Aggregation means announcing the address blocks received from the regional internet registry to the other ASes connected to your network. Sub prefixes of this aggregate may be also announced. They'll be used internally in the ISP network for ISP infrastructure or distributed for customer use. All these sub prefixes may also be announced to other autonomous systems to aid with multihoming as we'll see later on in this series; however, it seems to be that too many operators are still thinking about class C's classfull routing and Class A, Class B, Class C addresses were obsoleted in 1994 and what we're seeing as a proliferation of /24s in the internet routing table. When this recording was made in October 2019 we see four hundred forty eight thousand /24s in IPv4 table. This table has seven hundred seventy eight thousand prefixes in it. Interesting enough the same is happening for IPv6. We see thirty six thousand slash 48 in a recent table of 75,000 prefixes so how do you configure an aggregate we will be providing examples based on using the Cisco IOS imagine an isp has been allocated 166 0.0 /spaceafter 10 network statement under the address family ipv4 of the BGP configuration as shown on the screen he would then have to add an IP route statement of the prefix with an X table of non zero this static route is called a pull-up route connectivity to ISPs customers can be achieved by using more specific prefixes remember that the longest match is always preferred how would you want to announce this aggregate by using a filter on the ebgp peer a configuration example of how to properly announce an aggregate using filters on the ebgp pairs shown on the screen as you can see the router bgp statement denotes that the ISP has been assigned an ASN of 64 511 the aggregate is announced using the Network statement network 166 that 0.0 mask 255 255 - 2 4 0 remember that a pull-up route must be configured to announce the aggregate or the route must be present in the routing table an outbound filter called out - filter is applied to its neighbor 100 at 67 that 10.1 when an ASN of 101 which only permits its prefix of 100 at 6600 /19 to be announced and denies everything else and inbound fields are called default is applied to the same neighbor permitting only the default route from this neighbor.

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

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