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The first step in the Spanning Tree process is to elect the root bridge. Each switch has a base MAC address, also known as a burned-in address (BIA). Unlike an interface MAC address, the BIA is tied to the switch itself, not a particular interface. A bridge is elected as the root if

 It has the lowest priority—You must configure the priority per VLAN in multiples of 4,096. The reason is that the VLAN ID is appended to the configured priority. The default configured bridge priority is 32,768, so for VLAN 1, the priority would be 32,769 (32,768 + 1), the priority for VLAN 2 would be 32,770, and so on. The priority value can range from 0 to 61,440. All bridges have equal priority by default.

 It has the lowest base MAC address—Assuming the priorities of all the bridges are equal, the bridge with the lowest BIA becomes root.

Here's how the election process works. Initially, each switch assumes it is the root. It places all of its ports into a listening state, and every two seconds, sends a configuration/Hello Bridge Protocol Data Unit (BPDU) out of every nonblocking port. The BPDU is addressed to the multicast MAC address 0100.0ccc.cccd and sourced from the interface MAC. The BPDU itself contains a transmitting bridge identifier (ID) that uniquely identifies the sending switch. The transmitting bridge identifier includes

 Transmitting bridge priority

 Transmitting bridge BIA

 VLAN ID

The BPDU also contains a root identifier that includes the following:

 Root priority

 Root BIA

 VLAN ID

Because each switch considers itself the root, the root identifier and transmitting bridge ID information is initially identical. The following is a BPDU from a switch advertising itself as the root bridge:

Spanning Tree Protocol Protocol Identifier: Spanning Tree Protocol (0x0000) Protocol Version Identifier: Rapid Spanning Tree (2) BPDU Type: Rapid/Multiple Spanning Tree (0x02) BPDU flags: 0x3c, Forwarding, Learning, Port Role: Designated Root Identifier: 32768 / 1 / 00:15:f9:fb:1e:80 Root Bridge Priority: 32768 Root Bridge System ID Extension: 1 Root Bridge System ID: Cisco_fb:1e:80 (00:15:f9:fb:1e:80) Root Path Cost: 0 Bridge Identifier: 32768 / 1 / 00:15:f9:fb:1e:80 Bridge Priority: 32768 Bridge System ID Extension: 1 Bridge System ID: Cisco_fb:1e:80 (00:15:f9:fb:1e:80) Port identifier: 0x8001 Message Age: 0 Max Age: 20 Hello Time: 2 Forward Delay: 15 Version 1 Length: 0

Notice that the root ID and the transmitting bridge ID are the same, indicating that the switch believes it's the root. However, if a switch receives a BPDU with a superior root ID, it stops advertising itself as the root. Instead, it begins advertising the bridge with the superior bridge ID as the root. In the end, all switches in the topology will advertise the same switch—the one with the lowest bridge ID—as the root. That bridge wins the election and becomes the root.

An interesting implication of the election process is that older switches tend to be elected as the root. It's not unheard of for someone to purchase a used replacement switch and plug it into an existing network of newer switches, only to have the used switch take over as root and cause a temporary network outage.

Imagining this process is difficult, so let's look at an example. Consider the switches shown in Figure 2.2. Assuming all switches have equal priority, SW1 would be elected root because it has the lowest base MAC address.

SW1#show spanning-tree vlan 1 VLAN0001 Spanning tree enabled protocol rstp Root ID Priority 32769 Address 0015.f9fb.1e80 This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address 0015.f9fb.1e80 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 sec

Because this spanning tree instance is for VLAN 1, the priority for this bridge is 32,769—the default of 32,768 plus the VLAN ID. If you don't want SW1 to be the root, you can increase its priority in increments of 4,096, like so:

SW1#configure t Enter configuration commands, one per line. End with CNTL/Z. SW1(config)#spanning-tree vlan 1 priority ? <0-61440> bridge priority in increments of 4096 SW1(config)#spanning-tree vlan 1 priority 36864

Notice that 32,768 + 4,096 = 36,864. IOS won't allow priorities that aren't in increments of 4,096. In short order, SW3 (bridge ID 0015.fa83.e900) takes over as the root:

SW1#show spanning-tree vlan 1 VLAN0001 Spanning tree enabled protocol rstp Root ID Priority 32769 Address 0015.fa83.e900 Cost 4 Port 5 (GigabitEthernet1/0) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 36865 (priority 36864 sys-id-ext 1) Address 0015.f9fb.1e80 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 sec

Refer to Figure 2.4. Even though SW3's base MAC address is higher—0015.fa83.e900 is greater than 0015.f9fb.1e80—it's elected because it has a lower priority than SW1, and a lower base MAC address than SW2 and SW4.

Figure 2.4 Converged STP topology with SW3 as the new root

Alternatively, you could select the root bridge you want by decreasing its priority to less than that of the other bridges. For example, if you wanted to specifically choose SW3 as the root bridge, you could decrease its priority to 28,672 (32,768 – 4,096).

The bridge with the next lowest bridge ID to the root gets the status of the designated bridge and is also called the secondary root bridge. The designated bridge stands by, waiting to take over in case the primary root bridge fails or ceases to have the best bridge ID, for example, if you were to increase its priority.