# Implementation of RIPE-181++

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**Tony Bates**

*Tue Oct 4 11:03:50 CET 1994*

Daniel Karrenberg <Daniel.Karrenberg at ripe.net> writes: * Okay, thanks for your changes. I have incorperated them and added a small note on the slitting ASes. Find below he revised text. --Tony. P.S. Today is really the last day to comment before we send this document out as ripe-181. The interaction of interas-in/interas-out with as-in/as-out Although formally defined above, the rules associated with policy described in terms of interas-in and interas-out with respect to as-in and as-out are worthy of clarification for implementation. When using interas-in or interas-out policy descriptions, one must always make sure the set of policies described between two ASes is always equal to or a sub-set of the policy described in the global as-in or as-out policy. When a sub-set is described remember the remaining routes are implicitly shared across all connections. It is an error for the interas policies to describe a superset of the glo- bal policies, i.e. to announce or accept more routes than the global policies. When defining complex interas based policies it is advisable to ensure that any possible ambiguities are not present by explicitly defining your policy with respect to the global as-in and as-out policy. If we look at a simple example, taking just in-bound announcements to simplify things. If we have the following global policy: aut-num: AS1 as-in: from AS2 10 accept AS100 OR {10.0.0.0/8} Suppose there are three peerings between AS1 and AS2, known as L1- R1, L2-R2 and L3-R3 respectively. The actual policy of these connec- tions is to accept AS100 equally on these three links and just route 10.0.0.0/8 on L3-R3. The simple way to mention this exception is to just specify an interas policy for L3-R3: interas-in: from AS2 L3 R3 (pref=100) accept {10.0.0.0/8} The implicit rule that all routes not mentioned in interas policies are accepted on all links with equal preference ensures the desired result. The same policy can be written explicitly as: interas-in: from AS2 L1 R1 (pref=100) accept AS100 interas-in: from AS2 L2 R2 (pref=100) accept AS100 interas-in: from AS2 L3 R3 (pref=100) accept AS100 OR {10.0.0.0/8} ripe-181.txt October, 1994 ^L - 29 - Whilst this may at first sight seem obvious, the problem arises when not all connections are mentioned. For example, if we specified only an interas-in line for L3-R3 as below: aut-num: AS1 as-in: from AS2 10 accept AS100 OR {10.0.0.0/8} interas-in: from AS2 L3 R3 (pref=100) accept AS100 OR {10.0.0.0/8} then the policy for the other links according to the rules above would mean they were equal to the global policy minus the sum of the local policies (i.e. ((AS100 OR {10.0.0.0/0}) / (AS100 OR {10.0.0.0/0})) = empty) which in this case would mean nothing is accepted on connections L1-R1 and L2-R2 which is incorrect. Another example: If we only registered the policy for link L2- R2: interas-in: from AS2 L2 R2 (pref=100) accept AS100 The implicit policy for both L1-R1 and L3-R3 would be as follows: interas-in: from AS2 L1 R1 (pref=100) accept {10.0.0.0/8} interas-in: from AS2 L3 R3 (pref=100) accept {10.0.0.0/8} This is derived as the set of global policies minus the set of interas-in policies (in this case just accept AS100 as it was the L2-R2 interas-in policy we registered) with equal cost for the remaining connection. This again is clearly not what was intended. We strongly recommend that you always mention all policies for all interas connections explicitly, to avoid these possible errors. One should always ensure the set of the interas policies is equal to the global policy. Clearly if interas policies differ in complex ways it is worth considering splitting the AS in question into separate ASes. However, this is beyond the direct scope of this document. It should also be noted there is no direct relationship between the cost used in as-in and the preference used in interas-in. -------- Logged at Wed Sep 7 01:08:13 MET DST 1994 ---------

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