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Re: [RRG] getting rid of longest match



On 25 feb 2008, at 23:43, Scott Brim wrote:

What I'm thinking is that for any given part of the address space, there would be a fixed prefix length. So for 192.0.0.0/8 /24, for 64.0.0.0/8 /20, for 17.0.0.0/8 /8... So in theory, there would be no overlap. There could still be if the filters weren't set up properly, which would probably have to resolved when translating the RIB into the FIB.

Isn't this just what we did pre-CIDR?

Yes, except that it wouldn't be hardcoded into the IP stack but enforced through filters. So it could be /24 for 193.0.0.0/9 and /19 for 193.128.0.0/9.

The issue with overlap is: which one do you choose? Longest match says the smallest block is the good one, but I don't think YouTube appreciated that logic yesterday. If we ditch longest match we need another rule. Allowing only one prefix size for a certain address range is one way to resolve this, but others are also possible. For instance, S-BGP or soBGP-like security mechanisms.

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