What is STP Root Bridge?
In STP configured switched network or broadcast domain, a switch is designated as the root bridge. The root bridge serves as an administrative point for all spanning-tree calculations to determine which redundant links to block.
Selection of the Root Bridge:
The root bridge in an STP network is selected through an election process based on Bridge IDs.
The Bridge ID is a combination of a priority value and the MAC address of the bridge. The bridge with the lowest Bridge ID becomes the root bridge.
Administrators can influence the root bridge selection by configuring the priority values of bridges.
Role in the Network:
Reference Point for Path Calculations: All path calculations in the STP topology are made relative to the root bridge. This means that the network’s spanning tree is organized based on the shortest paths to the root bridge.
Loop Prevention: By defining a single logical root in the network, STP prevents data loops, which are detrimental in Ethernet networks.
STP Operation:
When STP is enabled, bridges (switches) in the network exchange BPDU (Bridge Protocol Data Unit) messages to elect the root bridge and to determine the network’s topology.
Each bridge determines the best (shortest) path to the root bridge. The port on each bridge that provides the best path to the root bridge is designated as the root port.
Spanning Tree Recalculation:
If the network topology changes (for example, if a bridge is added or removed, or a link fails), the spanning tree may need to be recalculated. This could potentially result in the election of a new root bridge.
Stability and Optimality:
The stability and performance of the spanning tree topology depend significantly on the location and stability of the root bridge. Ideally, the root bridge should be centrally located to optimize data paths across the network.
Root Bridge Failure:
In case of root bridge failure, a new root bridge election process takes place to maintain the loop-free topology.
An election process determines which switch becomes the root bridge.
Each switch has a Bridge ID (BID) that is made up of a priority value, an extended system ID, and the MAC address of the switch.
All switches in the network take part in the election process. After a switch boot up, it sends out BPDU frames containing the switch BID and the root ID every 2 seconds. By default, the root ID matches the local BID for all switches on the network.
The root ID identifies the root bridge on the network. Initially, each switch identifies itself as the root bridge after bootup.
Let us look at it this way, when switches A, B, C, and D are on the same network or broadcast domain boots up, the switches will forward their Bridge Protocol Data Unit (BPDU) frames to neighboring switches.
All switches in the network or broadcast domain will read the root ID information from the BPDU frame of all their neighbors.
After reviewing the entire root IDs from the BPDU received from each switch, the switch with the lowest BID ends up being identified as the Root Bridge for the spanning tree process. It may not be an adjacent switch, but any other switch in the broadcast domain.
Study the figure above and see if you can Identify the switch with the lowest priority.
Root Ports – Switch ports closest to the root bridge with the lowest cost path.
Designated Ports – All non-root ports that are still permitted to forward traffic on the network.
Non-designated ports – All ports are configured to be blocked to prevent loops.
Summary.
* Each switch has a bridge ID (BID) of priority value followed by a MAC address
* Switches exchange Bridge Protocol Data Unit (BDPU) to compare bridge IDs
* The switch with the lowest bridge ID becomes the root bridge.
* Eventually, all switches agree that the switch with the lowest BID is the root bridge.
In conclusion, the STP root bridge is a fundamental component of STP’s loop prevention mechanism in Ethernet networks. Its primary role is to act as a point of reference for all path calculations within the network, ensuring an efficient and loop-free network topology. Proper planning and configuration of the root bridge are essential for optimal network performance and stability.
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