernet frame cannot be sent directly to the destination device. Instead, the Ethernet frame is sent to the MAC address of the default gateway, the router.
Marcy and Vincent had no idea how their small business network operated. It’s not much more complicated than my own home network. I did not realize there were protocols that ensure how my devices interact with each other and with the internet. Think about your home network, or the network at school or work. Do you understand the difference between your device’s IP address and its MAC address? How does this knowledge help you better understand how your network operates?