We're now going to look at partial routes and what strategy we would adopt here. What we will do is ask one upstream provider for a default route because a default route says I know how to get to the will Internet and it's easy to originate a default route towards a bgp neighbor we've seen the one line configuration earlier in this series we still ask the other upstream provider for a full routing table and then we filter it based on the neighboring AS or other policies that we want to implement for example we want traffic to their neighbors to go over the link to that AAS and in fact what will happen is most of what the upstream sends is thrown away it's much easier than asking the upstream provider to set up custom BGP filters for you let's look at the router C configuration it bears a lot of similarities to the previous fool routing table example we still have an inbound prefix filter allowing all prefixes apart from the RFC 6890 blocks I'd Bank we still sin or aggregate but now the AOS pass filter we used earlier which allowed a s130 and a s 130s immediately adjacent asses the es pass filter is applied directly to the bgp neighbor it's no longer used within a write map we also have a route map tag default law which looks for the default route and tags it with a low local preference the route map looks for the default sets local preference 280 the second line of the rat map leaves all other prefixes alone and you can see the a s path access list in the example here looks exactly the same as it did previously the router D configuration is greatly simplified we now accept the default route in and we send our address block out the summary of this the router C configuration except the full BGP table from a s 130 oh we could get them to send less but it's very few operators these days will actually permit this it's usually foo BGP table or defaults we filter the a SNS we get from s 130 such that only s 130 and a s1 30s neighboring asses are accepted we allow the default route and set it to local preference 80 traffic to those a asses we'll go over a s 130 link traffic to the other a SS we'll go over the link to s 140 if the s 140 link fails we get back up here s 130 and vice-versa well let's take the table with full route entries as we saw earlier and add in the partial rights from es 140 we get on prefix just the default and we leave it local preference 100 from es 130 well we still get the 30,000 prefix as we had in the previous example originated by a s 130 and the neighbors but now we just leave them local prep 100 we also get the default route from a s 130 and we set it to local preference 80 so in this example a grand total of 30,000 and two paths compare that with a 1.3 million that we had before so the outcome of this is such is such that if the link to s 140 fails with partial routes it's just a default route that was is withdrawn from the local BGP table the whole network will fail over to the s 130 link very very quickly in a matter of seconds and same goes the other way around if the s 130 link fails the 30 thousand and one prefixes learned from es 132 be quickly removed and the default right from PS 140 will carry the traffic compare this with the full routes what if the s 140 link goes down we have 650,000 prefixes that have to be withdrawn from the local BGP table this will not happen in a few seconds even with the best will of the vendors and the best software and the fastest CPUs there'll be a significant amount of time required to remove all 650,000 prefixes learn from s 140 across the entire tournament system customers will see this as an artic or worst case funny routing or maybe looping routing or traffic and so forth customers will notice this outage the partial route example it's unlikely any user will notice it unless they're doing some really delay sensitive or time-sensitive traffic or content analysis so the power out solution is much preferred and indeed in the Operations community several operators have talked about the benefits to their network when they have moved from carrying full routes for multihoming in an example like this to simply using partial routes.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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