[AusNOG] Optus AS number prepend

[Dino Sosic]

Hi,

Thanks all for the advices. There are multiple approaches to this as you have all nicely pointed here. I'll go ahead and use the communities Optus provides and see if manipulating the local pref in their core will do the trick. If it does not, I'll try something else. 

Cheers
Dino


-----Original Message-----
From: Mark Smith [mailto:markzzzsmith at gmail.com] 
Sent: Thursday, 20 August 2015 9:06 PM
To: Dino Sosic <Dino.Sosic at datacom.com.au>
Cc: ausnog at lists.ausnog.net
Subject: Re: [AusNOG] Optus AS number prepend

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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