Why STP prefer lower priority but VRRP prefer higher priority

At the beginning of my journey as a network engineer, I was always confused when configuring STP. I often had to double-check and re-read the documentation over and over again. It wasn’t the command syntax that was difficult, commands are easy to remember naturally since we type them repeatedly. What I kept forgetting was:
"Hmm, should I set a lower or higher priority for this bridge? Okay, let's Google it… and try to remember it."

Then, a few months later, when I had to configure STP priority again, I’d do the same thing all over again. 😆

One day, someone told me:
"A lower value means higher priority."

That made sense—after all, if something is #1 priority, it means it's the top priority, right? But then, I suddenly realized:
"Wait, what about VRRP? In VRRP, a higher priority value is preferred!"

That contradiction got me thinking. So, I started digging through IEEE documentation and finally found something that cleared up my confusion:

IEEE 802.1D Standard: The IEEE 802.1D standard specifies that the root bridge is the switch with the lowest Bridge ID, which is a combination of bridge priority and MAC address. This ensures a deterministic and consistent selection process.

I couldn’t find the exact document again while writing this—maybe the link got moved—but you can search for it. The key takeaway here is "deterministic and consistent selection process."

This makes perfect sense. The default STP priority is 32768, and if we don’t explicitly configure it, the switch with the lowest MAC address becomes the root bridge.

Why does using the lowest value make STP more consistent?

Because hardware vendors increment MAC addresses over time as they manufacture new products. If we buy switches from the same vendor, rack them, and leave the priority at default, the root bridge will remain unchanged. This means we avoid unnecessary recalculations of the STP topology—no sudden changes in root ports, designated ports, or blocked ports.

Now, imagine if STP used the highest MAC address instead. Every time we added a newer switch, the root bridge would change (unless we manually set the priority). That would be a complete disaster!

What about VRRP?

VRRP, on the other hand, prefers higher priority values because a higher number represents greater capability, better specs, and so on. If there’s a tie, VRRP uses the highest IP address as the tiebreaker. This makes sense because we can assign IP addresses manually, meaning we control the outcome during the design phase.

This is different from STP, which relies on MAC addresses pre-assigned by hardware vendors. Yes, we can change a MAC address, but in practice, we usually don’t.

Other Protocols That Follow the "Higher is Preferred" Rule

The same logic applies to:

• OSPF DR (Designated Router) Election

• PIM DR (Designated Router) Election

• IS-IS DIS (Designated Intermediate System) Election

All of these protocols prefer higher values, just like VRRP, because priority and IP addresses can be set during design, rather than relying on factory-assigned MAC addresses.

Understanding why STP selects the lowest priority while protocols like VRRP prefer the highest helps eliminate confusion and makes network design more intuitive. STP relies on MAC addresses, which are fixed by hardware vendors, ensuring a stable and predictable root bridge selection. On the other hand, VRRP and other protocols prioritize higher values because they allow manual control over election processes.

By remembering these distinctions, we can make better design choices and avoid unexpected behavior in our networks. So, next time you configure STP or VRRP, you won’t need to Google it again—you’ll already know why! 🚀