We're now going to look at the next example. We're scaling up the previous one where the network is connecting to the upstream provider and several local peers through an internet exchange point. This again is a very common situation in many regions of the internet--you connect to your transit provider to see the whole internet and you connect to the local internet exchange point so that local traffic stays local for the same reasons as earlier. If we look at the diagram we now see a s 100 rata a is connecting to an Ethernet switch at the internet exchange point and the several other networks present there Roger C is connecting to s 130 the upstream provider so how do we configure all of this we're going to announce the slash 19-acre gate to every neighboring a s that's before we will accept the default route only from the upstream provider we don't need the foo BGP table and we accept all rights which are originated by the exchange point peers if we look at the router a configuration we can scale the example we saw earlier we can set up a peer group we call it I X PPS and in that we put in our ID bond prefix list that lets our address block out we strip out private e asses we could send the BGP community attribute if we require that and we can also set up a road map to set local preference for the prefixes that we hear from the exchange point peers this is actually a very common type of peer group used for operators peering at an internet exchange point and then we apply this peer group to the exchange point neighbors I've got some examples here on the slide each neighbor has the peer group applied to it we've got an inbound filter because we always filter bgp sessions in and out as we have learned before and the prefix list is set up for each peer the right map set local pref basically everything that we hear from the exchange point peer we set local preference high with made at 150 in this example I realize right now we don't need this because we're only here the default right from the upstream provider and the prefixes we hear from exchange point peers are more specific and therefore better paths however in future we may get a second upstream provider we may get prefixes learned from the two up streams and we may be a prior preferences there so we're starting to introduce the concept of applying local policy to prefixes depending on where the heard from so I've set local preference 150 for exchange point peers and that means there will be higher priority than say prefixes we learn from any other source note that router a does not generate to aggregate for a s 100 if router a becomes disconnected from the backbone then the aggregate would simply disappear from the exchange part and BGP failover would work as expected if router a did generate the aggregate and then if router a was disconnected from the backbone it would carry on announcing the aggregate to the exchange point what would happen is outbound traffic will go to the upstream provider it would find its way to one of the peers as expected but then the peer would see the best path back to you via your router a which is announcing the aggregator at the exchange point and because it has nowhere for the traffic to go it would simply be dropped in the null interface or discard interface on the router so you'd end up cutting yourself off from the local peers so the best policy and the best practice if the router is remote from your network do not generate the aggregate on that router secondly notice the inbound right map which sets the local preference higher than the default and it's a visual reminder that BGP best path the local traffic will be across the exchange point and as I mentioned earlier allows the future case for operator may take bgp rights from their upstream providers if we look around to see configuration this is the one connecting to the upstream we simply allow the default in and send an aggregate out as we did before and just to conclude notice the router a configuration the prefix list is higher maintenance but it's safer if a peer is it going to introduce a new prefix they will let all members of exchange point know that they have a new address block they want to introduce and so everybody can update their filters and we don't generate the aggregate for s 100 on router a either the exchange point traffic goes to and from the local ixb everything else goes to the upstream notes for the I XP peers that the peering ISPs the exchange point will only exchange prefixes they originate and sometimes they exchange prefixes from neighboring guesses as well usually their customers so as before be aware that the exchange by border router should carry only the prefixes you want the exchange point peers to receive and the destinations you want them to be able to reach if you got the foo BGP table there or if you have a default route there your peers could potentially transit your backbone to the rest of the internet also if the exchange point routers that the i-x are distant from your backbone it's really important not to originate your address block at the exchange point router.

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

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