I have a dedundant Internet access (with two different ISPs): two leased lines endind up on two different Cisco routers (both running HSRP for redundancy and BGP for routing).
Problem: the line from ISP1 is full while the line from ISP2 is almost empty.
My wish is to route every SMTP packet through ISP2, but I've been told that it's not possible to implement some "protocol routing" while BGP is running. Another solution for me would be to route according to the source address (every packet coming from a specific IP address would go through the ISP2).
Does someone have any idea about how I could proceed?
First of all, I hope that when you say that "both running HSRP for redundancy" you mean that there are a total of four routers. Otherwise I suspect that you will always have the phantom router idle.
You might be able to implement some type of policy routing to help if the above isn't an issue (to include policy by protocol and/or source address). But without knowing how your network is laid out, it would be pointless to make specific suggestions. Just remember that your policy applies to incoming packets. So if you wanted to test (match) traffic, you need to do it as it comes into a router. Then you can specify (set) which interface it leaves out on.
Also, with rare exceptions, I don't think you can influence the route that traffic from outside to inside takes. External networks will make those decisions on thier own. On that note, are both of your ISPs advertising both/all of your address spaces? If not, your inbound traffic will always be via the only ISP that is doing the advertising.
I found something today you might be interested in. Although this paper addresses load balancing (or, more accurately, load sharing), it also includes a way to build in redundancy as well. This is done by means of mainipulating preferences. Frankly, some of it is a little over my head but I think you might find it a nice alternative to using HSRP - which I still think will prevent you from ever doing any kind of load balancing.
Also note that this paper includes a way to influence traffic coming into your network by using some pretty wild access list techniques. Learn something new every day.
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