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Creating a redundant array of inexpensive links
RAID for the Network
Stable Internet connections (uplinks) are mission critical in many enterprises. Unfortunately, they often break down. If you want to connect two uplinks redundantly using two or more providers, you will typically experiment with the Border Gateway Protocol (BGP). This solution can be a fairly expensive, though, because providers charge dearly for enterprise-level connections. With a few restrictions, you can achieve redundancy far less expensively by opting for Linux and the Fault Tolerant (FT) Router [1].
A Linux host typically sends its packets with the help of a routing table. All packets that do not belong to a specific route follow the default route, which usually leads to the Internet. If this link fails, all the users on the inside are cut off from the Internet. The reasons for failure can be many, including bulldozers digging up cables, Layer 2 or 3 software failures, or routers that fail one hop downstream on the provider's network.
To avoid hard disk failures, administrators have relied on RAID for a long time; in the simplest case, this means simply doubling the number of disks in a mirroring RAID [2]. This isn't quite as easy for access lines. In the classic setup for this scenario, at least two Internet providers safeguard the network; that is, your own connection has two uplinks.
The administrator needs to inform the rest of the world using BGP (on internal networks, this can also be an internal routing protocol such as OSPF, or Open Shortest Path First). If one link fails, the protocols notice this and stop sending packets over the dead link.
The protocols detect failures automatically. If the Internet Protocol (IP) fails even though the link is working perfectly at the lowest level, the routing protocol notices this through active monitoring. Although it can take a while, at least the changeover happens without
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