what is the root bridge in STP?

One switch is elected as the root switch.

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# The shortest distance to the root switch is calculated for each switch.

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# A designated switch is selected. This is the switch closest to the root switch through which frames will be forwarded to the root.

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# A port for each switch is selected. This is the port providing the best path from the switch to the root switch.

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# Ports included in the Spanning-Tree Protocol are selected.

All of the bridges in the network will send and receive Bridge Protocol Data Units, and the bridge with the lowest Bridge ID is selected as the Root. The most important part of the Bridge ID is the Bridge Priority, the default value is 8000hex, or 32768 decimal (some vendors will use 8 as the configuration value for default priority), if all of the bridges have the default value then the one with the lowest MAC address will become the Root Bridge. Many networks do not work properly because of the location of the bridge with the lowest priority is on an edge bridge rather than one near the core.

All of the ports on the Root bridge are designated Ports, a designated port is a port that a bridge downstream from the Root Bridge will be connected to provide a path to the Root.

When the Root Bridge is selected then the root bridge sends out BPDUs indicating that it is root. Each bridge will then calculate the cost to get to the root bridge, this will be the number of hops from the bridge to the Root Bridge.

Each link speed is given a cost. The cost for a Link that is using basic STP is one value, the cost for a Link that is using RSTP (Rapid STP) is a different value and if it is using MSTP (Multiple Spanning Tree Protocol) it is a third value. This is because when STP was envisioned there was only Ethernet and Fast Ethernet. When RSTP was envisioned there was Ethernet, Fast Ethernet and Gigabit Ethernet. Now with MSTP there is 10 Gig Ethernet with the expectation that it will go higher. (Fast Ethernet could be 19 or 200000) If there is a mixture of STP versions in the network this could cause issues. So when a bridge transmits out BPDUs on its designated ports, it will take the cost that it receives on its Root Port and add the cost of its Designated Port. This should mean that if there are two hops of FastEthernet then its old cost would be 38, and the other was only one hop then the cost would be 19, so the lowest hop count will be chosen first, if not then a Gig connection would be 4 and it would be chosen as the best route as it has the lowest cost.

If the link costs are equal then the bridge will compare BPDUs to see which transmitting port has the best port identifier. The first part of the Port Identifier is a Port Priority, the default value for Port Priority is 80hex or 128 decimal. If the port priorities are equal, then the port numbers are compared and the transmitting port with the lowest number is selected as the root port. Many people believe that the bridge trying to determine the best path to the Root will use its lowest port number. But testing and verification of the IEEE 802.1D-2004 standard will show that it is the bridge with the lowest number transmitting to this bridge. The bridge then picks that port as the Root Port or the best path to the Root Bridge. The port on the upstream bridge will be a designated port. The other port that connect to an upstream bridge will be an alternate or back-up port.

If you have any questions, send me an email and I will help. I have spent a great deal of time learning about the operation of the IEEE 802.1D-2004 and 804.1Q-2005 standards to be able to teach courses on Bridges, Hubs, Routers and various other legacy protocols.