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Lab 2: Basic Spanning Tree Topology Calculation

February 12, 2012

This topology can be seen all over study guides and for good reason as it is a basic topology which distills how STP works on its base level. Find the root switch, and label each interface as a root port, designated port, or blocking port.

Answer

This was pretty straight forward.  SW1 becomes the root bridge because it has the lowest root bridge ID (16384.0200.0000.1111). Not only is its MAC address lower, but its priority is lower as well.

Immediately both ports on the root bridge become designated ports since all ports on root bridges are designated. Default settings indicate a Fast Ethernet link has a STP cost of 19. Fa 0/2 has a SW2->SW1 cost of 19 (0 incoming, 19 added at Fa 0/2). Fa 0/1 on SW3 also has the same value because its SW3->SW1 has the same Fast Ethernet link settings back to SW1. This leaves two ports, SW2’s Fa 0/1 and SW3’s  Fa0/2 to find out which is blocking and which is designated.

STP uses four criteria to determine links:

1. Lowest root bridge ID

2. Lowest root path cost to root bridge

3. Lowest sender bridge ID

4. Lowest sender port ID

1 and 2 both are ties so we’ll have to go to the third item to break the tie. In this case, SW2 has a lower bridge ID than SW3 so SW2’s Fa0/1 becomes the designated port leaving SW3’s Fa 0/3 to go into blocking mode.

STP should now be fully converged.

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