ISPs who don't and won't aggregate are generally held in poor regard by the internet operations community. The regional internet registries publish the minimum allocation sizes. At the time of making this recording for all registries the minimum allocation for IPv4 was a slash 24 and for IPv6 it was a slash 4 to 8 for an assignment. Basically going to an end user or slash 32 for an allocation which is what goes to a network operator under 2010 there really wasn't any reason to see anything longer than a slash twenty two ipv4 prefix and Internet but now it's quite likely that the ipv4 run out is having a serious impact on the internet routing table and is determining what is PS are announcing to the global Internet so what happens if aggregation is not properly done on the Internet we will now show you an example of what happens if aggregates are not properly announced as you can see on the screen a customer has been assigned a slash twenty tree network from a s 100 slash 19 address block you see that only a s 100 announces the customers individual networks to the internet and not its aggregate what happens when the customer link goes down their slash 22 your network becomes unreachable and is withdrawn from the a s 100 cybg P since their ISP doesn't aggregate its slash 19 network block the slash twenty tree network withdrawal is announced to its peers and this starts rippling through the internet this causes added load on all Internet backbone routers as the network is removed from routing tables when the customer link returns the ax / 23 network is now visible to their ISP and is riad ver ties to peers this starts rippling through the internet again and adds the load on internet backbone routers as network is reinserted into the routing table some ISPs suppress the flaps an internet may take 10 to 20 minutes or longer for this network to be visible this means that quality of service is not guaranteed for this network in the good example for example imagine the same example where the customer has a slash 23 net worth assigned from the same AS 100 slash 19 block in this scenario a s 100 announces it slash 19 aggregate to the internet if the customer link goes down there slash 23 network becomes unreachable and it's we drawn from s 100 cybg P the slash 19 aggregate is still announced the Internet so there are no BGP hold on problems or propagation delays as well as no damping by other ISPs when the customer link returns their slash 23 network is visible again and re-injected into a s 100 ibgp the whole internet becomes visible immediately and the customer has quality of service perception in conclusion the good example is what everybody should do as it adds the Internet stability as well as reduce the size of the routing table this reduces routing churn thereby improving Internet QoS for everyone. That example is what many ISPs do due to either lack of knowledge or laziness. This must be avoided.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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