Jasni,
according to the diagram you sent me:
Your network is very complex and I can't give you step-by-step instructions.
But the idea is following:
Put your users to different VLANs.
Make a different STP root for each of these VLANs.
Example:
In the left part of your diagram there is a Switch with VLAN3 defined. Let's suppose one of the 6509s is a root for VLAN3.
Create a new VLAN4 with root beeing the other 6509.
Move some hosts connected to the Switch from VLAN3 to VLAN4. The Switch is connected via two trunks to two 6509s. One of these 6509s is a root for VLAN3, the other one for VLAN4. So while connecting to the Internet through the Cisco 7507 the VLAN3 users will use one trunk and VLAN4 users the other trunk.
You can even go further:
Use HSRP on your 7507 routers.
Tuning priorities in particular HSRP groups you can make one router active for VLAN3 and standby for VLAN4 and the other active for VLAN4 and standby for VLAN3.
This aproach will make your nework more complex - it requires additional routing between VLANs, changing user PCs IP addresses and default gateways, etc.
But finally the load balancing can be improved.
Note: Read carefully the URL I'd provided in the previous message. It explains the STP tunning. It is sometimes not enough to move STP roots. Sometimes you need to play with port costs or priorities.
Regards,
Milan
