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Figure 4.8 Frames sent from Host A to Host B are forwarded from Segment 1 to Segment 2 by Switch 2, which is then forwarded from Segment 2 to Segment 1 by Switch 1 and then re-forwarded by Switch 2. This process continues ad infinitum.
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3. The original frame is also received by Switch 2 on Segment 1. Switch 2 also records the source MAC of Host A to be on Segment 1. Since Switch 2 does not know where Host B is, like Switch 1 it replicates the frame and sends it out the port connected to Segment 2. 4. Switch 2 receives the replicated frame from Switch 1 in Step 2 above via Segment 2. Switch 2 removes the existing entry for Host A in the MAC FDB and records that Host A belongs to the port attached to Segment 2. Switch 2 then replicates the frame and transmits it out the port attached to Segment 1, where it will be received by Switch 1 on Segment 1. 5. This process continues indefinitely as both Switch 1 and Switch 2 replicate the original frame from Host A onto Segments 1 and 2, causing excessive flooding and MAC FDB instability. Figure 4.9 further illustrates this process at the port level. Switch 1 and Switch 2 will map the MAC address of Host A to Port 0 when it receives the original frame from Host A to Host B. Later, when the replicated frame from Switch 1 arrives at Port 1 of Switch 2, Switch 2 must remove its original entry for Host A and replace it with the new entry for Host A, mapping it to Port 1. This activity causes an unstable database as Switch 2 tries to keep up with the perceived location of Host A.
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4.4 E T H E r N E T PAT H r ED u N DA NC Y: S T P
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Figure 4.9 A further illustration of a loop created in a switched network when there are multiple active paths. The frame from Host A to Host B will continually circulate around the network.
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The problem should now be clear: When there are active redundant paths in an Ethernet switch network, frames can be forwarded indefinitely owing to the nature of the way switches learn MAC addresses and flood frames with an unknown destination. What is needed is a way to preserve redundant paths while avoiding this problem. In the next section, we will examine such a solution.
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Using STP to Prevent Loops
The Spanning Tree Protocol (STP) was developed to solve these instability and broadcast-storm issues. It was invented in 1985 by radia Perlman and was first published as a standard by the IEEE as 802.1D. revisions to STP were published in 1998 and 2004, and the rapid Spanning Tree Protocol (rSTP) was introduced in 1998 as IEEE 802.1w. In 2004, the IEEE incorporated the changes of rSTP into the Spanning Tree Protocol and obsoleted previous versions. This version was published as IEEE 802.1D-2004. Because the current version of STP incorporates all the advances of rSTP, for the remainder of this chapter, we will just use the term STP. You should keep in mind that our use of the acronym from this point forward refers to the current version of STP published in 2004 and not previous, obsolete versions. A few points about STP should be noted at the outset. First, STP is intended to prevent loops in an Ethernet switched network. It does this by selectively blocking ports
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to achieve a loop-free topology. That is, it determines what ports it can put into a nonfunctioning state to prevent loops from occurring, while still allowing frames to reach every destination in the Ethernet network. Second, STP uses a root/branch/leaf model, which determines a single path to each leaf spanning the entire switched network. Finally, because end stations such as workstations or servers rarely act as Ethernet switches, they are usually not part of the STP protocol and do not participate in it. The first version of STP was slow at converging because of various issues with the protocol design. Many enhancements were introduced to improve convergence time with STP version IEEE 802.1d-2004, and the following discussion covers only the procedures in the current version. The exact mechanisms that STP uses to achieve a loopfree topology are the subject to which we now turn. You should keep in mind throughout this discussion that the sole purpose of STP is to build an active loop-free topology (active in the sense that the ports that are blocked can change in response to changed network conditions). In Figure 4.10, STP would create a loop-free topology out of the physically looped network by blocking the ports that connect Switches C and E. All of the processes that make up STP are used to accomplish this seemingly trivial goal. Spanning Tree topology can be thought of as a tree that includes the following components: A root (a root bridge/switch) Branches (LANs and designated bridges/switches) Leaves (end nodes)
Figure 4.10 STP will block the ports between Switches C and E, ensuring a loop-free topology in the switched network.