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New Member

Can I connect one router to two different Internet Link?


I administer my university's LAN. Currently our Internet connection to the ISP is a VSAT link through a cisco 1750 router with one ethernet port and two serial ports. The Ethernet port is connected to the Internal LAN and one serial port is connected to the VSAT modem. Now we are acquiring a leased line connectivity from a different ISP. Physically we can connect the leased line to the unused serial port. However, the IP numbers being alloted by the different ISPs are different and both are telling that they will allow only packets generated from their alloted IP number range to pass through. Since there is only one Ethernet port we can configure it with one IP number only. Within this scenario is there any way by which can use the same router to connect ot the Internet through two different ISPs through two different type of links. Any suggestion advise will be highly welcome.


New Member

Re: Can I connect one router to two different Internet Link?

You can connect this router to two different ISP's, they put with this equipment you won't get to accomplish the load balancing among these links, with this equipment you will get to set up a failover architecture (activate / passive), in other words, when the main link is down the second link it will assume, for this you should use static routes with different costs and NAT.

New Member

Re: Can I connect one router to two different Internet Link?

Yes you can put a secondary ip adddress on the ethernet interface and use two default routes. Using that scenario you will do per packet load balancing.

New Member

Re: Can I connect one router to two different Internet Link?

Could you please suggest how?



Re: Can I connect one router to two different Internet Link?

Yes, you can do this; however, the traffic won't be evenly balanced between the two Internet connections.

1. Use ISP #1's assigned network block for half of your hosts and ISP #2's assigned network block for the second half.

2. Use Policy Based routing to route ISP #1 assigned IPs to ISP #1 and ISP #2 assigned IPs to ISP #2:



Re: Can I connect one router to two different Internet Link?

You actually have several choices in connecting to both ISPs.

You can run BGP and load balance (sort of) with full resilience. This is the best way if you are providing externally accessed services, as it is the only way you can serve up the same IP address through both ISPs. Load sharing with BGP is possible, but takes work and is rarely exactly even.

The other option is to use policy routing to split your traffic between the two ISPs, as suggested by mkmodl. This provides load balancing (to the extent you can evenly divide traffic by assigning IPs from the appropriate ISP to internal systems. This option may or may not allow you to provide resilience to link failure (and does not provide resilience to ISP failure).

If you can reliably detect link failure at the link level (such as by using keep alives on an point-to-point T1), you can use NAT to make your inside users on the failed link's IPs appear as if they are in the remaining ISPs address space. This is often satisfactory for clients surfing the net, but is rarely adequate for web servers or other services you are providing to the outside world (other than SMTP, which can automatically handle multiple servers).

There is a brief white paper on my web site which goes over all the options for adding resilience to an ISP connection as well as extensive discussion and example configurations (including both BGP and NAT approaches) in chapter 8 of my book High Availability Networking with Cisco.

Good luck and have fun!

Vincent C Jones