Hello NANOG,
I noticed that more than 3K prefixes are with 2 Origin ASes.
Are they the simplest cases of anycast? Or they are mainly due to misconfiguration?
The third (and probably more likely) option is that the prefixes are advertised by two providers as the customer wants redundancy with their own IP space, but does not have a public ASN. Ie. the customer has a circuit and possibly a BGP feed to two different providers.
i honestly don't remember the requirement for single-origin
AS in a prefix announcement. Not sure why anyone would think that
multiple origin announcements are a "misconfiguration".
--bill
Ah, that's very possible. So I suppose the 90 prefixes with 3 origin ASes are due to the same reason..
Then there is basically no inter-As anycast besides the anycast prefix for DNS root, since I only noticed like 8 prefixes that are announced by more than 3 ASes..
There's lots of strangeness out there, for instance:
Bill lets anyone who has an IP assignment from an ep.net /24 announce that /24. The term 'anycast' has some vagueness at the edges.
Kris
I never said that was the only reason, I'm sure plenty of people are doing anycast with different originating ASes.
For example, check the 192.88.99.0/24 prefix.
Zhenkai Zhu wrote:
Then there is basically no inter-As anycast besides the anycast prefix for DNS root, since I only noticed like 8 prefixes that are announced by more than 3 ASes..
I presume you are using route-views or some such to get a larger picture of the BGP geography? I believe that many anycasts utilize the same ASN for their anycasted address space. I would expect that a few of them have an ASN specifically for their anycasted space.
Given that the networks are duplicates, there's no requirement that one part of the AS needs to receive routes from the other part of the AS. For management and such of the devices, I presume there are separate netblocks advertised with a different ASN (possibly even statically assigned by a hosting network).
Jack
Or you could just allow as-loops... not saying it's a good thing, but it
could be done 
Stefan Fouant
Then there is basically no inter-As anycast besides the anycast prefix for
DNS root, since I only noticed like 8 prefixes that are announced by more
than 3 ASes..
...but inter-domain anycast is often achieved by using a single origin
AS, which is then transited through the 'provider' autonomous systems.
In which case, you may be looking for the wrong thing.
Rob
Rob Evans wrote:
Then there is basically no inter-As anycast besides the anycast prefix for
DNS root, since I only noticed like 8 prefixes that are announced by more
than 3 ASes..
...but inter-domain anycast is often achieved by using a single origin
AS, which is then transited through the 'provider' autonomous systems.
Hi Rob,
Do you mean multihoming here?
Zhenkai
Jack Bates wrote:
Zhenkai Zhu wrote:
Then there is basically no inter-As anycast besides the anycast prefix for DNS root, since I only noticed like 8 prefixes that are announced by more than 3 ASes..
I presume you are using route-views or some such to get a larger picture of the BGP geography?
Yes.
I believe that many anycasts utilize the same ASN for their anycasted address space. I would expect that a few of them have an ASN specifically for their anycasted space.
Given that the networks are duplicates, there's no requirement that one part of the AS needs to receive routes from the other part of the AS. For management and such of the devices, I presume there are separate netblocks advertised with a different ASN (possibly even statically assigned by a hosting network).
I just want to make sure if I understand correctly. You mean that the anycasted address space can be announced in different places yet with the same origin AS?
Zhenkai
Zhenkai Zhu wrote:
I just want to make sure if I understand correctly. You mean that the anycasted address space can be announced in different places yet with the same origin AS?
Yes, and it is commonly done.
Jack
I was under the impression anycast services with homogeneous origin AS was far more common than the heterogeneous. Almost all the instances I know of use homogeneous origin AS.
I'd be interested in statistics either way.
Patrick W. Gilmore wrote:
I was under the impression anycast services with homogeneous origin AS was far more common than the heterogeneous. Almost all the instances I know of use homogeneous origin AS.
I'd be interested in statistics either way.
The original question provides a good statistic, I think. Only 8 prefixes that were announced by more than 3 origin AS.
Jack
Patrick W. Gilmore wrote:
Kevin Loch wrote:
Patrick W. Gilmore wrote:
Zhenkai Zhu wrote:
I just want to make sure if I understand correctly. You mean that
the anycasted address space can be announced in different places yet
with the same origin AS?Yes, and it is commonly done.
I was under the impression anycast services with homogeneous origin AS
was far more common than the heterogeneous. Almost all the instances
I know of use homogeneous origin AS.I'd be interested in statistics either way.
192.88.99.0/24, 2002::/16, and 2001::/32 are some
notable examples of heterogeneous origin AS.
And those prefixes (6to4 & Teredo) all come with annoying problems as
one never knows which relay is really being used and it is hard to debug
how the packets really flow.
Greets,
Jeroen
I know examples of both, although I will admit I did not know any v6 ones. 
However, I'm just interested in global stats.
[snip]
The original question provides a good statistic, I think. Only 8
prefixes that were announced by more than 3 origin AS.
And the overall message is that only the (prefix holder|originating
ASn[s]) can tell you if it is intended or not. Sadly, this is not a
useful metric for a third-party to use to determine prefix annoucnement
legitimacy. Perhaps an update to RPSL to allow for intentional multiple
origins is overdue?
[insert thread about folks who do/don't use tools regardless of
appropriateness]
Joe Provo wrote:
And the overall message is that only the (prefix holder|originating ASn[s]) can tell you if it is intended or not. Sadly, this is not a useful metric for a third-party to use to determine prefix annoucnement legitimacy. Perhaps an update to RPSL to allow for intentional multiple origins is overdue?
Legitimacy no, but it does lead one to believe that most anycast (given > 3 locations) is homogeneous. I would love multiple origin support.
Jack
There are tons which have 2 origin ASes. I realize that's not a lot of locations by today's anycast standards, but it is probably still common.
OTOH: "Serious" anycast obviously requires more than 2, so I grant your point. The overwhelming majority of anycast is done from homogeneous origin AS.
Which means I was right.