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Loadbalancing in STP

The below question was asked in the interview that i faced..

How come load balancing possible in STP?

Hall of Fame Super Blue

Re: Loadbalancing in STP


CS1 = core switch 1

CS2 = core switch 2

CS1 & CS2 connected via a L2 trunk. These switches are responsible for inter-vlan routing. You have 10 vlans on the switches - vlans 1 - 10.

AS1 = access-layer switch.

AS1 is connected to CS1 and CS2 with L2 trunk links. All 10 vlans are present on AS1.

To load balance using STP you would do the following

1) For vlans 1 - 5 set the STP root to be CS1 and the STP secondary to be CS2.

2) For vlans 6 - 10 set the STP root to be CS2 and the STP secondary to be CS1.

3) Ensure that the trunk link between CS1 & CS2 is not blocked by STP.

With this scenario AS1 will forward vlans 1 - 5 using CS1 uplink and block vlans 6 - 10 on this same uplink. And AS1 will forward vlans 6 - 10 using CS2 uplink and block vlans 1 - 5 on this same uplink.

Note that when you use this setup you generally match up the HSRP active gateway as well ie. HSRP active gateway for vlans 1 - 5 would be CS1 and for vlans 6 - 10 it would be CS2.