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Spanning Tree Electing Root Bridge

Per-VLAN Spanning Tree Concepts

Let’s began with how Spanning tree work in enterprise network. From the figure we can identified who will be the root bridge and config to choose who will be the root bridge in our network.

Let’s start with default state of Spanning tree. A real world Spanning tree example. We have 3 tiered network structure of enterprise network. Top of Switch is Access, which is directly connected with the user and middle is Distribution then core switch, at bottom server FARM switches. We don't change anything priority and mac address are the same; now guess who will be the Root Bridge.

Obviously switch0, because it has the lowest mac address. That access layer switch became the Root Bridge. Now do we want that switch became Root Bridge in our network? No we didn't want it to be Root Bridge. That isn't the center of the network.

Remember that all switch finds best way to reach Root Bridge and block all other redundant links. Switches think that's the center of the network. Now switches are selecting their root port with the lowest cost path to reach the Root Bridge.

Switch2 directly connected port became Root Port because it has 100M link with cost 19. Just like Switch2, Switch3, Switch4 and other also elect Root Port. Block all other port left over. 

Okay let’s remove block port and see the network. That's the real topology of the switch look like. Take a look at this, the problem is what links got block. e.g the major link of distribution(Switch3 to Swtich2) link has been cut off as well as core switch(Switch4 to Swtich2) also cutoff, that’s what we really don't want.

Distribution Switch have a flood of traffic which is forwarded to the Root Bride, the limit of network congestion, that access switch might be low performance with lower quality like Catlyst 2900xl. It may have 100M links with definitely can't handle so much traffic coming from the distribution switch. This could be bottle-necking of the entire network. All is working fine, switch led are blinking green but user feels slow because congestion of the link. A tone of traffic is coming from the distribution switch which causes Root Bridge crashing. That could happen complete network down for 10 to 30 or more. This could depend on how big the network is? That happen because bad Root Bridge has been elected, then what switch had to be the Root Bridge.

The answer is, Core Switchs in the network should be the Root Bridge. So everybody finds the best way to reach the Root Bridge and block other redundant link. That’s the good way to configure the Spanning tree. Make sure you change the priority to lower, so the Core switch can wins the Root Bridge election, don't let mac address break the tie between the switches.


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