12-10-2009 06:46 AM - edited 03-04-2019 06:55 AM
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?
12-10-2009 10:24 AM
The longest mask I've seen is /24
The shortest mask I've seen in AT&T's looking glass has been /16
I suggest contacting your ISP to verify what are they willing to accept
Regards
Edison.
12-10-2009 10:46 AM
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.
Hope that helps.
Tim
12-10-2009 11:53 AM
Hello Daniel,
/24 is the most specific route that can be advertised
for example see (for Europe the RIR is RIPE)
it is true that mininum prefix advertisable on the internet is /24
see
http://www.ripe.net/ripe/docs/ripe-484.html
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
Hope to help
Giuseppe
12-10-2009 10:00 PM
Today, /24 is the minimum allowed by most ISP.
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).
12-14-2009 01:36 PM
/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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