> Why are inconsistent AS-paths bad? Operationally, I mean. (I
> hadn't had any operational mishaps in all my years at UUNET
> because of inconsistent AS-paths, so I'm wondering if
> something else causes an issue now?)
>
> Technically there is nothing wrong with them. As you
> observed, it's simply an organisation multihoming using a
> private ASN - hopefully their upstreams are stripping out the
> private ASN before they propogate the originated prefixes
> further. (Mind you, I find it rare for two ISPs to agree
> which private ASN to use for a mutual customer to multihome -
> often so much easier to obtain a real one.)
>
> And yes, people can quite easily use public ASNs. It's simply
> a case of an APNIC member obtaining an ASN so their customer
> can multihome. And that can't be too hard, right?
>
> philip
> --
>
Hi Philip,
You're quite right. Technically, you can still be pass traffic to the
client's endpoint, although the path selected may not be optimal.
What can cause issues is if you then start creating routing policies on
your route reflectors based on AS-path, especially if you don't have a
direct peer with both of the providers that are carrying that customer's
traffic. With multiple peering points, you can end up backhauling the
traffic over your own links because the routing decisions passed via
your IGP are no longer necessarily optimal e.g. I may wish to pass a
10Mbit stream from a Wellington customer to somewhere else in
Wellington, via another provider, but the route reflectors see that
destination as having a shorter AS path via Auckland. We don't have a
destination AS that a metric can be applied to. We can of course apply
different costs to the WAN links (and should have done), but the
fine-grained control of traffic isn't achievable because the router
can't be given much to go on.
Policy routing and tagging is an option, but relies on the destination
site, who is not necessarily a customer of ours, informing us of changes
in their topology so our rules can be updated.
APNIC have always been very good when it comes to getting ASN's; I've
always found the justification of additional address space to be an
ordeal though :-)
In my opinion, it would be far more preferable if APNIC allocated an ASN
when a customer was given an independent address assignment. After all,
the reason that most want independantly allocated space is to
multihome...
Cheers,
Gordon
