Specifications

CHAPTER 3

Spanning Tree

Table 3-1 Spanning Tree Costs

Link Speed Previous IEEE Specification Current IEEE Specification

10 Mb/s 100 100

100 Mb/s 10 19

1 Gbps 1 4

10 Gbps 1 2

The STP Election

Spanning Tree builds paths out from a starting point, the “root” of the tree. The first step in selecting paths is to identify

this root device. Then each device selects its best path back to the root, according to the criteria laid out in the previous

sections (lowest root BID, lowest cost, lowest advertising BID, lowest port ID).

Root Bridge Election

Looking at Figure 3-1, first select the root bridge. Assume each switch uses the default priority.

n Switch A BID = 80–00–00–0c-11–11–00–11

n Switch B BID = 80–00–00–0c–26–78–10–10

n Switch C BID = 80–00–00–0c-32–1a-bc-de

n Switch D BID = 80–00–00–0c-81–81–11–22

n Switch E BID = 80–00–00–0c–26–79–22–22

Switch A has the lowest BID, so it is the root. Each nonroot switch must now select a root port.

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CCNP SWITCH 642-813 Quick Reference by Denise Donohue

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