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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