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Embed code for: 20.10 Physical Redundancy in a LAN
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20.10 Physical Redundancy in a LAN
Physical Redundancy in a LAN Section 20: Implementing VLANs and Trunks Loops can affect performance in a switched LAN, and STP is a solution. Loops may occur in the network as part of a design strategy for redundancy. Adding switches to LANs can add the benefit of redundancy. Connecting two switches to the same network segments ensures continuous operation if there are problems with one of the segments. Redundancy can ensure the availability of the network at all times. However, when switches are used for redundancy in a network, loops are a potential problem. When a host on one network segment transmits data to a host on another network segment, and the two are connected by two or more switches, each switch receives the data frames, looks up the location of the receiving device, and forwards the frame. Because each switch forwards the frame, each frame is duplicated. A loop results, and the frame circulates between the two paths without being removed from the network. The MAC address tables may also be updated with incorrect address information, resulting in inaccurate forwarding. In the topology that is shown in the figure, suppose that host A sends a frame to host B. Host A resides on network segment A, and host B resides on network segment B. Redundant connections between hosts ensure continuous operation if a segment fails. For this example, it is assumed that none of the switches have learned the address of host B. Switch 1 receives the frame that is destined for host B and floods it out to switches 2 and 3. Switch 2 and switch 3 both receive the frame from host A (via switch 1) and correctly learn that host A is on segment 1 and 2. Each switch forwards the frame to switch 4. Switch 4 receives two copies of the frame from host A, one copy through switch 2 and one copy through switch 3. Assume that the frame from switch 2 arrives first. Switch 4 learns that host A resides on segment 3. Because switch 4 does not know where host B is connected, it forwards the frame to all of its ports (except the incoming port) and therefore to host B and switch 3. When the frame from switch 3 arrives at switch 4, switch 4 updates its table to indicate that host A resides on segment 4. It then forwards the frame to host B and switch 2. Switches 2 and 3 now change their internal tables to indicate that host A is on segment 3 and 4. If the initial frame from host A was a broadcast frame, both switches forward the frames endlessly. They would use all of the available network bandwidth and block transmission of other packets on both segments. This situation is called a "broadcast storm." The solution to loops is STP, which manages the physical paths to given network segments. STP provides physical path redundancy while preventing the undesirable effects of active loops in the network. STP is on by default in Cisco Catalyst switches. STP behaves as follows: STP forces certain ports into a standby state so that they do not listen to, forward, or flood data frames. The overall effect is that there is only one path to each network segment that is active at any time. If there is a problem with connectivity to any of the segments within the network, STP reestablishes connectivity by automatically activating a previously inactive path, if one exists. Up Next: InterVLAN Routing