Editing Spanning Tree Protocol (section)

===Root bridge and the bridge ID===
[[Image:Spanning tree protocol at work 2.svg|thumb|right|250px|An example network. The numbered boxes represent bridges, that is switches in a LAN. The number is the bridge ID. The lettered clouds represent [[network segment]]s. The smallest bridge ID is 3. Therefore, bridge 3 is the root bridge.]]

The ''root bridge'' of the spanning tree is the bridge with the smallest (lowest) bridge ID. Each bridge has a configurable priority number and a MAC address; the bridge ID is the [[concatenation]] of the bridge priority and the MAC address. For example, the ID of a bridge with priority 32,768 and MAC {{MACaddr|0200.0000.1111}} is {{MACaddr|32768.0200.0000.1111}}. The bridge priority default is 32,768 and can be configured only in multiples of 4096.{{efn|Spanning tree incorporated 802.1t, and per 802.1t, uses the 4 most-significant bits of the 802.1d two-octet priority field as priority, and the least-significant 12 bits of that field as the extended system ID.}} When comparing two bridge IDs, the priority portions are compared first and the MAC addresses are compared only if the priorities are equal. The switch with the lowest priority of all the switches will be the root; if there is a tie, then the switch with the lowest priority and lowest MAC address will be the root. For example, if switches ''A'' (MAC = {{MACaddr|0200.0000.1111}}) and ''B'' (MAC = {{MACaddr|0200.0000.2222}}) both have a priority of 32,768 then switch ''A'' will be selected as the root bridge.{{efn|The original 802.1d envisioned the possibility of the root bridge having more than one port on the same [[network segment]], and in that case, the port with the lowest port ID would become the designated port for that network segment, and put into forwarding mode, while its other ports on that same network segment became non-designated ports put into blocking mode. Not all bridge manufacturers follow that rule, some making all ports designated ports and putting them all into forwarding mode.}} If the network administrators would like switch ''B'' to become the root bridge, they must set its priority to be less than 32,768.{{efn|Alternatively the network administrator can configure the switch as a spanning tree root primary or secondary. When configuring the root primary and root secondary the switch will automatically change the priority accordingly, 24,576 and 28,672 respectively with the default configuration.<ref>{{cite web |url=https://www.cisco.com/c/m/en_us/techdoc/dc/reference/cli/nxos/commands/l2/spanning-tree-vlan.html |title=spanning-tree vlan |publisher=[[Cisco Systems]] |access-date=2020-05-04}}</ref>}}