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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