31 Days Before Your CCNA Exam

44 31 Days Before Your CCNA ExamIn a LAN where all nodes are connected directly to the switch, the throughput of the networkincreases dramatically. With each computer connected to a separate port on the switch, each is in aseparate collision domain and has its own dedicated segment. The three primary reasons for thisincrease are the following:■■■Dedicated bandwidth to each portCollision-free environmentFull-duplex operationSwitching LogicEthernet switches selectively forward individual frames from a receiving port to the port where thedestination node is connected. During this instant, the switch creates a full bandwidth, logicalpoint-to-point connection between the two nodes.Switches create this logical connection based on the source and destination Media Access Control(MAC) addresses in the Ethernet header. Specifically, the primary job of a LAN switch is toreceive Ethernet frames and then make a decision: either forward the frame or ignore the frame. Toaccomplish this, the switch performs three actions:1. Decides when to forward a frame or when to filter (not forward) a frame, based on the destinationMAC address2. Learns MAC addresses by examining the source MAC address of each frame received bythe bridge3. Creates a (Layer 2) loop-free environment with other bridges by using Spanning TreeProtocol (STP)To make the forward or filter decision, the switch uses a dynamically built MAC address tablestored in RAM. By comparing the frame’s destination MAC address with the fields in the table,the switch decides how to forward and/or filter the frame.For example, in Figure 27-1 the switch receives a frame from Host A with the destination MACaddress OC. The switch looks in its MAC table and finds an entry for the MAC address and forwardsthe frame out port 6. The switch also filters the frame by not forwarding it out any otherport, including the port on which the frame was received.In addition to forwarding and filtering frames, the switch will also refresh the timestamp for thesource MAC address of the frame. In Figure 27-1, the MAC address for Host A, OA, is already inthe MAC table. So the switch refreshes the entry. Entries that are not refreshed will eventually beremoved (after 300 seconds in Cisco IOS).Continuing the example in Figure 27-1, assume another device, Host E, is attached to port 10.Host B then sends a frame to the new Host E. The switch does not yet know where Host E islocated. So it forwards the frame out all active ports except for the port on which the frame wasreceived. The new Host E will receive the frame. When it replies to Host B, the switch will learnHost E’s MAC address and port for the first time and store it in the MAC address table.Subsequent frames destined for Host E will only be sent out port 10.