Contents
Why root bridge is used in STP?
Redundant links are used to provide a backup path when one link goes down but a Redundant link can sometimes 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.
What is root bridge in 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.
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.
How is STP cost calculated?
SW3 receives BPDUs on its 10 Mbit interface (cost 100) and on its 1000 Mbit interface (cost 4). It will use its 1000 Mbit interface as its root port (shortest path to the root bridge is 19+19+4=42). SW3 will forward BPDUs to SW4. The root path cost field will be 100….Spanning-Tree Cost Calculation.
| Bandwidth | Cost |
|---|---|
| 100 Mbit | 19 |
| 1000 Mbit | 4 |
How is STP cost calculation?
It will use its 1000 Mbit interface as its root port (shortest path to the root bridge is 19+19+4=42). SW3 will forward BPDUs to SW4. The root path cost field will be 100. SW4 receives a BPDU from SW2 with a root path cost of 19….Spanning-Tree Cost Calculation.
| Bandwidth | Cost |
|---|---|
| 10 Mbit | 100 |
| 100 Mbit | 19 |
| 1000 Mbit | 4 |
Which switch should be root bridge?
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.
What are the rules for STP root port selection?
Here are the basic rules of STP. 1. Lowest bridge ID (Priority:MAC Address) switch becomes the Root-Bridge. 2. Each non-root bridge should have ONE root port (RP) which is the port having lowest path-cost to Root Bridge.
How to elect root port in spanning tree protocol?
In this tutorial we will learn in detail how Spanning Tree Protocol (STP) elect root port after choosing a root bridge. Root port is the port that is closest to the root bridge, which means it is the port that receiving the lowest-cost BPDU from the root.
How is root port selection based on cost?
Root Port selection is based on the port having lowest cost to the Root Bridge (CAT1). For PVST (Per VLAN Spanning Tree) path cost will depend on bandwidth of links and cost value is as shown below for most commonly used links. Also it is important to understand how path cost calculate.
Which is the best port for root bridge?
Designated port – The port which sends best BPDU i.e ports on the root bridge will be in forwarding state. Root port – The port which receives the best BPDU on non root bridge. Criteria for selecting root port: Lowest path cost to reach root bridge