Now RFC 1998 is okay for simple multihoming situations where mostly you have just one or two links to the same ISP and you have very simple needs to set local pref. Now ISPs create backbone support for many other communities to handle more complex situations. And the goal is to simplify the ISP BGP configuration and initially it looks a little bit more complicated but the idea is the moment you have decided how things work you set it up once and you don't have to keep fiddling each time so this gives a customer more policy control over what's happening. Now there are no recommended ISP BGP communities apart from what was documented in RFC in 1998 and the five well-known communities which are mostly implemented by the router software but there have been a couple of efforts to document a few from time to time, a couple of NANOG tutorials and some communities listed at the website shown on the screen and not that much more. However, each ISP, especially the larger transit providers, tend to publish their ISP policy either on the ISPs website or it will be in the autonomous system object inside the IRR, which is the internet routing registry. Now typically you'll have these communities on the slide in front of you so you'll have wherever I see an X that would be the s number of the is V and X 80 would mean set a local preference of 80 which means this is a backup path X 120 would mean set a local preference of 120 which is a primary path remember the default local preference is 100 so you want to override the default x1 would mean prepend the autonomous system number once when announced upstreams x2 would mean prepended twice to the upstream x3 would mean prepended three times when you announcing to the upstream and in a special one which came up recently is the 666 which means you set the next hope to one i 2.0 2.1 and this is a black hole root it's very useful for dos attack mitigation and it can be propagated along the different up streams pretty quickly so that if you're facing a DDoS you just attach that community to the network that is facing the attack and everybody upstream knows to blackhole it so let's look at a couple of examples the first one that we are going to look at is on the screen in front of you we have a Rooter inside s100 and a couple of customers right customizing s130 and upstream is on s 200 so for the customer who's on s130 we are going to use the customer policy on the inbound direction and when we have propagated into the upstream in s 200 we are going to use the upstream - out route map in the outbound direction as usual we create our community lists that we can easily match the different communities that could be attached to any advertisement coming towards us so community least one matches one hundred : one currently lists 200 : 2 all the way to community released 6 which is the black hole route and much is 100 : 666 and then we create a static route route to 1/9 to the 0 to 2.1 - now 0 this is convention so if we look at the customer policy on the inbound this is what you have on the screen so for the first statement it's a permit 10 and we match community for which remember community for matches 100 : 80 and we set the local preference to 18 for the next statement we match community 5 set a local preference to 120 and for the next one we match the community list 6 which sets the IP next hope to 1 9 to the 0 2.1 and we've already told the router to reach that address by shunting it to now 0 and it goes on and on and on to match these different communities and do exactly what would expect them to do now to the upstream we have this route map that you have on the screen route map upstream - out on permit 10 for community 1 we wanted to set the prepend just once so we prepared our s 100 once for match in community - we prepared twice so we have set s path prepend 100 100 and then the third time we have a match community 3 and we prepare three times it is e and this is a way to quickly implement the policy that we described in the previous slides now if you look at an example of sprint which is what you see on the screen this is what they publish as they are routing policy and you can get it from the sprint website on sprint studnets and you look at the policy and that's a URL at the bottom and as you can see there is number is on the left the string and the different communities that they want and what they're going to do when you send them those communities this is an TN t--'s example and you can see it's very very similar in the way they construct it and it looks very much like our examples that we have just gone through and more information can be looked at that URL that's on the screen right now which is their website and it's their routing policy on their website Verizon Europe this is out of the internet routing registry and you can get this output from Whois and this is an example of the kind of data and the kind of way they shall specify what they expect you to do and they just use their remarks field to do the same description that we've seen on the previous slides Talia has this one as well and you have very many more that have been snipped from this output and this is also from the IRR so you can see that output inside who is Beatty ignite for the European backbone also looks like this and it's slightly different in the way they specify what they want you to send compared to the other two. And level three looks like this. They have different sections and they're just using the remarks field and that's it.

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

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