[cisco-nas] OT - routing ARIN /22 blocks

[Pierre Nepveu]

Hello Adam,

first of all, congrats on receiving your own IP allocation!  It's the first time
I see an allocation of /22. It used to be that /21 and even /20 were the minimum
allocations, but it must have changed... as many things have changed in the last
few years.

AFAIK, this provision for "least likely to be routable across the Internet" in
RFC-2050 refers to those blocks that were issued directly by ARIN (and other
IR's) _before_ CIDR blocks became the norm. (RFC-2050 is pretty old). Some
organizations have their own /24, for example. These are the networks least
likely to be routable, because some ISPs filter on prefix length so as to keep
their BGP table smaller. For example, most will not accept a prefix longer than
/24.

Some organizations (Verio is most famous for this) have (or used to have) pretty
stringent rules as far as what was accepted into their network, which made for
very weird behavior. However, they usually ajust according to ARIN's rules of
allocations, which means that your /22 should make it all over (since this is
the smallest that ARIN will allocate).

But, if you announce it as two /23, it might not. I've seen once an organization
who legitimately held a /16 announce just part of it (was it /24 or even
smaller? I don't remember). This small block didn't make it into Verio's
network. For class B, they were filtering on something like /20 or /21 Anything
smaller was simply filtered out of their BGP table. (Verio people : please don't
be offended by the above. I simply used you as an example of what Mr Greene
might expect.)

So, as a rule, always announce your /22, even if you decide to announce smaller
blocks. Those places that filter out smaller nets will still keep your /22 and
you won't be cut off. Fear not, and enjoy.

One last thing : there's a group (Team Cymru, http://www.cymru.com) that
maintains a list of networks that should not be on the Internet, mostly because
they are not released by IANA yet. They're called bogon prefixes (or martian
networks). Many organizations filter out these networks, based on what Team
Cymru publishes. If the block you received was recently released by IANA, it is
possible that not all such organizations have updated their filter, and you may
be filtered out of some places.

HTH,

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Le 2004-07-23 à 16:33, Adam Greene a écrit:

AG> Hi,
AG> 
AG> This is not exactly a cisco-nas issue, but I thought perhaps someone on this
AG> list might be able to help me out on this one, as I know there's lots of ISP
AG> folks listening....
AG> 
AG> We are a multi-homed ISP and have just gotten approval from ARIN for a /22
AG> IP allocation. This is fabulous for us, as it means that finally after all
AG> these years, we will stop being tied to our upstream provider(s) for IP
AG> space. However, before we break out the champagne and renumber for the nth
AG> time, I must pose the question: is this /22 going to be globally routable?
AG> 
AG> According to RFC 2050, "addresses issued directly from the IRs (non-provider
AG> based), are the least likely to be routable across the Internet."
AG> 
AG> My current upstream providers (AT&T and a local one called Hudson Valley
AG> Datanet) both assure me that they will have no problem routing & advertising
AG> our /22. In the case of the local provider, they will also have their
AG> upstream providers adjust their filters accordingly. I'm just concerned that
AG> some other provider out there will be unwilling to listen to such a small
AG> advertisement.
AG> 
AG> Does anyone have real-world experience that might ease my fears (or confirm
AG> them)?
AG> 
AG> --Adam
AG> 
AG> ---
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AG> 
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