I am peered with two ISPs at our headquarters. I am advertising a /22 to both ISPs. Is there a minimum subnet size that you can advertise to an ISP? At one point I though you could only advertise a /22 or larger. Is this dependant on the ISP?
It varies with each ISP, as they set their own rules for what length prefix customers can advertise. I've seen most ISPs permit /24 as the smallest block they will let you advertise up. While you may only be able to get a /22 from ARIN (or the appropriate numbers authority), you can advertise as little as a /24.
this is the current IPv4 assigment policy at RIPE RIR.
>> The minimum size of a sub-allocation is /24. This is the smallest prefix length that can be reverse delegated and allows for a reasonable number of small assignments to be made by a downstream network operator.
You can take advantage of this to try to influence return paths for specific subnets within your own block
However, it depends on your ISP BGP peering policy. If you have a /22, they may not allow you to break them to /24. They may require you to advertise aggregated.
Do take note also of security regarding advertising and receiving routes in BGP peering.
1. If you are not a transit, make sure you don't advertise routes received from ISP-A to ISP-B (vice versa).
2. Sometimes, your ISP can make mistakes. They may accidentally advertise to you chunks of /30 from their BB network. I see this happens. You can put a policy in your router not to receive longer than /24 (your mileage may vary).
/24 is the smallest any ISP will accept from my past experience. I have had to break up a /22 into /24's to announce to my carrier who then summarized my announcement back to the /22 to the peers. This is something that will need to be discussed with them in reference to their BGP policy.
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