Overview of Layer 2 Networking
Layer 2, also known as the Data Link Layer, is the second level in the seven-layer OSI reference model for network protocol design. Layer 2 is equivalent to the link layer (the lowest layer) in the TCP/IP network model. Layer2 is the network layer used to transfer data between adjacent network nodes in a wide area network or between nodes on the same local area network.
A frame is a protocol data unit, the smallest unit of bits on a Layer 2 network. Frames are transmitted to and received from devices on the same local area network (LAN). Unilke bits, frames have a defined structure and can be used for error detection, control plane activities and so forth. Not all frames carry user data. The network uses some frames to control the data link itself..
At Layer 2, unicast refers to sending frames from one node to a single other node, whereas multicast denotes sending traffic from one node to multiple nodes, and broadcasting refers to the transmission of frames to all nodes in a network. A broadcast domain is a logical division of a network in which all nodes of that network can be reached at Layer 2 by a broadcast.
Segments of a LAN can be linked at the frame level using bridges. Bridging creates separate broadcast domains on the LAN, creating VLANs, which are independent logical networks that group together related devices into separate network segments. The grouping of devices on a VLAN is independent of where the devices are physically located in the LAN. Without bridging and VLANs, all devices on the Ethernet LAN are in a single broadcast domain, and all the devices detect all the packets on the LAN.
Forwarding is the relaying of packets from one network segment to another by nodes in the network. On a VLAN, a frame whose origin and destination are in the same VLAN are forwarded only within the local VLAN. A network segment is a portion of a computer network wherein every device communicates using the same physical layer.
Layer 2 contains two sublayers:
- Logical link control (LLC) sublayer, which is responsible for managing communications links and handling frame traffic.
- Media access control (MAC) sublayer, which governs protocol
access to the physical network medium. By using the MAC addresses
that are assigned to all ports on a switch, multiple devices on the
same physical link can uniquely identify one another.
The ports on a switch operate in access mode; ports connect to a network device such as a desktop computer, an IP telephone, a printer, a file server, or a security camera. The port itself belongs to a single VLAN. The frames transmitted over an access interface are normal Ethernet frames.
Including the sublayers, Layer 2 on the QFX Series supports the following functionality:
- Unicast, multicast, and broadcast traffic.
- Bridging.
- VLAN 802.1Q—Also known as VLAN tagging, this protocol allows multiple bridged networks to transparently share the same physical network link by adding VLAN tags to an Ethernet frame.
- Extension of Layer 2 VLANs across multiple switches using Spanning Tree Protocol (STP) prevents looping across the network.
- MAC learning, including per-VLAN MAC learning and Layer 2 learning suppression–This process obtains the MAC addresses of all the nodes on a network
- Link aggregation—This process groups of Ethernet interfaces at the physical layer to form a single link layer interface, also known as a link aggregation group (LAG) or LAG bundle
- Storm control on the physical port for unicast, multicast, and broadcast
- STP support, including 802.1d, RSTP, MSTP, and Root Guard
