Spanning Tree Concepts

STP Operation

Each switch in the broadcast domain initially assumes that it is the root bridge for a spanning tree instance, so the BPDU frames sent contain the BID of the local switch as the root ID. By default, BPDU frames are sent every two seconds after a switch is booted; that is, the default value of the Hello timer specified in the BPDU frame is two seconds. Each switch maintains local information about its own BID, the root ID, and the path cost to the root.

When adjacent switches receive a BPDU frame, they compare the root ID from the BPDU frame with the local root ID. If the root ID in the BPDU is lower than the local root ID, the switch updates the local root ID and the ID in its BPDU messages. These messages indicate the new root bridge on the network. The distance to the root bridge is also indicated by the path cost update. For example, if the BPDU was received on a Fast Ethernet switch port, the path cost would increment by 19. If the local root ID is lower than the root ID received in the BPDU frame, the BPDU frame is discarded.

After a root ID has been updated to identify a new root bridge, all subsequent BPDU frames sent from that switch contain the new root ID and updated path cost. That way, all other adjacent switches are able to see the lowest root ID identified at all times. As the BPDU frames pass between other adjacent switches, the path cost is continually updated to indicate the total path cost to the root bridge. Each switch in the spanning tree uses its path costs to identify the best possible path to the root bridge.

The following summarizes the BPDU process:

Note: Priority is the initial deciding factor when electing a root bridge. If the priorities of all the switches are the same, the device with the lowest MAC address becomes the root bridge.

1. Initially, each switch identifies itself as the root bridge. S2 forwards BPDU frames out all switch ports. (Figure 1)

2. When S3 receives a BPDU from switch S2, S3 compares its root ID with the BPDU frame it received. The priorities are equal, so the switch is forced to examine the MAC address portion to determine which MAC address has a lower value. Because S2 has a lower MAC address value, S3 updates its root ID with the S2 root ID. At that point, S3 considers S2 as the root bridge. (Figure 2)

3. When S1 compares its root ID with the one in the received BPDU frame, it identifies its local root ID as the lower value and discards the BPDU from S2. (Figure 3)

4. When S3 sends out its BPDU frames, the root ID contained in the BPDU frame is that of S2. (Figure 4)

5. When S2 receives the BPDU frame, it discards it after verifying that the root ID in the BPDU matched its local root ID. (Figure 5)

6. Because S1 has a lower priority value in its root ID, it discards the BPDU frame received from S3. (Figure 6)

7. S1 sends out its BPDU frames. (Figure 7)

8. S3 identifies the root ID in the BPDU frame as having a lower value and, therefore, updates its root ID values to indicate that S1 is now the root bridge. (Figure 8)

9. S2 identifies the root ID in the BPDU frame as having a lower value and, therefore, updates its root ID values to indicate that S1 is now the root bridge. (Figure 9)