LAN Switching Forwarding
LAN switches can be characterized by the forwarding method that they support. In the store-and-forward switching method, error checking is performed and erroneous frames are discarded. With the cut-through switching method, latency is reduced by eliminating error checking.
With the store-and-forward switching method, the LAN switch copies the entire frame into its onboard buffers and computes the cyclic redundancy check (CRC). The frame is discarded if it contains a CRC error or if it is a runt (less than 64 bytes, including the CRC) or a giant (more than 1518 bytes, including the CRC). If the frame does not contain any errors, the LAN switch looks up the destination address in its forwarding, or switching, table and determines the outgoing interface. It then forwards the frame toward its destination.
With the cut-through switching method, the LAN switch copies only the destination address (the first 6 bytes following the preamble) into its onboard buffers. It then looks up the destination address in its switching table, determines the outgoing interface, and forwards the frame toward its destination. A cut-through switch provides reduced latency because it begins to forward the frame as soon as it reads the destination address and determines the outgoing interface.
Some switches can be configured to perform cut-through switching on a per-port basis until a user-defined error threshold is reached, when they automatically change to store-and-forward mode. When the error rate falls below the threshold, the port automatically changes back to store-and-forward mode.
LAN switches must use store-and-forward techniques to support multilayer switching. The switch must receive the entire frame before it performs any protocol-layer operations. For this reason, advanced switches that perform Layer 3 switching are store-and-forward devices.
LAN Switching Bandwidth
LAN switches also can be characterized according to the proportion of bandwidth allocated to each port. Symmetric switching provides evenly distributed bandwidth to each port, while asymmetric switching provides unlike, or unequal, bandwidth between some ports.
An asymmetric LAN switch provides switched connections between ports of unlike bandwidths, such as a combination of 10BaseT and 100BaseT. This type of switching is also called 10/100 switching. Asymmetric switching is optimized for client/server traffic flows in which multiple clients simultaneously communicate with a server, requiring more bandwidth dedicated to the server port to prevent a bottleneck at that port.
A symmetric switch provides switched connections between ports with the same bandwidth, such as all 10BaseT or all 100BaseT. Symmetric switching is optimized for a reasonably distributed traffic load, such as in a peer-to-peer desktop environment.
A network manager must evaluate the needed amount of bandwidth for connections between devices to accommodate the data flow of network-based applications when deciding to select an asymmetric or symmetric switch.
LAN Switch and the OSI Model
LAN switches can be categorized according to the OSI layer at which they filter and forward, or switch, frames. These categories are: Layer 2, Layer 2 with Layer 3 features, or multilayer.
A Layer 2 LAN switch is operationally similar to a multiport bridge but has a much higher capacity and supports many new features, such as full-duplex operation. A Layer 2 LAN switch performs switching and filtering based on the OSI data link layer (Layer 2) MAC address. As with bridges, it is completely transparent to network protocols and user applications.
A Layer 2 LAN switch with Layer 3 features can make switching decisions based on more information than just the Layer 2 MAC address. Such a switch might incorporate some Layer 3 traffic-control features, such as broadcast and multicast traffic management, security through access lists, and IP fragmentation.
A multilayer switch makes switching and filtering decisions based on OSI data link layer (Layer 2) and OSI network layer (Layer 3) addresses. This type of switch dynamically decides whether to switch (Layer 2) or route (Layer 3) incoming traffic. A multilayer LAN switch switches within a workgroup and routes between different workgroups.
Layer 3 switching allows data flows to bypass routers. The first frame passes through the router as normal to ensure that all security policies are observed. The switches watch the way that the router treats the frame and then replicate the process for subsequent frames. For example, if a series of FTP frames flows from a 10.0.0.1 to 192.168.1.1, the frames normally pass through a router. Multilayer switching observes how the router changes the Layer 2 and Layer 3 headers and imitates the router for the rest of the frames. This reduces the load on the router and the latency through the network.
Review Questions
Q—A multilayer switch mimics the actions of a router when an initial frame passes through a router. What things does the multilayer switch do to the Layer 2 and Layer 3 headers to thoroughly imitate the router?
A—The switch must modify the source and destination MAC addresses in the Layer 2 header so that the frame appears to come from/to the router/workstation. Furthermore, the switch must change things in the Layer 3 header such as the IP time-to-live value.
Q—A LAN switch most closely resembles what type of internetworking device?
A—A LAN switch behaves like a multiport bridge.
Q—Two trunk protocols were described. For what situation would you use the IEEE 802.1Q mode?
A—Whenever you deploy a hybrid of switches from multiple vendors and need to trunk between them. All other trunk protocols work within specific vendor equipment environments.
Q—Which switching method protects network segment bandwidth from errored frames?
Store-and-forward transmits frames only if the frame's integrity is assured. If the switch receives an errored frame, then the switch discards it.
Q—How does a store-and-forward switch know if a frame is errored?
A—The switch uses the CRC to determine whether any changes occurred to the frame since the source generated it. The switch calculates CRC for the received frame and compares it with the CRC transmitted with the frame. If they differ, the frame changed during transit and will be discarded in a store-and-forward switch.
Q—Do VLAN borders cross routers?
A—No. VLANs are broadcast domains and describe the extent that broadcast frames transit the network. Routers do not pass broadcasts. Therefore, the same VLAN cannot exist on two ports of a router.
Q—How does a trunk link differ from an access link?
A—An access link carries traffic for a single VLAN. The traffic on an access link looks like any other Ethernet frame. A trunk link transports traffic for multiple VLANs across a single physical link. Trunks encapsulate Ethernet frames with other information to support the multiplexing technology employed.
Q—Before switches and VLANs, administrators assigned users to a network based not on the user's needs, but on something else. What determined the user network assignment?
A—Administrators previously assigned users to a network based upon the user's physical proximity to a network device or cable.