We're now going to look at the case of
the local exchange point with upstream
provider is also appearing at the IXP.
This again is a relatively common
example where all the operators wish to
participate in the local exchange point
but perhaps two or three operators
serving the economy where the IXP is
located are also the transit providers
for everybody else. So how do we ensure
that transit traffic goes in the transit
link
and peering traffic goes on the peering link?
If you look at the diagram
we've now got AS100 connecting to the
exchange point as before and the AS100
connecting to the transit provider
through router c-- and the transit
provider participates in the exchange
part i plant traffic fromAS100 is
quite simple to set up
open traffic from AS100 follows the
following the upstream will send the
full BGP table to AS100 safe upstream
provider will send domestic routes to
the exchange point peers. AS100 will use the
exchange point for domestic traffic and
will use the upstream link for
international traffic so I'd ban traffic
was quite simple to achieve what about
inbound traffic towards a s 100 well
ears 100 will send the address block to
the IXP piers and it will send the
address block to the upstream the best
path from the upstream to a s 100 has
two paths it has one path going through
the transit link and another path
through the peering link so how do we
separate international and domestic
inbound traffic if we want domestic
traffic to go over the peering link and
international over the transit link what
do we do
well the solution is to separate the a
SS art there are other solutions but
they are actually very fiddly and really
not best practice the best solution is
to separate the autonomous systems after
all autonomous system represents routing
policy it's not the fence around the
company's network is to separate the
policy applied to different parts of the
infrastructure so the domestic Network
which peers of the exchange point
clearly belongs in a separate autonomous
system from the transit part of the
infrastructure so what the operator
needs to do is separate the
infrastructure into domestic and transit
as the diagram shows as150 is the
domestic network where all the access
and content is hosted and as160 is the
transit part of the network which sells
transit to other network operators in
the area so let's look at how this is
set up inbound traffic to s100 now
follows this s100 sends address block to
IXP peers including as150
which is their upstream providers
domestic network es-100 sends address
block to its upstream as160 router D in
as150 does not pass prefixes learn from
exchange point peers to as160 that's the
important rule now the best path from
the upstream to AS100 will be preferred
via the transit link the domestic s will
use the IX the transit a s will use the
direct transit link to AS100 so
transit providers who peer with the
customers at an exchange point for local
routes need to split the air signs into
two one is for domestic business and
domestic routes and another is for
international transit France
and to a s numbers are entirely
justifiable from the regional registries
because these two a asses have
completely different routing policies a
domestic s appears at the exchange point
and the transit s connects the transit
customers and up streams and this
solution is much easier to implement on
other solutions such as complex source
address policy routing remember an
autonomous system is used for
representing a distinct routing policy
and it doesn't necessarily map on to an
organization a transit business will
have different routing policy from a
nexus business or a hosting business and
therefore will quite likely have
a different autonomous system number
for each.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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