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New Member

Routing between 2 circuits

I have an SMTP server that I would like to send outbound email via two different T1s. How can I load balance the outgoing email between two links? Is is possible in a router or firewall? Do I have to do it on the host?




Re: Routing between 2 circuits

Depends on where the two T1's terminate -- same router, or two different routers? And do they go to the same ISP, two different ISPs, or are they private leased lines to other locations?

Also, are you sending SMTP to any e-mail server on the Internet, or to specific SMTP hosts within your private network?

Need more info to be able to give you a more useful answer. But it is possible to do things on hosts, routers, and firewalls to send some traffic in one outbound direction, and other traffic in the other direction.

For example, if you have two routers, and they have a LAN connection to the same network your SMTP server is on, you could do crude load balancing by programming two static routes on that host: mask to one router (covers all IPs 0.x.x.x through 127.x.x.x) mask to the other router (covers all IPs 128.x.x.x through 255.x.x.x)

NOTE: The second static route here effectively is limited to 128.x.x.x through 223.x.x.x because IP multicasts run from 224.x.x.x through 239.*.*.*; and stuff 240.x.x.x and up is reserved.

Anyway, the SMTP receives a message, queries a local DNS server for domain name to MX record IP address, and sends the message to the appropriate router.

That's one simple way to do it, not very elegant. There are more sophisticated ways, but it depends on what you have, how it's connected, and where the mail's going. For example, if you're connected to two different ISPs with two different routers, you could run BGP routing protocol from each router to its connected ISP, and between your routers. Then, run HSRP on the routers, and have your SMTP server use the HSRP shared IP address as its default gateway. Then, when your e-mail message goes out, it goes to the active HSRP router, which looks up the route in its tables and decides whether it's quicker to send the message out its T1 to its ISP, or if a more direct path would be via your other router's T1. But it depends on what routers you have, whether they'd be able to handle BGP (big route tables), your ISP situation, etc.

So, there are many ways something along these lines can be done. Just need more info so we can point you in the right direction for you.

Hope this helps.

New Member

Re: Routing between 2 circuits

This is very helpful. Thanks for the post. The best solution sounds like a variation of your last suggestion. The setup is:

1. Two differenet 2600s connected each with its own T1 connected to 2 separate ISPs.

2. SMTP server is on the same LAN as both ethernet interfaces of the 2600s.

3. SMTP traffic goes out to any smtp server on the Internet

4. Return SMTP traffic is sent to another SMTP server not on local LAN.

I would like to use HSRP for the two internal fa0/0s on the LAN which are on the same subnet as the SMTP server. Using this I would like to be able to send out SMTP traffic using load balancing with the two T1s. Is this possible just to enable HSRP for only the SMTP server and not other hosts on the LAN?

So, the default gateway of all hosts except the SMTP server would be a the real IP of one of the routers. Then the SMTP server would use the HSRP shared IP for its gateway.

1. Would this cause routing problems on the LAN?

2. Also, can you load balance a router and a (PIX)firewall inside interface using HSRP?

3. Since this setup would only be used for outgoing SMTP traffic, is BGP necessary?