200 301 braindumps

200 301 braindumps.1.2.2 Spanning Tree Protocol (IEEE802.1D)Without Spanning Tree Protocol (STP), a LAN with redundant links causes Ethernet frames to loop indefinitely within the network. When STP is enabled, some switches block ports so that those ports do not forward frames.STP intelligently selects the ports to block, but always keeps the following two things in mind.. all devices in the VLAN can send frames to all other devices. in other words. STP does not block too many ports. it just truncates some parts of the LAN from other parts. STP achieves some balance by allowing frames to be presented to each device without creating the problem of frames looping back and forth across the network indefinitely.STP performs an additional check on each interface before the switch sends or receives user data using the interface to avoid the appearance of loopback frames. This check is as follows: if the port is in the STP forwarding state in that VLAN, then it is used normally, if the port is in the STP blocking state, then all user data is blocked and no user data is sent or received on that interface in that VLAN. Note that these STP states do not change the other information you already have about the switch interface. The connected/unconnected interface state does not change. The operational state of the interface (access or trunk) also does not change. the STP simply adds this additional STP state to completely shut down the interface in question with a blocking state. In most cases, the last two sentences summarize the purpose of STP. As for the specifics of how STP works it can take quite a bit ofof time to study and practice. The second part of this chapter first explains the need for STP and how STP resolves loopback frames, followed by how STP chooses which switch port should be blocked to accomplish the task of STP.1. The Need for Spanning TreeIf the LAN wants to have redundant links and does not use STP, then there are three common problems that arise, and STP can avoid all three of these problems in an Ethernet LAN. In fact, these three problems are a side effect of the situation, i.e. without STP, some Ethernet frames will loop back in the network repeatedly for a considerable period of time (hours, days, strictly speaking, if the LAN devices and links do not stop working, these frames may loop back indefinitely).A frame that loops back and forth in the network can result in what is called a broadcast storm. Broadcast storms occur when a broadcast or multicast frame or a unicast frame with an unknown destination loops back indefinitely in the LAN. Broadcast storms can fill some links with useless copies of many identical frames, crowding out the truly useful frames, and can severely impact PC performance by requiring end-user PCs to handle too many broadcast frames.For ease of understanding, Figure 1-3 shows a simple example of a broadcast storm, where Bob first sends out a broadcast frame. The dashed portion of the figure identifies the process by which the switch forwards frames when STP is not in use. Hint: Bob's initial broadcast makes a copy of the original frame to port Gi0/1 via SW3, which is also forwarded in the other direction. The frame is not shown in the figure for ease of observation.Remember the LAN switch logic that you reviewed at the beginning of this chapter? This logic tells the switch to send broadcast frames outbound on all interfaces of the same VLAN (except the ingress port of that broadcast frame). In this diagram, this means that it is SW3 that forwards Bob's frame to SW2, which forwards the frame to SW1, which then forwards the frame to SW3, which forwards it again to SW2, etc., and so on and so forth in a loop.When a broadcast storm occurs, the frame loops back repeatedly as shown in Figure 1-3 until one of the following three scenarios occurs and the frame loopback stops: one, one of the switch interfaces is shut down; two, the switch is restarted; and three, the loop is broken for some reason. Of course, the above can happen in the opposite direction, with Bob sending the initial frame, SW3 forwarding" a copy to SW1,SW1 forwarding it to SW2,and so on