BGP subnet size

Unanswered Question
Dec 10th, 2009

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? 

I have this problem too.
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Edison Ortiz Thu, 12/10/2009 - 10:24

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.

tprendergast Thu, 12/10/2009 - 10:46

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

Giuseppe Larosa Thu, 12/10/2009 - 11:53

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

Danilo Dy Thu, 12/10/2009 - 22:00

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).

Rick Morris Mon, 12/14/2009 - 13:36

/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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