ccna r&s certification cost

ccna r&s certification cost.4.4 Switching FeaturesAlthough bridges and switches have many differences, they are both Layer 2 devices and both perform the following three basic network functions. . Learning: learning which port the device is connected to. . Forwarding: intelligently forwarding frames to the port to which the destination device is connected.. Avoid Layer 2 loops: Use Spanning Tree Protocol (STP) to avoid loops so that frames do not loop endlessly through the network. These are the features of transparent bridges. Other bridge types include source-route bridging, source-route transparent bridging, and source-route translation bridging, with source-route translation bridging used in mixed-media networks such as those with a mix of Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI). the CCNA exam focuses on transparent bridging, and Token Ring and FDDI are almost deprecated LAN technologies, so only transparent bridging is covered in this book .The term transparent appropriately describes a transparently bridged network: devices connected to the network are unaware that the network contains bridges and switches that forward frames to their destinations. From a physical perspective, a transparently bridged network is essentially a series of star topologies connected together; however, transparent bridges allow the connected devices to assume that all devices within the broadcast domain are located in the same logical network segment, as shown in Figure 4.2.Figure 4.2 Depicting a transparent bridged network from a physical perspective and a logical perspectiveThe next few subsections discuss the three main functions of bridges and switches in more depth. In these subsections, the term switch refers to a Layer 2 device, while the terms bridge and switch have the same meaning when referring to the three main functions.4.4.1 Learning FunctionsOne of the three main functions of a transparent switch is to learn which device is connected to each active port of the switch. As a frame enters a switch port, the switch looks at the source MAC address of the frame and compares it to the port address table (also called the content addressable memory (CAM) table or MAC address table). In the early days of bridge technology, CAM was a special high-speed memory that helped the bridge perform the switching function one forwarding frames out of the correct port. Nowadays, switches use RAM to store MAC addresses, but the term CAM is still commonly used.After a frame is received on a port, the switch looks at the source MAC address in the frame; if there is no corresponding entry in the CAM table, it adds that address (and the source port number) to the CAM table. If the address is included in the CAM table, the switch compares the incoming port with the port specified in the table. If they are different, the switch will update the CAM table with the new port information. This is important because when a device moves from one port to another, you want the switch to be informed of its new location and to forward frames to it correctly (rather than forwarding them to the original port) oWhen you update an entry in the CAM table, the switch also resets the timer for that entry. The switch uses the timer to age the old information in the CAM table to make room for storing the new MAC address. The default setting for the aging timer varies with the switch. Aging is important because the switch will not be able to store new addresses when the CAM table is full. For MAC addresses that are not stored, traffic sent to those addresses will be flooded, which in turn affects the bandwidth of all interfaces on the switch and bridge. After the switch receives traffic whose source MAC address matches a CAM table entry, it also resets the timer for that entry. Therefore, if a device keeps sending information, its MAC address will always be included in the CAM table; if a device keeps not sending information, its MAC address will eventually be removed from the CAM table.