calculating Bandwidth Utilization for the two networks

Hi

My Problem is

A 512 kbps internet link is terminated on a Cisco 2600 router.
The Router and the PIX with failover are connected to a Cisco
catalyst Switch 4507 with 3 VLANS.

VLAN 2 is for PIX, VLAN 3 is for Back office network and VLAN 4
is for Client Network.
Inter VLAN Communication is blocked between Back office and
Client Network.

(I.e Inter VLAN Communication is blocked between VLNA 3 and VLAN 4)

My objective is to calculate the Internet Bandwidth utilization separately for Back office network and Client network I am using MRTG (Multi Router Traffic Graph) to calculate the Bandwidth. Which interface I have to monitor for calculating Bandwidth Utilization for the two networks separately.

Since your two office VLANs don't intercommunicate and both access the internet through VLAN2 you should be able to measure the traffic for the two VLANs at the switch and get the answer you need. To get graphs in the format you want you might need to use the MRTG options for custom scaling of the graph axes and/or a muliplier