Which switch should be root bridge?

The switch with the lowest priority becomes the root bridge. If there’s a tie, then the switch with the lowest bridge ID number wins. The ID number is typically derived from a MAC address on the switch. The problem is that, by default, every switch has the same priority value (32768).

What is root bridge priority?

The bridge priority determines which bridge is elected as the root bridge. If two bridges have the same path cost to the root bridge, the bridge priority determines which bridge becomes the designated bridge for a LAN segment.

How do I set priority on root bridge?

To ensure that a switch has the lowest bridge priority value, use the spanning-tree vlan vlan-id root primary command in global configuration mode. The priority for the switch is set to the predefined value of 24,576 or to the highest multiple of 4096 less than the lowest bridge priority detected on the network.

What is the default bridge priority?

32768
The default priority is 32768, and additional roots are set below this number. STP increments priority by 4096, so the next priority is 4096 below 32768. The lower the number, the higher the priority.

What is the purpose of a root bridge?

A Root Bridge is a reference point for all switches in a spanning-tree topology. Across all connected switches a process of election occurs and the Bridge with the Lowest Bridge ID is elected as the Root Bridge.

Why root bridge is used in STP?

Redundant links are used to provide back up path when one link goes down but Redundant link can sometime cause switching loops. The main purpose of Spanning Tree Protocol (STP) is to ensure that you do not create loops when you have redundant paths in your network.

How do you calculate the root bridge?

The root bridge is selected by manually configuring its bridge priority to a low value. 32768 is the default value out of a range from 0 to 61440. If all switches in a single spanning tree have the same bridge priority, the switch with the lowest MAC address will become the root bridge.

Which priority makes a device most likely to become a root bridge?

In that case, the value can range from 0 to 61,440, but only as multiples of 4,096. A lower bridge priority is preferable. This command is actually a macro on the Catalyst that executes several other commands. The result is a more direct and automatic way to force one switch to become the Root Bridge.

Why is default bridge priority 32768?

The default is 32768. The lower the number the more likely the switch will be chosen as the root switch. Valid priority values are multiples of 4096. Spanning-tree priority xxx configures the priority in RSTP if RSTP is running, or configures the CIST priority if MSTP is running.

How is bridge ID calculated?

Every switch has an identity when they are part of a network. This identity is called the Bridge ID or BID. It is an 8 byte field which is divided into two parts. The first part is a 2-byte Bridge Priority field (which can be configured) while the second part is the 6-byte MAC address of the switch.

What is root path cost in STP?

Path cost – The path cost is the metric stp uses to calculate the shortest path to elect root port to reach the root-bridge . The path cost is based on the speed of the interface of switch .

What is a root bridge STP?

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 32768 and MAC 0200.0000.

What’s the default priority for a root bridge?

The following example shows four switches with default priority value. Here Switch 1 is the Root Bridge (Switch) since it has the lowest MAC address. If you want one particular switch to be the Root Bridge (Switch), change the priority to a lower value than 32,768.

When does a switch become a root bridge?

As all the switches have default priority therefore there is a tie on the basis of priority. Now, the switch with the lowest Mac address will become a root bridge. Here, switch A will become the root bridge as it has the lowest Mac address. Therefore, the ports of switch A will be in forwarding state i.e designated port.

Where is the root bridge in a network?

The Root bridge (switch) is a special bridge at the top of the Spanning Tree (inverted tree). The branches (Ethernet connections) are then branched out from the root switch, connecting to other switches in the Local Area Network (LAN). All Bridges (Switches) are assigned a numerical value called bridge priority.

What is the bridge priority of a switch?

All Bridges (Switches) are assigned a numerical value called bridge priority. The Bridge (Switch) priority value is used to find the Bridge (Swtich) ID. The Switch ID is made from two values. • The Switch Priority, which is a numerical value defined by IEEE 802.1D, which is equal to 32,768 by default.