We're now going to talk about receiving prefixes from peers. A peer is an ISP with whom you agree to exchange prefixes you originate into the internet routing table. There is usually a peering agreement between you and your peer, and this means that prefixes you accept from a peer are only those you have indicated they will announce. Prefixes you announce to your peer are only those you have indicated you will announce. Agreeing what each will announce to the other can be done using emails as part of the peering agreement. And then updates or use of the internet routing registry and configuration tools such as IRRToolSet, the URL is shown on the screen. An example of how to receive prefixes from a peer is shown on your screen using Cisco IOS. As you can see, the peer has prefixes 220.127.116.11/16, 18.104.22.168/18, and 22.214.171.124/17. On your local router you would create a prefix-list called "my-peer" and permit these prefixes in the prefix-list and deny everything else. This deny is done by the last statement which says deny everything greater than a /32. This prefix-list "my-peer" is then assigned to the inbound filter of the peer configuration using the second neighbor configuration statement shown on the screen. As you can also see, your prefix in this case is configured on the prefix list called "my-prefix" and assigned to the outbound filter of the peer configuration using the third neighbor configuration statement, ensuring that only that prefix will be announced to the neighbor.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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