On Mar 9, 2012, at 12:50 PM, Bernhard Schmidt wrote: > On 09.03.2012 20:31, Owen DeLong wrote: > > Hi, > >> Let us not forget that there is also the issue of PA /48s being >> advertised (quasi-legitimately) for some end-user organizations that >> are multi-homed but choose not to get PI space. It is not uncommon to >> obtain a PA /48 from provider A and also advertise it from Provider >> B. > > While I agree it's not uncommon, I'm not a big fan of this setup. Also, > provider A should still have his aggregate announced, which would allow > strictly filtering ISPs to reach the destination anyway. >
I'm not a big fan, either, but, I think that the concept of "be conservative in what you announce and liberal in what you accept" has to apply in this case. Since it is a common (quasi-)legitimate practice, arbitrarily filtering it is ill-advised IMHO. The statement about the covering aggregate assumes that there are no failures in the union of {site, loop, provider A}. In the event that there is such a failure, the aggregate may not help and may even be harmful. Since one of the key purposes of this kind of multihoming is to provide coverage in the event of such a failure, filtration of the more-specific seems to defeat the purpose. > Announcing /48s from a PA block without the covering aggregate calls for > trouble. No question. However, the covering aggregate alone is also insufficient. Owen