Exam Tutorial Cisco CCNP / BCMSN Changing Root Bridge Election Results

Exam Tutorial Cisco CCNP / BCMSN Changing Root Bridge Election Results
Each switch will have a Bridge ID Priority value, more commonly referred to as a BID. This BID is a combination of a default priority value & 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 & 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.

Your BCMSN & CCNP studies will include mastering the details of Spanning Tree Protocol (STP). While you learned some of these details in your CCNA studies, a bit of it may be old to you. Before going on to the intermediate & advanced STP features, let's review the root bridge election process & learn how to change these results.

If STP is left alone, a single switch is going to be the root bridge for every single VLAN in your network. Worse, that single switch is going to be selected because it has a lower MAC address than every other switch, which is not exactly the criteria you require to use to select a single root bridge.

Switches are a lot like people - when we first arrive, we announce that we are the center of the universe! Unlike some people, the switches will soon get over it. BPDUs will be exchanged until one switch is elected Root Bridge, & it is the switch with the lowest BPDU that will end up being the Root Bridge.

The time will definitely come when you require to determine a particular switch to be the root bridge for your VLANs, or when you will require to spread the root bridge workload. For instance, if you have 50 VLANs & five switches, you may require 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 ?

hello-time Set the hello interval for the spanning tree

forward-time Set the forward delay for the spanning tree

priority Set the bridge priority for the spanning tree

max-age Set the max age interval for the spanning tree

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

root Configure switch as root

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

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

SW2#show spanning vlan 20

VLAN0020

Root ID Priority 24596

Spanning tree enabled protocol ieee

This bridge is the root

Address 000f.90e2.1300

SW2#show spanning vlan 30

VLAN0030

Spanning tree enabled protocol ieee

Root ID Priority 24606

This bridge is the root

Address 000f.90e2.1300

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

In the next CCNP / BCMSN tutorial, we'll take a look at more STP features.

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