Spanning Tree – Root Bridge

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Spanning Tree Protocol Root Bridge

Every spanning tree switched network or broadcast domain has a switch designated as the root bridge. It serves as the reference point for all spanning-tree-enabled switches. The spanning tree algorithm determines which redundant paths to block. It is selected as an election process.

The figure below illustrates the bridge ID (BID) fields. BID is a unique identity when the switch is part of a network. It is a 64-bit field divided into three parts: the 4 4-bit Bridge Priority field, the 12 12-bit Extended System ID field, and the switch’s 48-bit MAC address. The Bridge Priority field is configurable, while the MAC address is unique among all switches. The sum of these two ensures a unique Bridge ID.

Root Bridge election

All switches in the broadcast domain join the election process. When switches complete their booting process, they send out BPDU frames every two seconds containing the switch BID and the root ID. The adjacent switches receive the BPDU frames and read the root ID information from them. If the root ID of the BPDU sending switch is lower than the root ID on the receiving switch, the receiving switch updates its root ID, identifying the adjacent switch as the root bridge.

Spanning Tree Protocol Root Bridge

It doesn’t need to be the adjacent switch. It could be any other switch in the broadcast domain. The switch then sends new BPDU frames with the lower root ID to the adjacent switches. Finally, the switch with the lowest BID is selected as the root bridge for the spanning tree instance.

Now, look at the election process from another angle. Before manually configuring the bridge priority, all the switches have a default priority. Therefore, it is a tie based on priority. The switch with the lowest Mac address will become a root bridge. In the figure below, switch-3 is elected as the Root-Bridge because the bridge priority ID is the default, which is 32769; so, the switches elect the root-bridge based on the MAC address. The switch-3 is the lowest MAC address, so all the root-bridge ports become in the forwarding state, i.e., designated port.

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Asad Ijaz

NetworkUstad's lead networking architect with CCIE certification. Specializes in CCNA exam preparation and enterprise network design. Authored 2,800+ technical guides on Cisco systems, BGP routing, and network security protocols since 2018. Picture this: I'm not just someone who writes about tech; I'm a certified expert in the field. I proudly hold the titles of Cisco Certified Network Professional (CCNP) and Cisco Certified Network Associate (CCNA). So, when I talk about networking, I'm not just whistling in the dark; I know my stuff! My website is like a treasure trove of knowledge. You'll find a plethora of articles and tutorials covering a wide range of topics related to networking and cybersecurity. It's not just a website; it's a learning hub for anyone who's eager to dive into the world of bits, bytes, and secure connections. And here's a fun fact: I'm not a lone wolf in this journey. I'm a proud member and Editor of Team NetworkUstad. Together, we're on a mission to empower people with the knowledge they need to navigate the digital landscape safely and effectively. So, if you're ready to embark on a tech-savvy adventure, stick around with me, Asad Ijaz Khattak. We're going to unravel the mysteries of technology, one article at a time!"

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5 Comments

  1. Spanning Tree Algorithm - Path Cost » Networkustad
    October 19, 2019 @ 6:29 pm

    […] is determined using the sum of the individual port costs along the path from the destination to the root bridge. Each “destination” is a switch port. The default port costs are defined from the speed of the […]

  2. 802.1D Bridge Protocol Data Unit (BPDU) Frame Format » Networkustad
    October 22, 2019 @ 2:06 am

    […] spanning tree algorithm exchange Bridge Protocol Data Unit (BPDU) to elect Root Bridge. It is a frame containing 12 different fields with different information.  The information in the […]

  3. Types of Spanning-Tree Protocols » Networkustad
    October 24, 2019 @ 2:56 pm

    […] of using only one Root Bridge, the CPU and memory requirements for CST are lower than for the other protocols. However, because […]

  4. Spanning-Tree Bridge ID configuration » Networkustad
    October 27, 2019 @ 5:24 pm

    […] configuring a specific switch as Root Bridge, the bridge ID priority value must be adjusted to make sure as lower than all the bridge priority […]

  5. PVST+ Load Balancing » Networkustad
    October 29, 2019 @ 6:07 pm

    […] configured in the network, VLAN 100 and 200. The purpose is to configure Switch-1 as the primary root bridge for VLAN 100 and the secondary root bridge for VLAN 200 and Switch-2 as the primary root bridge for […]