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Spanning Tree Protocol
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FIGURE 14-9 RP
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LAN Segment-B A Switch-5 00000001A134 10 Switch-2 000100010001 B 10 DP LAN Segment-D RP G Switch-4 000000011787 40 Switch-3 00010000AC1D H 20 RP LAN Segment-A D 20 DP C F RP LAN Segment-C 10 10 DP 10 I J E 20 DP 10
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Root and designated ports
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Switch-1 000000011234 ROOT
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Going back to our network example, let s start with the easiest segments: B and C For Switch-1, the accumulated path cost for LAN Segment-B is 0, Switch-2 is 20, and Switch-5 is 10 Since the root switch (Switch-1) has the lowest accumulated path cost, its local port (J) becomes the designated port for LAN Segment-B This process is also true for LAN Segment-C the root switch has the lowest accumulated path cost (0), making port I on Switch-1 the designated port for LAN Segment-C LAN Segment-A has two choices: Switch-3 s D port and Switch-4 s H port Switch-3 has the lower accumulated path cost: 10 versus Switch-4 s 50 Therefore, Switch-3 s D port becomes the designated port for LAN Segment-A LAN Segment-D also has two choices for a designated port: Switch-5 s B port and Switch-4 s G port Switch-5 has an accumulated path cost of 10, and Switch-4 has a cost of 30 Therefore Switch-5 s B port becomes the designated port for LAN Segment-D Figure 14-9 shows the updated STP topology for our network, where DP represents the designated ports for the LAN segments
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Changing Port States
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After the designated ports are chosen, the switches will move their root and designated ports through the various states: blocking, listening, learning, and forwarding, whereas any other ports will remain in a blocked state Figure 14-10 shows the ports in a blocking state, designated by an X Remember that on Switch-2,
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14: Switches and Redundancy
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FIGURE 14-10 RP
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LAN Segment-B A Switch-5 00000001A134 10 Switch-2 000100010001 B 10 DP LAN Segment-D RP G Switch-4 000000011787 F RP LAN Segment-C 10 E 20 DP 10
Ports in a blocking state
10 DP 10 I
X 40
20 RP
C Switch-3 00010000AC1D D LAN Segment-A
Switch-1 000000011234 ROOT
20 DP
only Port F (the root port) is in a forwarding state: Port E will remain in a blocking state In this example, two ports are left in a blocking state: Switch-2 s E port and Switch-4 s G port STP guarantees only a layer 2 loop-free topology it does not guarantee an optimal topology! For example, in the network shown in Figure 14-10, networking devices on LAN Segment-A would have to go through Switches 3, 1, and 5 in order to reach LAN Segment-D, since Switch-4 s G port is in a blocked state
CERTIFICATION OBJECTIVE 1403
Rapid Spanning Tree Protocol
The 8021d standard was designed back when waiting for 30 to 50 seconds for layer 2 convergence wasn t a problem However, in today s networks, this can cause serious performance problems for networks that use real-time applications, such as voice over IP (VoIP) or video To overcome these issues, Cisco developed proprietary bridging features called PortFast (discussed earlier), UplinkFast, and BackboneFast The problem with these features, however, is that they are proprietary to Cisco
Rapid Spanning Tree Protocol
The Rapid Spanning Tree Protocol (RSTP) is an IEEE standard, defined in 8021w, which is interoperable with 8021d and an extension to it With RSTP, there are only three port states:
Discarding Learning Forwarding
A port in a discarding state is basically the grouping of 8021d s blocking, listening, and disabled states The following sections cover some of the enhancements included in RSTP
Additional Port Roles
With RSTP, there is still a root switch and there are still root and designated ports, performing the same roles as those in 8021d However, RSTP adds two additional port types: alternate ports and backup ports These two ports are similar to the ports in a blocking state in 8021d An alternate port is a port that has an alternative path or paths to the root but is currently in a discarding state A backup port is a port on a segment that could be used to reach the root switch, but an active port is already designated for the segment The best way to look at this is that an alternate port is a secondary, unused root port, and a backup port is a secondary, unused designated port Given these new port roles, RSTP calculates the final spanning tree topology the same way as 8021d Some of the nomenclature was changed and extended, and this is used to enhance convergence times, as you will see later in the RSTP Convergence Features section
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