So the first example we will look at is dealing with the case where one link is going to be used as the primary link and the other link is the backup link and this will be for two links to the same ISP and it applies when the end site has bought maybe a large primary one link to the upstream provider and a small secondary one link which the users backup. For example the primary path might be an E1 2 megabits per second the backup might be 64 kilobits per second or something similar obviously to apply to your situation scale the bandwidths appropriately if you look at the diagram you'll see that a s 100 is connected by two paths to a s 6 5534 the link between router a and C is the primary path and the link between router B and D is the backup path as you can see in the diagram we've used a s 6 5534 for the N sight that is multihoming this is a private a s as we learned earlier there's no need for a public air snubber for this type of multihoming the end site has only got two external connections and only connecting to the same upstream provider s 100 will remove the private e s it hears from its customer and of course it will remove any customer sub prefixes from internet announcements so let's have a look at see how we configure this well to start off we announce the slash 19 aggregate on each link the primary link will receive the slash 19 the backup link will receive the slash 19 this means the upstream provider will see the slash 19 on both paths and if either link fails because we're announcing the 19 on the alternative path as well it ensures continued connectivity for the inside for inbound well the upstream provider simply announces a default route on both paths so the insight will see the default route on both links and again if either link should fail the default being heard on other paths ensures continued connectivity but how do we make one path the primary and the other one the backup well as we learned when we looked at the BGP attributes we can use the metric and the local preference to achieve this if we do all the traffic engineering from the insight perspective if the insight announced the slash 19 on the backup path with an increased metric then the upstream provider will see the two paths one with metric zero which is the default and the other one with a larger metric larger non zero metric which makes this the backup path for the other way around the insight will receive the default route on both links one path will have the default local preference of 100 and the other path the end sight will reduce the local preference say make it 80 this lower local preference means the backup path is less preferred hence making it the backup so will this set up the backup link having the slash 19 announced with increased metric and the inbound default route being tagged with a lower local preference will ensure that the backup link functions as designed and of course if either link fails the other one becomes the primary path ensuring continued connectivity between the and site and the operator let's look at the configuration for router a we've included Cisco IOS configuration showing how this might be configured router a is the primary path on the end site and you see router is announcing the aggregate and is accepting that fault drought in from the upstream provider router here of course is also originating the slash 19 address space if we look at router B we see the same prefix list letting the aggregate out to the upstream provider and we see another prefix list which also accepts the default route in from the upstream provider so this is the same configuration as on router a but we now have to write Maps one route map which I've called med tank out is applied on the outbound announcement and the other route map LP low in is applied on the incoming announcements if we look at the next slide we'll see what the write Maps actually are doing right map med turn out sets the metric on all prefixes being sent out to the upstream provider we're only announcing the slash 19 so the slash 19 gets a med of 10 set on it the route map LP low in matches all prefixes and sets local preference 90 inbound and then the N site will see the two paths one with local preference 100 the default on router a and this one on router B with local preference 90 and we achieve the setup that we were aiming for if we look at the router C configuration on the upstream provider or the upstream provider does is originated default route sets up a prefix list to let that default outbound to the customer and creates another prefix list to allow the customer slash 19 address block in that it's a very simple configuration and indeed it's exactly the same as for router D again a default originated allowing the customer prefix in and allowing the default route outbound the upstream provider has a very simple configuration which they don't need to adjust if the customer chooses to swap around which link becomes Brown which were think becomes backup if we look at rotary router II was the upstream providers connection to the rest of the internet and I want to point out to the remove private es command the upstream provider there is stripping out the private a s six five five three four in the announcement of the customer prefix out to the Internet as we saw earlier missus best practice private a s number should not be seen on the public internet so the result of this is router II will announce the slash nineteen prefix learned from the customer end site out to the internet as though it was originated from AS100.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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