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Background: We own /24 as well as /23 blocks provider independent.

I am talking to an ISP, say ISP-A for a 10G line with a IPVPN line (2 links - actual inbound + IPVPN link for clean traffic when there is a attack) for DDOS mitigation. When there is a attack my ISP filters the attack the send the clean traffic through IPVPN. This is a proposal.

For redundancy purpose, we are planning to have an another ISP-B but still want to use the ISP-As DDOS mitigation. When we asked this, ISP-A said, we cannot do mitigation for blocks less than /24, technically they cannot advertise less than /24 due some memory issues etc and said it is kind of internet limitation not BGP or theirs(ISP-A).

Can anyone explain more on it?

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Most transit provider will refuse to announce tough BGP a prefix longer than /24. (I.E. /25 for example)

It's related to the size of the routing tables in the routers.

You have to understand that the prefix you announce will be propagated to almost all routers on the Internet, and that's a lot.

Each prefix announced take some memory in each router, and computational resources when the router has to pick up a route to forward a packet.

Allowing people to announce long prefixes would dramatically increase the numbers of routes in each router all over the world, which is not desirable.

There's actually many many prefix longer than /24 announced on the Internet, even many /32, because each ISP doesn't enforce this rule and there's no practical way to force them to do so, and partially because of historical reason (from time when this good practice didn't exist).

At the beginning of the Internet, there was very few routes and no central authority, no rules. Then the Internet grow bigger and bigger and it started to be an issue when some routers were no more able to handle the routing table in memory.

The fact that the Internet routing table grow so big lead to this consensus that we shouldn't anymore allow it.

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  • So technically, I cannot use my /23 block as it is? I need to split it, if so I need more than 1 drops in my location. Is it correct?
    – user88975
    Commented Apr 14, 2016 at 11:04
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    /23 is bigger than /24, not smaller (in terms of network size, I.E. hosts within the network). You can announce a /23, and one or two /24 but you cannot announce a /25 or a /28, for example.
    – JFL
    Commented Apr 14, 2016 at 11:06
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    I think you are confusing the issue with, "Most transit provider will refuse to announce a BGP prefix smaller than /24." Properly it would be any prefix larger than /24. Since you used the word, "prefix," you are referring the the number of bits in the mask, so a larger number of mask bits is not allowed. You may want to edit the answer.
    – Ron Maupin ♦
    Commented Apr 14, 2016 at 15:53
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    The wording "smaller prefix" is used here for a smaller block of addresses, but "smaller prefix" could also be read as "shorter prefix", meaning a larger block of addresses. Using the terms "shorter prefix" and "longer prefix" are less ambiguous and cause a lot less confusion though, so it would probably be better to reword the answer using those :)
    – Sander Steffann
    Commented Apr 14, 2016 at 16:37

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