We are now going to talk about the best practices for receiving prefixes. There are three scenarios for receiving prefixes from other ASNs. The first one is using BGP to talk to your customers, the second scenario is when BGP is needed to talk to peers, finally, the third scenario is using BGP to talk to your upstream or transit provider. Each has different filtering requirements and need to be considered separately. We're now going to talk about receiving prefixes from customers. ISPs should only accept prefixes which have been assigned or allocated to their downstream customer. If an ISP has assigned address space to its customer, then the customer is entitled to announce it back to his ISP. If the ISP has not assigned address space to its customer, then it must check in the five RIR databases to see if this address space really has been assigned to the customer using the "whois" tool. An example of how to use the "whois" command to check if a customer is entitled announced address space is shown on the screen. As you can see the output, the assigned IP address is displayed as "inetnum", with the name of the organization that was assigned the prefix as "netname". The status of the registration is shown as "ASSIGNED PORTABLE", which means that it is an assignment to a customer and the customer can announce it to you. If the assignment is for an ISP as shown in this output, the status would indicate "ALLOCATED UNSPECIFIED", this means that this prefix should not be announced if it was provided to the ISP by its customer. We'll now give an example of how to receive prefixes from a customer using Cisco IOS. As indicated on the screen, the IP address block of 100.69.0.0/20 is allocated to the customer and should only be announced to the upstream provider. The upstream provider should only accept this from the customer and nothing else. As you can see from the upstream provider's BGP configuration, the customer prefix is configured under "prefix-list customer", and assigned as an inbound filter under neighbor configuration. Since the upstream provider is only providing a default route to the customer, the default route is assigned to the prefix-list default, and assigned to the outbound filter of the upstream provider's neighbor configuration.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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