BGP Internet Multihoming

1. What is Internet Multihoming ?
   
   Multihoming allows you to be connected to more than one Internet Service Provider. The reason why you may need multihoming is essentially for redundancy purposes. Assuming you have ISP_A and ISP_B, a major issue can occur at the ISP_A backbone or on the ISP_A circuit but you still have ISP_B routing your public IP block. In other word when one of the ISP's stops providing the Internet connectivity you still have the other ISP.
   
   
   
2. Why do I need for setting up a multihomed Internet service ?
   
   I will explain here what I use when planning and provisioning a such design, keep in mind there is more than one way to do it.
   
   First you need to have two Internet circuits provided by two different ISP's. I strongly recommend making sure both ISP's circuits comes to your building with different physical paths.You also need two Internet routers, we call them the CE (Customer Edge) routers. You can have multihoming with only one CE router but it would be a Single Point of Failure. I personally use Cisco 7204VXR with NPE-G2 and 1Go RAM. The reason why you need RAM is because of the Internet routing table.You need your own BGP AS Number, you will get it from the RIPE or from the other Internet Registry Associations depending of your region.You need your IP block, in my case I have a /20
   
   
   
3. How is the traffic balanced over ISP's circuits ?
   
   Well this THE challenge in multihoming.