Two networks, One Server

Static, RIP, EIGRP, OSPF, BGP, IBGP, IS-IS and any other routing.
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Two networks, One Server

Post by silverskymedia » Wed May 11, 2011 8:42 am

I have a set of servers that I want to set on two distinct networks.

The justification is: 1) Last Mile Network Redundancy 2) Load Balancing

It is very unlikely we will be allowed to run BGP for external routing so we're going to just do round-robin DNS. So we will have two distinct IP address ranges for each network. For the purposes of this question well say (Net1 = and (Net2 = I would also like to run OSPF (or some other internal routing) to balance between the two lines. I have a Cisco 2800 (2811) to route between the networks.

The Cisco router is currently configured to use 1 to 1 NAT (ip nat inside source static network /28). With this configuration I am working fine on Net1. The problem is that when I bring in the second network I anticipate trouble. For one if I run the command ip nat inside source static network /28 (if it will let me do that) the NAT is overlapped. And while the server won't know which network the traffic has originated (good thing) the router will not know which IP address to put back on the IP header which would make requests coming from the secondary network unable to receive a response.

If, however I were to do ip nat inside source static network /28 instead (creating a new internal network) and just assign the server a 172.16.1.x and a 172.16.2.x IP address then that would fix the NAT problem but at that point I'm not sure the router will be able to effectively route packets through the best network available. The networks have very different line speeds and because the internal routing is round-robin DNS there could be a better return route. That is to say it should be fine for a packet that originated from the network to be routed back through the network so long as I preserve the originating IP address header.

Any ideas on the best way to configure the network / Cisco 2800 router to achieve my goal?


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Re: Two networks, One Server

Post by john » Fri May 27, 2011 12:58 pm

two nics on each server.

Nic 1
(Net1 =

(Net2 =

The router can do the rest. traffic going into the server will be sent out the same interface. (lining up the ips)

With round robin if one line is down wont you still be dropping half your traffic?


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