We're going to now carry on our look at the basic principles of multihoming. The first one we want to think about is this: announcing address space attracts traffic. We learned that at the start of the series, of course, unless policy and upstream providers will interfere with this but basically announcing address space will attract traffic. Announcing the ISP aggregate out a link will result in traffic for that aggregate coming in that link. Announcing a subprefix of an aggregate out a link means that all traffic for that subprefix will come in that link even if the aggregate has announced some other path. It's important to remember the most specific announcement always wins. It is not possible to override it. To split traffic between two links we announce the aggregate on both links and that ensures redundancy. We also announce one half of the address space on each link. The first half on the first link, the second half on the second link. And this is just the first step. This is not the final solution. We see how this works. Traffic for the first half of the address space should come in the first link. Traffic for the second half of the address space should come in the second link. If either link fails the fact that we are announcing the aggregate ensures that there is a backup path. The keys to successful multihoming configuration is to keep traffic engineering prefix announcements independent of prefixes we learn by iBGP from access routers, the prefixes used by our customers. We need to understand how to announce aggregates. We need to understand the purpose of announcing subprefixes of aggregates. We need to understand how to manipulate BGP attributes and we need to realize that having too many up streams, too many transit paths or external paths will make multihoming harder as mentioned earlier, two or three transit providers is enough.

© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.

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