Two identical fiber optic broadband lines working as one

Solution 1:

Depending on the technology you are employing, there may be many ways to achieve what you are trying to do:

• if your fiber links are Ethernet links and you can control the configuration of switches at both ends of the link, you may use 802.3ad link aggregation (also known as Trunking or Fast Etherchannel) to transparently aggregate at Layer 2
• if you are working with some kind of Internet Service Provider and your links are employing PPP as the encapsulation protocol, MPPP or Multilink PPP would do something rather similar at an encapsulation layer
• if no Layer 2 option is available, multipath routing might be an option
  • either with the help of routing protocols as done for BGP multipath load sharing
  • or with some additional routing logic as described in the LARTC howto using a feature implemented in Linux, but in no ways unique to it

The whole task would of course involve "Cisco learning" if the device you have is a Cisco router, but other router vendors would provide this kind of feature set as well.

Solution 2:

Are these lines coming from the same provider? What is your goal? Is it resiliency or performance?

If it's performance and a single provider, lobby for a higher-bandwidth link from the provider (on a GigE port).

If these are diverse links, I think the path of least resistance is to use of a link balancer unit. This was covered recently in: Load balancing with Cisco router

But the idea is that you use an external appliance that interfaces with your two (or more) ISP handoffs. I use the example of the Elfiq link balancer, which can aggregate the bandwidth of multiple ISP's and load balance traffic using a variety of algorithms. Other offerings by Barracuda, Peplink, etc. are as capable. If a link goes down, the other available lines will take over. It's transparent to your users.

For inbound services, balancing is either done by DNS or with the protocol's atributes (e.g. multiple MX records for mail).

In the Elfiq case, you would still use a single firewall behind the device to handle NAT translation. Your firewall config does not have to change. You can insert this load balancer into your environment without any disruption.