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CCNA And CCNP SWITCH Exam Training:

Root Bridge Elections And STP

By Chris Bryant, CCIE #12933

Your CCNA and CCNP SWITCH exam studies will demand mastering the details of Spanning Tree Protocol (STP).

While you learned some of these details in your CCNA studies, the stakes are raised for your CCNP SWITCH studies.

Of course, mastering any skill requires mastering the fundamentals before you get to the advanced material. In this tutorial, we'll review the overall process of a root bridge election.

Each switch in your network will have a Bridge ID Priority value, more commonly referred to as a BID. This BID is a combination of a default priority value and the switch's MAC address, with the priority value listed first.

For example, if a Cisco switch has the default priority value of 32,768 and a MAC address of 11-22-33-44-55-66, the BID would be 32768:11-22-33-44-55-66. Therefore, if the switch priority is left at the default, the MAC address is the deciding factor in the root bridge election.

Switches are a lot like people - when they first arrive, they announce that they are the center of the universe! (Unlike some people, the switches will soon get over this misconception.)

BPDUs will be exchanged between the switches until one switch is elected the root, and it's the switch with the lowest BID that will end up being the root bridge.

If STP is left to its default settings, a single switch is going to be the root bridge for every single VLAN in your network. In a small network, you might not care about that. In most network, though, you're going to want your root bridge to be one of your more powerful switches with regards to CPU and memory.

Generally speaking, your root bridge should also centrally located in the network as opposed to a switch at the edge of your network.

The time will definitely come when you want to determine a particular switch to be the root bridge for your VLANs, or when you will want to spread the root bridge workload.

For instance, if you have 50 VLANs and five switches, you may want each switch to act as the root bridge for 10 VLANs each. You can make this happen with the spanning-tree vlan root command.

SW1(config)#spanning-tree vlan 1 ?
forward-time Set the forward delay for the spanning tree
hello-time Set the hello interval for the spanning tree
max-age Set the max age interval for the spanning tree
priority Set the bridge priority for the spanning tree
root Configure switch as root

In this example, we've got two switches, and SW1 has been elected the root bridge for VLANs 10, 20, and 30. We'll use the spanning-tree vlan root command on SW2 to make it the root bridge for VLANs 20 and 30.

SW2(config)#spanning-tree vlan 20 root primary
SW2(config)#spanning-tree vlan 30 root primary

SW2#show spanning vlan 20

VLAN0020
Spanning tree enabled protocol ieee
Root ID Priority 24596
Address 000f.90e2.1300
This bridge is the root

SW2#show spanning vlan 30

VLAN0030
Spanning tree enabled protocol ieee
Root ID Priority 24606
Address 000f.90e2.1300
This bridge is the root

SW 2 is now the root bridge for both VLAN 20 and 30. Notice that the priority value has changed from the default of 32768.

Take the next step to CCNA and CCNP SWITCH exam success with our exclusive CCNA Tutorials and CCNP SWITCH Tutorials pages!

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