I have 3 internet links 2x25M and 1x50 M .I have 3 different LAN subnets(Public).How can I assure that traffic to and from first subnet use first link second subent use second link and thrid subnet use thrid link .whats the config requured for this
To use those links as efficiently as possible I recommend using BGP and receiving the full routing table from your upstream. Cisco OER is a great feature that works will with balancing traffic between disparate circuit sizes.
You can use PBR to influence the traffic outgoing (you cannot influence the inbound traffic with this method)
-make an ACL matching source subnet-1 to destination ANY
-match the ACL in a route-map
-in the route-map "set ip next-hop POP1-IP"
-apply the route-map under the LAN interface with "ip policy route-map NAME-OF-ROUTE-MAP"
More about PBR:
Then with BGP, you can influence inbound and outbound traffic.
The procedure is something like:
-match interesting traffic in ACL1 (for subnet-1) ACL2(for subnet-2)
-match ACL in route-maps (one for inbound and one for outbound / peering)
-in each route-map modify the some BGP attributes (depending if inbound or outbound one as-path, MED, local-preference....) so the one subnet is more prefered on the primary POP than on the rest
-apply route-maps in and out to the 3 peerings corresponding to the three POPs.
More on BGP attributes:
I didn't asked, but you have theree point to point links there on the WAN link, right? One for each POP.
Let me know if this helps.