[AusNOG] dual home - set one carrier to secondary with AS prepend - not working

Tue Dec 22 03:03:02 EST 2015

Hi there,

LocalPref implementations always override AS-path length as you have
already discovered. For this reason you will receive "some" traffic on
the link because the network in question is providing transit for you.
However, some transit providers will allow you to tag your prefixes with
a special BGP community which lowers the import LocalPref of the
prefixes you are advertising to them below that of the prefixes they
receive from their peers. You should probably ask your transit providers
if they support this feature.

- If the backup ISP sees your prefixes via the other transit provider,
those routes will be Best Elected and the backup ISP will not act as an
upstream of your network.
- If the backup ISP no longer sees your prefixes via the other transit
provider the routes with lower LocalPref will become Best Elected. Due
to the customer communities tagged upon import it will start acting as
an upstream transit provider for your network.
- Once your primary ISP starts providing transit for your prefixes again
those become Best Elected in your backup ISP's network and as a result
it views the prefixes as "peer" instead of "downstream".

The advantage of this implementation is that you don't have to muck
about with more specific prefixes, as you know routing table growth is
bad. There's also going to be zero traffic coming in on your backup
circuit while your primary circuit is healthy.

The downside of this implementation is that you will have a
reconvergence hit until the routes via your primary ISP completely
disappear from the perspective of your backup ISP. It also means that if
there's only partial visibility of your prefixes in the worldwide
routing table (e.g. your primary ISP still announces in AU, but not to
its US transits) your fallback ISP may not pick up on that.

Best regards,
Martijn

On 12/21/2015 05:28 AM, Nik Geyer wrote:
> In response to a specific comment you made in your original post... " Using the looking glass tools, it looks like places like iinet are learning networks only via nextgen."
>
> The looking glasses are peering with routers that have already made their best path decision, therefore those routers are only advertising their best selected path to the looking glass. So in the case of the iiNet looking glass, the routers have already made best path selection between the Telstra and Nextgen received routes, Nextgen received routes won and is what is then advertised to the LG.
>
> Looking glasses are only meant to present information on how that AS routes to your prefixes at that very point in time, all best paths by the actual routers forwarding traffic have been calculated and what you are seeing is the end result. So the results you are seeing are normal.
>
> Specific to your situation, a lot of Australia will be preferring to send you traffic via Nextgen as they peer at IX's - it's cheaper than sending the traffic to you through a "Gang of Four" (GoF) company, i.e. Telstra, who don't. The only way you're going to be able to "fix" this is as per an earlier suggestion - advertise an aggregate via Nextgen and longer prefixes via Telstra. Best path selection algorithms won't even come into play then as longest prefix always wins.
>
> Cheers,
> Nik.
>
> -----Original Message-----
> From: AusNOG [mailto:ausnog-bounces at lists.ausnog.net] On Behalf Of Mitchell, Peter
> Sent: Monday, 21 December 2015 2:14 PM
> To: AusNOG at lists.ausnog.net
> Subject: Re: [AusNOG] dual home - set one carrier to secondary with AS prepend - not working
>
> Thanks all for responses.  We are in change freeze so it is difficult to test failover.
>
> Two of the /24's are part of a /23.  I will investigate sending different masks to different carriers as suggested.  
>
> Appreciate everyone's assistance.
>
> Peter.
>
> From: Ben Hohnke [mailto:settra+ausnog at gmail.com]
> Sent: Monday, 21 December 2015 1:00 PM
> To: Mitchell, Peter; AusNOG at lists.ausnog.net
> Subject: Re: [AusNOG] dual home - set one carrier to secondary with AS prepend - not working
>
> Is tearing down the nextgen session for testing a possibility? That way you'd be able to find out if Telstra are advertising your prefixes correctly.
> Also, does the looking glass results show the AS prepending correctly?
> Ben
>
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