So the last example showed how to multi-home to the same upstream provider. What we're going to do now is how we're going to multi-home two different upstream providers again using two links from the end site to two different service providers. There are few differences here. First off we're going to use a public AS number. We have to use a public AS number because the end site is now going to appear on the global Internet as its own entity it is possible to use a private a s but what will happen then is bearing in mind the previous examples the upstream providers are going to strip the private s from the announcements of Internet and will appear as though the end site is originated their address space from two different autonomous systems there's nothing technically wrong with that but operationally it might be challenging and that even some operators out there who don't really like this appearance of this inconsistent origin a s address space used could come from both app streams but more likely and more commonly it will come from the regional internet registry technically multihoming with address space from both app streams will work just fine but operationally it's next to impossible most upstream providers these days have specific requirements or policy enforce not permitting their end users or end customers from using the address space for anything else apart from connecting to their networks so the recommendation is very much if you wish to multi-home between two different upstream providers is to get independent address space from the regional internet registry the configuration concepts we're going to look at are actually very similar to those used for to links to the same and we'll look at some examples in a minute first off I want to just show you what the inconsistent s looks like so if you look at the diagram we have two autonomous systems a s 200s two-time with a common customer sitting at a are six five five three four who is multihoming between those two operators now of course as I said earlier we're stripping out the private a s from what goes to the internet so to appear as though the address space used by this n site is being originated by both a s 200 and AS 210 and the many examples of this across the Internet today several vendors will even have command line that shows you how to display those inconsistent autonomous systems it's not bad to do this and it's not illegal to do this but a lot of people don't like it and as I mentioned they're more likely to be operational challenges in trying to make this work so we're now going to look at two links to different ISPs and using one link as the primary and the other link as a backup only so following the example we did earlier with the connection to the same upstream provider if you look at the diagram we've got a s 100 connecting to s 1 10 and 2 is 120 and what we want to do is make s 110 the primary upstream provider for inbound and outbound traffic and we want to make s 120 the backup for inbound and outbound traffic so how do we configure this well as before we announce the slash 19 aggregate on each link in fact every example we do will always be announcing aggregates the primary link to a s 110 will make the standard announcement the aggregate gives out and the default right will come in from the upstream the backup link will lengthen the air path on the announcement of the aggregate by using the a s path pre-planned command so when we take a view from the internet we'll see a very long path through a s120 and a much shorter path through a s110 so this will make the a s120 pathway back up the default right we hear from a s120 will simply mark with a lower local preference to make it the backup path for outbound traffic when one link fails the announcement of the xix aggregate via the other link ensures continued connectivity as we've learned before let's look at a configuration example so we're looking at router a which is the connection to the primary upstream provider we're announcing our aggregate prefix list aggregator it will send send the aggregate out to the upstream and we accept the default right from them and that is all it's a very simple configuration again if we look at router B then we have the prefix list aggregate out allowing Negra gate I plant and we have another prefix list default in or allowing the default route in but but in addition we have to write Maps we have one right map s120 prepend which will do the S path prepend I burned and we have another right map LP low which will do the setting of the low local pref inbound let's look at the two right maps as you can see an example right map is 120 prepend has added three more instances of a s 102 the air span so if we go to a s 120 we'll actually see the aggregate coming from this network with four of s 100 in the air span so it's a much longer s path than if we sat in a s 110 I looked at the prefix announcement for the inbound default all we're doing is setting the local preference to 80 so es 100 will see two default routes one with the local prep 100 and the other one with a local prep eating and the latter is law which makes it the backup path now this is not a common situation as most sites tend to prefer to use whatever capacity they have but this can be used when maybe two competing ISPs agree to provide mutual backup to each other ordinarily they don't want their traffic to go through the competitor but if the main link to the Internet fails then they can get backup through their competitor or neighboring provider but what it does show is the basic concept of how to use local preference and a s path prepends for traffic engineering in particular directions s path prepend is commonly used on the Internet today to make an incoming path through an operator look much longer than it really is.

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

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