Using multiple ISP lines together??

Hello,
I have a Cisco 1721 router with 2 wic cards, hence two different lease lines from two different isps. (let me call the A and B)

what I want to do ( and cant find a way to do) is to manage the return traffic based on ISPs. Behind this router there are couple of servers for authenticated users to login. Some come from isp A, some from isp B.

If the connection comes from isp A, I want the reply to go back from the line of isp A and the same for isp A.

At the moment default gateway is isp B so the traffic coming from line A gets replied from line B.

Is this possible?

I would like to see your router config so I can see what you are doing right now. Are you running a routing protocol such as BGP with both ISP peers (muti-homed BGP with a higher administrative distance for one interface for failover)? or are you using statics? basicly to balance outgoing traffic between them you will need to run a routing protocol that will load balance with your ISPs that way you will have routes out to the people coming in to reply back and the routing protocol will make the decision on which interface to send back out by cost. the problem here is that your router is very small and can not handle a full internet routing table which is I believe 200,000 routes by now. If you know what their source IP address is then you could put statics in for each destination but this is just insane hehe.
To answer your question yes you can load balance by source, the router needs to have routes to them though. I would not use anything smaller then a 7000 series cisco with VIP 250 cards if running CEF.

Do you own the IP subnet your servers are using or they are part of the space provided by one of the ISPs. If the later, which ISP is providing you that space? Is it /24 or greater than that?

Thanks.

Thanks for the answers
Here is the system:

the router has two serial interfaces with lines connected to two different isps. both isps also provided subnets and from these I used one from each on a server on the inside as primary and secondary addresses. lets say these are 192.168.1.2 (isp A) and 10.0.1.2 (isp B). (imagine these are real if some internet user tries to reach 10.0.1.2, regardless of his/her own network, somehow his/her request packets will enter isp B 's network and they will reach the server through the serial connected to isp B. they will go through the router and its single internal ethernet interface and reach the server's ethernet interface (or to be more specific they will reach 10.0.1.2) the server will naturally geneate respond packets with source 10.0.1.2. once these packets with the soucr ip 10.0.1.2 reach the router the router should (somehow) know that these came from isp B and send them through serial connect to B. normal "route add" does it for destination.

-if you want to go somewhere go from here.

ip route "somewhere" "somewhere's netmask" "go from here"

I want to do it for the "source":

-if you are coming from this ip, go to this gateway... and vice versa.

I hope I was able to explain

ahhh a poor mans load balancing. look at your router what are you using for routing? and what is its default gateway?
Dont have to worry about route aggregation now do ya Rais hehe

basicly its like this if the router doesnt have a route for the destination address (the address that its sending the packet to) then it will push it out its default gateway. whith that being said what is in your routing table and what is making the routing decisions.

Its default gateway is isp A, hence even packets generated from 10.0.1.2 gets to be sent through the isp A 's line (192 network)

Ok so now we are getting somewhere you are not using a routing protocol at all so the only thing that router has in its routing table are the two directly connected links and its default gateway. what this meens is that if you are not sending to ISP B direct link network everything out goes through ISP A even if you are sending back to one of ISP B's clients. You will need to create static routes for networks you would like to go out ISP B's interface or start excepting routes from ISP B via a routing protocol you could tell them to send only their routes to you and thats it no peer routes(this is why you use routing protocols hehe) cause you could give it a route that is longer than going through ISP A.