Since its inception STP has became the most critical component of any LAN, We cannot even imagine a LAN without STP, Infact this one feature is enabled by default by 99% of the OEMs. But leaving it to the default configuration can sometimes worsten the network performance. STP requires deep understanding and fine-tuning when clubbed with various other features likes HSRP. Lets see how the STP should be fine-tuned to add to your network performance instead of taking toll on it. Generally this type of issues arises when layer 2 STP boundries fails to sync with layer 3 gateways. lets have a look at one similar example.
In the diagram above (Sorry about the poor drawing) STP Root function and Active gateway function is performed by separate switches. Now whenever a packet is trying to move out of the network, It will first report to the STP root but since it is not the active HSRP forwarding device, it will then be forwarded to another router which is actively forwarding traffic. So, this kind of sub-optimal traffic forwarding will unnecassary add delays to your traffic.
This applies to scenarios with Multiple VLANs as well. If we are doing HSRP load balancing by creating multiple groups, The router/switch should be made STP root for the appropriate VLANs.
So, This was a brief description of the issues we genrally face on the LAN side. if some wish to discuss on any particular scenario - Your comments are always welcome !!!
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