So the previous example showed how to connect a single customer to the upstream service provider network with two links. Now it's quite unusual for an ISP to just have one dual homed customer. It's actually quite a valid and indeed valuable service offering for an ISP with multiple points of presence to actually make this a proper service for lots of end-users. It's probably better for the ISP than having the customer multi-home with another provider although when we talked about multihoming as introduction we did talk about connecting to two different upstream providers but very often network operators certainly those with multiple points of presence will offer this as a service as well so what we want to do is look at scaling the configuration for the upstream provider and by scaling I mean simplifying it as well so we'll use our chosen equipment vendors best practices for making the configuration easier to manage on the router so that means using templates and in Cisco's case using peer groups you want every customer to have the same configuration more or less so let's look at the diagram now we have a s 100 with links to in this case three customers and the first thing that you'll notice that each customer is living in the same autonomous system we've chosen es6 5534 for each of the customer guesses router e in the diagram is a s1 hundreds up stream or one of the upstream links and that again as before removes the private ESS we're using and any customer sub prefixes from internet announcement so how do we handle this one well the customer announcements are exactly the same as in previous example we follow the same techniques using the same private es deep the same a s is perfectly fine in fact the IETF RFC 2270 documents this particular example as was used by some network operators back in the late 90s we can do this because address space is not overlapping each customer only hears the default route so we don't run into any issues of BGP loop detection one customers prefix will go to the upstream provider but the upstream provider is only sending the default route to the other customers there's no need for one customer to see their address space that's being used by the other one hence the loop detection in BGP doesn't cause any issue and the nice thing about this is the upstream provider can template the configuration for all of the router A's and router B's they go to the customer site so this could help the customers with their configurations and they're the same as in the examples we saw earlier we've concluded some here just for reference router a1 might be using this slash 19 and a slash 20 and of course the prefix list is the same as in the earlier example allowing the 19 aggregate out as well as 1/20 same with the router B configuration it allows the 19 and the other slash 20 out but our focus is on the upstream provider now let's have a look at router see what we've done here is created a peer group called BGP customers a peer group if you recall is a way of grouping neighbors that have the same configuration so in this peer group were setting up a s 6 5 5 3 4 which is what we're using for each customer each customer is getting a default originate because they all get a default route and they're also getting an outbound filter which only allows the default right out to the customer once we've created the peer group we simply apply to all the customers that are connecting so in the examples here we have the three customers customer one gets the peer group bgp customers and then we also have a prefix list customer one in which is filtering the prefix we hear from our customer it's very important as we have learned earlier to always filter outbound and inbound announcements never assume that your peer is going to do your filtering for you so we have customer one we have customer two and with customer three and so on it goes each time this operator adds another customer they just add that customer to the peer group create a prefix list for their address space and it's done and this helps scale the configuration for the network operator if we look at router D configuration it's the same in fact it is exactly the same and this is the nice thing for the network operator they again don't need to worry but which link the customer sending the sub prefixes of the aggregate we put on a standard configuration the same peer group and apply to the three customers as the slide show what about router e connecting to the rest of the Internet well if we assume that the customer address space is not part of the upstream providers address block then the outbound prefix list would of course need to list each prefix that each customer has and we strip out the private s as we've done before however if the customer prefixes actually come from the ISPs address block then we don't need to announce these sub prefixes to the internet the ISP aggregate is more than sufficient because the rest of internet again does not care about the detailed traffic engineering between the customer and the service provider so as the example shows the router e configuration is just a simple announcement of the ISPs aggregate out so let's just summarize this if we're multihoming to the same upstream provider we're going to use a private a s there really is no need for a public s number and it's quite possible that the regional registry policy may not allow the delegation of an s number for this purpose we're going to leak suppressant prefixes to the upstream provider only to aid our load sharing it is really important as a customer that we announce our address block to the upstream provider and also that as we've noticed the upstream providers border configuration to the rest of the internet is identical across all these situations we're announcing only aggregates to the global Internet and we' re making sure that the private AS numbers used for the customer multihoming are not leaked to the internet either.

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

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