Note that in the figure there is one bridge of priority 16, two of priority 32, and one of priority 64. As shown, the bridge with priority 16 will become the root bridge

because of its low priority. The election calculation for the root bridge by STP in Figure 4.11 is described in the following list: 1. After bridges/switches have initialized, root bridge election begins. 2. The bridge priority can be manually assigned. Since the priority field is 2 bytes or 16 bits, valid values range from 0 to 216 = 65,535 (the default value for AlcatelLucent switches is 32,768). 3. Each bridge/switch sends its BID to every other bridge/switch. As described, the BID is 8 bytes: 2 bytes for bridge priority and 6 bytes that contain the MAC address of the lowest-numbered port on the bridge/switch. 4. Election of the root bridge is determined using the BID, which is made up of the priority and MAC address. The switch with the lowest BID value is selected as the root. In practice, this means that the bridge with the lowest configured bridge priority will become the root bridge. 5. Any subsequent physical change in the network after election of the root bridge will cause an STP recalculation.

Each port on a bridge/switch has a path cost value assigned automatically, depending on the bandwidth. The accumulated path cost from the bridge to the root determines the total cost to reach the root. The default path costs for various link types are shown in Table 4.1.

In the STP calculations, each switch must establish a single path to the root. By doing this, all loops are eliminated. Three values are used in the STP port calculations: Port priority (has a default value but is configurable) Per port cost (the default value is dependent on bandwidth but is configurable) Port MAC address After bridges/switches have initialized, and root and leaf bridges have been selected based on the bridge with the lowest bridge ID, each bridge port participating in the Spanning Tree is assigned either a root, designated, alternate, or back-up role. The designated port is the only port on a given LAN segment that can forward frames to/from that segment in order to avoid loops. The process that ports undergo to determine if they are a root port, designated port, or should go into a discard state is described below: 1. All ports on the root bridge automatically become designated ports. 2. A switch/bridge that is not a root bridge and has ports participating in STP is referred to as a designated bridge/designated switch. 3. The port on a designated bridge that is closest (least path cost) to the root bridge is elected as the root port. To determine the least path cost of each port, Bridge Protocol Data units (BPDus) are received from the root. 4. Non-root ports on each designated bridge providing the least cost path from a particular LAN segment (that the port is connected to) to the root bridge are elected as designated ports. 5. Non-root ports on the designated bridge that do not provide the least path cost from the LAN segment (that the port is connected to) to the root bridge are elected as alternate ports and go into the discard state. This description can seem a little overwhelming, and STP can be confusing with all of its terminology. You should study Figure 4.12 and the explanation that follows it carefully as it illustrates all of the concepts mentioned in the previous steps.

Figure 4.12 The port is blocked because the path through that link to the root bridge is higher than the other path to the root bridge.

Root bridge Designated port Priority 16 Cost Root 10 port Designated bridge Priority 32 Designated port Designated port