Load Balancing on 3 ISP with 3 routers invlove

Unanswered Question
Sep 9th, 2007


I want to implement a load balancing. I have 3 ISP and each have a 3 cisco routers. It is connected to one 2900 switch. All of our servers and workstations are private IP addresses and i am using NAT to go out. Right now im just changing all the servers and workstations GW IP to route to different ISP in case one of them goes down. Is there any best solution for this? I just need a guide where to start. I hope you could help me.

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Joseph W. Doherty Sun, 09/09/2007 - 12:30

What might work for you is making a subnet that appears between your LAN subnets and the Internet. This would allow your routers to see the other two routers as paths to/from the Internet. Not only could it balance the load, but it could also be implemented to handle router failures without manual changes.

If this is unclear, let me know and I'll see if I can post some diagrams.

ullasupendran Sun, 09/09/2007 - 23:12


You can use PBR in a subnet wise manner and point each subnet to different isp .

worldcalltel Mon, 09/10/2007 - 07:23

Can you show me a diagram? Does it mean i need to use a routing protocol?

We have 3 Private IP address = Workstations and Servers = IP Phones = workstations

Joseph W. Doherty Mon, 09/10/2007 - 14:43

(BTW: I'm assuming you're not advertising the same NAT to all your ISPs.)

The attached diagram show logical paths from router1's viewpoint. Router2 and router3 could also logically be configured as router1, but you likely need PBR (as Ullas noted in his post).

Also not shown is router2 and router3 would still be connected to your three existing subnets. The there routers could be configured with HSRP.

Not shown are you existing connections from each router to the Internet.

Do you need a routing protocol, no. Could be done with static routes. On router1, you indicate 3 outbound path, direct to the Internet, and via router2 or router3.


Another possible method, if your router support GLBP, is have each defined as a member of the gateway address on all your internal VLANs.

worldcalltel Tue, 09/11/2007 - 07:52

Hi, i have attached my network topology. i modified it. The 3 ISP are connected to one NAT Router. and the NAT Router is connected to a 2900 switch. To support the 3 public networks and 3 private networks. I created a 6 subinterface the three subinterface ip address are from my 3 ISP subnet. and the other three are from my 3 Private subnet. All translation will be handle by the NAT router.

Nat Router

int fa0.1 = ISP 1

int fa0.2 = ISP 2

int fa0.3 = ISP 3

int fa0.4 = = workstations

int fa0.5 = = workstations and servers

int fa0.6 = = Cisco IP Phones

The network will be translated to ISP 2 to go out to the internet. The half of will be translated to ISP 1 to go out. And the other half is translated to ISP 3 network. Lastly, for the IP Phones it is connected to a SIP Proxy Server which is translated to ISP 3 network.

Here are my queries.

1. If one of the ISP Router goes down. Let say the ISP 2 goes down. The will automatically cannot go out to the internet. How do i manage this subnet to automatically fail over to either ISP 2 or ISP 3? Do i prepare another nat translation for to translate to ISP 2 or ISP 3 for backup?

2. I used the dotq.1 on this setup. If 3 ISP network is a member of VLAN 1 and the other 3 private subnet each have its own VLAN. Can i still create a nat translation whether its static or dynamic NAT and use the ISP IP addresses without changing its VLAN member?

a.alekseev Wed, 09/12/2007 - 09:34

according to your diagram.

routers ISP1, ISP2, ISP3 are under your control. Am I right?

a.alekseev Fri, 09/14/2007 - 13:13

Run EIGRP on ISP1, ISP2, ISP3, NAT routers.

Remove "delay" from EIGRP cost calculations

Run object tracking on ISP1, ISP2, ISP3 to check reachability providers' routers

From ISP1, ISP2, ISP3 advertise defaut-route to NAT router.

Do nat on ISP1, ISP2, ISP3

Do no nat on NAT router :))

worldcalltel Sun, 09/16/2007 - 07:34

Why do I still need to do NAT on ISP1 ISP2 and ISP3? All NAT translation will handle by NAT Router. Can you show me some examples?

worldcalltel Sun, 09/16/2007 - 07:41

I made my initial configuration. But instead of using an EIGRP, i only used static routes. See attached file. I want to know your comments about my work. So I would know what else is missing. Thank you in advance for your support


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