Cisco CCNP BCMSN Exam Tutorial Changing Root Bridge Election Results
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Cisco CCNP BCMSN Exam Guide: Modifying Root Bridge Elections
Overview
Becoming a CCNP requires a deep understanding of the Spanning Tree Protocol (STP) and root bridge elections. Discover these crucial concepts with insights from Chris Bryant, CCIE
12933.
Key Concepts
- CCNP
- BCMSN
- STP
- Root Bridge
- VLAN
- Priority
- Default Value (32768)
Introduction
In your BCMSN and CCNP studies, one vital area is mastering the Spanning Tree Protocol (STP). While some basics were covered in your CCNA coursework, there's a lot more to discover. Let's revisit how root bridge elections work and explore how to influence these outcomes.
Understanding Bridge ID Priority
Each switch is assigned a Bridge ID (BID), which consists of a priority value combined with the switch's MAC address. For instance, a default priority is 32,768. If a switch has this default and a MAC of 11-22-33-44-55-66, the BID is 32768:11-22-33-44-55-66. If the priority isn't customized, the MAC address becomes the deciding factor.
The Root Bridge Election
Initially, every switch acts like it’s the center of the network universe. They exchange Bridge Protocol Data Units (BPDUs), and the switch with the lowest BPDU becomes the Root Bridge.
If STP is left on its own, a single switch becomes the root bridge for every VLAN based on the lowest MAC address?"hardly ideal. Therefore, it's often necessary to designate specific switches as the root bridge or to distribute the workload more evenly.
Configuring Root Bridges
When you need specific switches to serve as root bridges for your VLANs, use the `spanning-tree vlan root` command. Let's say you have 50 VLANs and five switches; you might want each switch to be the root for 10 VLANs:
```plaintext
SW1(config)
spanning-tree vlan 1 ?
```Options include setting forward time, hello interval, max age, priority, or declaring the switch as root.
In our example, SW1 is the root bridge for VLANs 10, 20, and 30. You can change the root bridge for VLANs 20 and 30 to SW2 using:
```plaintext
SW2(config)
spanning-tree vlan 20 root primary
SW2(config)spanning-tree vlan 30 root primary
```Check the status with:
```plaintext
SW2
show spanning vlan 20
``````plaintext
VLAN0020
Spanning tree enabled protocol ieee
Root ID Priority 24596
Address 000f.90e2.1300
This bridge is the root
```
Similarly, for VLAN 30:
```plaintext
SW2
show spanning vlan 30
```SW2 is now the root bridge for both VLANs 20 and 30, with updated priority values from the default 32768.
Conclusion
Modifying root bridge elections in STP allows more control and efficiency in your network. Stay tuned for our next CCNP/BCMSN tutorial, where we'll explore further STP features.
You can find the original non-AI version of this article here: Cisco CCNP BCMSN Exam Tutorial Changing Root Bridge Election Results.
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