[AusNOG] Optus AS number prepend

[Mark Smith]

On 20 August 2015 at 11:52, Dino Sosic <Dino.Sosic at datacom.com.au> wrote:
> Hi,
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> If I’m eBGP peering with Optus ( Evolve ) and I want them to prefer another
> path to me ( via Amcom provider ) would prepending my AS number 5-6 times
> work?

AS pre-pending is unlikely to work, Optus are most likely going to be
preferring (via LOCAL_PREF) your (and other customer) routes over all
others (transit and peering). The only way they wouldn't be is if they
had some sort of a backup link product that only used the link if the
path to you stopped being available via their Internet transit or
peering links (in this case, your customer routes would have a lower
LOCAL_PREF than Internet and Peering routes). You'd know if that
product existed and you'd bought it.

There are a couple of other options, although they're bit debatable,
as they can cause issues, concern, or are just frowned upon.

Firstly, you could advertise two more specifics over your preferred
link, and a less specific over your Optus link (e.g., 2 x /24s over
preferred link, 1 x /23 over Optus link). The longest match rule, even
in Optus's network, would match traffic to the /24s before the /23,
and follow where ever the /24s lead. This is frowned upon because
you're now taking up two global Internet route table slots for your
address space instead of one.

The only other option is conditional advertising, where you would only
announce your routes to Optus if your non-Optus link fails. That's not
a standard feature of BGP, it is a BGP implementation feature, so it
may not be available on the BGP speaker you're using. This may confuse
Optus if they were using your route announcements as a or the method
to monitor your link/service status.

> Or it would never get that far down the path selection? I’m unsure how
> Optus core is setup, so maybe someone can shed some light. J
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> Thanks,
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> Dino
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