After the Root Bridge has been elected, it’s time for bridges to designate what are known as Root Ports. Before we can look at how Root Ports work, you need to know something about port costs. Obviously some ports are faster than others, and usually a faster port will be used to interconnect switches. As such, ports have what is known as a cost value, based on their speed. The lower a cost value, the faster a port. Table 3-1 outlines cost values used in STP calculations. It’s worth noting that when originally defined by the IEEE, 1 Gbps seemed like the fastest possible port speed. As such, there are two cost ranges that you’ll find on switches. The first column shows the original IEEE Spanning Tree port costs, and the second shows the new cost numbers. The Cisco 1900 switch uses the original IEEE values by default. Note that port cost values can also be changed.
Table: Original and new IEEE port costs.
|
Port Speed |
Original IEEE Port Cost |
New IEEE Port Cost |
|
1 Gbps |
1 |
4 |
|
100 Mbps |
10 |
19 |
|
10 Mbps |
100 |
100 |
Remember that a cost is associated with a port. These cost values are used in calculating which port will be the Root Port for any given bridge. All Non-Root Bridges will have one Root Port.
The figure below outlines why designating a Root Port is important. In it, bridges are exchanging BPDUs to try and find the lowest cost to the Root Bridge. Note that Bridge A is the Root Bridge in this case. Because it is the Root Bridge, both of its ports have a cost of 0. In this example, all of the bridges are connected using 100Mbps links. The port cost outlined in the table for a 100Mbps link is 19 (using the new IEEE costs).
Let’s walk through the process step by step. The Root Bridge will send out BPDUs with a cost of 0. These BPDUs will go to the 100Mbps port 1 on both bridges B and C. Since these ports have a cost of 19, the cost associated with port 1 on switches B and C reaching the Root Bridge is 19.
Figure: Port costs and Root Ports.
Now since B and C are also connected on segment BC with 100Mbps ports, these ports will also forward out BPDUs. Let’s assume that B is sending a BPDU to C. In it, it will announce a cost of 19 to reach the root. When it reaches C, this bridge will add its port cost of 19 to the value in the BPDU. As such, bridge C knows that it can reach the Root Bridge using port 2 with an aggregate cost of 38, or it can reach the Root Bridge with a cost of 19 using port 1. For bridge C, the Root Port becomes port 1, as it does for Bridge B as well. For both bridges B and C, port 1 represents the lowest cost to the root.
To summarize, a Root Port is the port on a switch that has the lowest cost path to the Root Bridge.
