RE: Pitfalls of annoucing /24s

What about the /24's that many ISPs (especially tier 2-3) are assigning
to multi-homed customers? What about an IX or "critical infrastructure
providers" that may be issued a /24 from ARIN (Policy 2001-3)?

Although it may be rare that a large aggregate would become unreachable
to a "large" network, doesn't the possibility exist that a customer with
a /24 would become unreachable (to some) due to the aggregate dropping
out even though the /24 should still be reachable? That scenario may
not be very likely, but the question of assymetric routing and one's
ability to balance traffic become issues. Assigning a lower preference
to /24's would be a lot friendlier than just throwing them away.

If I am way off base, I fully expect to be corrected (with volume). My
flame retardant suit is in place.


As long as it's provider assigned, and your provider announces the supernet that the /24 is from, it will still work. If you announce PI space out of the old class A space in /24's, many networks wont be able to reach you.

I have a /24 allocated to my by XO Communications in Phoenix, AZ
(67.X.X.0/24). I am currently announcing it to Verio in Europe. A
friend of mine that is an XO customer in Phoenix with BGP to XO can get
to that address block within XO's network.

But on the flip side. I also have a /22 from AT&T (12.X.X.0/22). When
I announce that network block to Verio in Europe (and nowhere else),
only certain places get to the Europe location. Networks that prefer
AT&T go to AT&T's network and die since the route isn't there. I don't
know if I am missing something but it think it may have to do with how
the network's peering/filter schemes work.

I may just be walking around the problem since I am a transit customer
of Verio and they normally filter.


Hi Andy,

Verio says they accept old class-a space at the /22 orshorter level so that
isn't it. I am fairly certain you can not successfully multihome with PA
class-A space.. If you are not announcing that /22 to AT&T then anyone that
is single-homed to AT&T (or preferring them) will probably not be able to
reach your /22. I ran into this problem with some 4/8 space that Level3
assigned to me by mistake. So you are dealing with more of a Policy issue
rather than general prefix filter.