[vc_row][vc_column][vc_column_text]Being connected using multiple circuits to the same ISP is a lot better than having to depend on a single circuit. But depending on a single ISP still allows for several risks:
- Physical outages. The ISP’s network may not have sufficient internal redundancy.
- Maintenance windows. If there is maintenance that impacts all of your connections, you’ll be unreachable during the window.
- Network management issues. If a problematic configuration or software update is rolled out, it may affect all of your connections.
- Routing problems. If the ISP runs into an issue with their internal routing or BGP, this can impact your reachability.
- Business continuity. There have been examples of ISPs going bankrupt and their customers being disconnected. (Usually there is some lead time when this happens.)
- Peering disputes. Sometimes ISPs “depeer” because of peering disputes, so that customers of ISP A can no longer reach customers of ISP B, even though both are reachable from ISP C.
These risks are reason enough to connect to at least two ISPs at the same time. An additional benefit of multihoming is that once you’re set up for it, it’s very easy to switch ISPs, so you’re in the position to negotiate for better deals. For the remainder of this guide, we’ll assume multihoming towards two ISPs. However, it’s entirely possible to connect to three or more ISPs at the same time.
In order to multihome towards two ISPs, you need the following:
- Connectivity to two BGP-capable ISPs
- Your own or at least semi-independent address space
- An AS number
- BGP-capable routers
In addition, you’ll need to monitor the status of your BGP connectivity and you’ll probably want to do at least some traffic engineering to balance incoming and/or outgoing traffic over both your ISPs.
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