[AusNOG] AARNET and Aggregates

[Jonathan Thorpe]

Hi All,

Thanks for the replies.

AARNET are favouring a less specific path (i.e. preferring the /20 over 
the /24). Not only that, the AS paths for the /20s are also longer than 
those that they see for the /24, so it doesn't make a lot of sense.

The aggregation is internal - we have a large number of /28-27s that we 
aggregate as /24s and export those routes (and only /24s) to the peering 
networks (e.g. Equinix). The transit providers (e.g. AS703) receive the 
full advertisement of /20s only (no /24s).

I tried advertising the /20s in addition to the /24s to Equinix to no avail.

The only clue I have at the moment is that this affects other networks 
that peer with Equinix that also have routes that have been aggregated 
at some point (in other words, they show up as "aggregated by <AS>").

Has anyone else observed this behaviour?

Kind Regards,
Jonathan









Sean K. Finn wrote:
> Do you mean that if you advertise a /20 and a /24 that the route that is preferenced is the /24 ?
>
> I.e. the more-specific route (/24) takes precedence over the aggregated route (/20) ?
>
> -Sean.
>
> -----Original Message-----
> From: ausnog-bounces at lists.ausnog.net [mailto:ausnog-bounces at lists.ausnog.net] On Behalf Of Jonathan Thorpe
> Sent: Wednesday, 11 November 2009 10:12 AM
> To: AusNOG at ausnog.net
> Subject: [AusNOG] AARNET and Aggregates
>
> Hi All,
>
> I've been trying to track down an odd problem that I've noticed over the 
> past few days and hope someone can shed some light on this.
>
> Looking at AARNet's looking glass (http://lg.aarnet.edu.au/cgi-bin/lg), 
> it seems that routes that are flagged as aggregated are ignored when 
> there are routes available that are not aggregated, despite having 
> longer AS paths.
>
> This results in inbound traffic that we would prefer to come in over 
> peering networks (e.g. Equinix) entering our network over transit 
> instead. This is not ideal because it results in using more expensive 
> bandwidth and is a bit more asymmetrical than we'd like.
>
> At first, I thought it was just us (we advertise two /20s as a series of 
> /24s - e.g. 122.202.64.0/24 and 203.124.176.0/24 under AS37996 which are 
> aggregates of smaller prefixes), but looking at other peers that show up 
> over our Equinix link, others that also have aggregated routes seem to 
> experience the same phenomenon.
>
> I've written to the AARNET NOC already, however I'm wondering if this 
> could be by design or simply a side effect?
>
>
> Thoughts anyone?
>
> Kind Regards,
> Jonathan
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>   


-- 
Jonathan Thorpe
Conexim Australia
www.conexim.com.au
Phone: 1300 133 900
Direct: +61 2 8214 5804


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