Electing Root Ports

Electing Root Ports

Now that a reference point has been nominated and elected for the entire switched network, each nonroot switch must figure out where it is in relation to the root bridge. This action can be performed by selecting only one root port on each nonroot switch. The root port always points toward the current root bridge.

STP uses the concept of cost to determine many things. Selecting a root port involves evaluating the root path cost. This value is the cumulative cost of all the links leading to the root bridge. A particular switch link also has a cost associated with it, called the path cost. To understand the difference between these values, remember that only the root path cost is carried inside the BPDU. (Refer to Table 2.2.) As the root path cost travels along, other switches can modify its value to make it cumulative. The path cost, however, is not contained in the BPDU. It is known only to the local switch where the port (or“path” to a neighboring switch) resides.

Path costs are defined as a 1-byte value, with the default values shown in Table 2.3. Generally, the higher the bandwidth of a link, the lower the cost of transporting data across it. The original IEEE 802.1D standard defined path cost as 1000 Mbps divided by the link bandwidth in megabits per second. These values are shown in the center column of the table. Modern networks commonly use Gigabit and 10-Gigabit Ethernet, which are both either too close to or greater than the maximum scale of 1000 Mbps. The IEEE now uses a nonlinear scale for path cost, as shown in the right column of the table.

Table 6-3 STP Path Cost

                                                                                                                   

Link Bandwidth                                              Old STP Cost                                                   New STP Cost                 

4 Mbps                                                 250                                                 250                     

10 Mbps                                               100                                                 100                     

16 Mbps                                                63                                                   62                      

45 Mbps                                                22                                                   39                      

100 Mbps                                              10                                                   19                      

155 Mbps                                               6                                                    14                      

622 Mbps                                               2                                                     6                       

1 Gbps                                                    1                                                     4                       

10 Gbps                                                  0                                                     2                       

The root path cost value is determined in the following manner:

1. The root bridge sends out a BPDU with a root path cost value of 0 because its ports sit directly on the root bridge.

2. When the next-closest neighbor receives the BPDU, it adds the path cost of its own port where the BPDU arrived. (This is done as the BPDU is received.)

3. The neighbor sends out BPDUs with this new cumulative value as the root path cost.

4. The root path cost is incremented by the ingress port path cost as the BPDU is received at each switch down the line.

5. Notice the emphasis on incrementing the root path cost as BPDUs are received. When computing the spanning-tree algorithm manually, remember to compute a new root path cost as BPDUs come in to a switch port, not as they go out.

After incrementing the root path cost, a switch also records the value in its memory. When a BPDU is received on another port and the new root path cost is lower than the previously recorded value, this lower value becomes the new root path cost. In addition, the lower cost tells the switch that the path to the root bridge must be better using this port than it was on other ports. The switch has now determined which of its ports has the best path to the root: the root port.

Figure 2.4 shows the same network from Figure 2.3 in the process of root port selection.

Figure 2.4 Example of Root Port Selection

The root bridge, Switch A, already has been elected. Therefore, every other switch in the network must choose one port that has the best path to the root bridge. Switch B selects its port gi1/0/1, with a root path cost of 0 plus 19. Port gi1/0/2 is not chosen because its root path cost is 0 (BPDU from Switch A) plus 19 (path cost of A–C link), plus 19 (path cost of C–B link), or a total of 38. Switch C makes an identical choice of port gi1/0/1.