[c-nsp] LAC for PPPoE with multiple links to LNS

[Arie Vayner (avayner)]

Please see inline.
Arie

-----Original Message-----
From: Warwick Duncan [mailto:warwick at frogfoot.com] 
Sent: Wednesday, November 28, 2012 12:46
To: Arie Vayner (avayner)
Cc: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] LAC for PPPoE with multiple links to LNS

Hi Arie

On Wed, Nov 28, 2012 at 08:06:31PM +0000, Arie Vayner (avayner) wrote:
> The main issue would be that a single L2TP tunnel would hash to a 
> single uplink, and would never load share across different uplinks.

So if I understand correctly, the load sharing between LAC and LNS is simply a question of multipath IP routing and nothing to do with PPP?
[Arie Vayner (avayner)] The LAC-LNS leg is based on L2TP tunneling, so you need to make sure you have multiple tunnels, and they are load shared using the regular IP multipath solutions. Just remember many PPP sessions can be tunneled on the same L2TP tunnel...
Also, multiple tunnels between the same IP pairs still look as the same "flow" on the IP layer...

I think this could be a good reference:
http://www.cisco.com/en/US/tech/tk801/tk703/technologies_tech_note09186a008021fd87.shtml 

> What you most likely would have to do is to have multiple IP addresses 
> on the LAC and LNS (loopbacks...), and route them through different 
> links.
> 
> The LAC can loadshare the sessions between different L2TP tunnels...

We only get a single (L3 provider) IP address per layer 3 link so I'm guessing we would have to build something like a GRE tunnel per link and do some OSPF to distribute routes for loopback addresses in our own (probably RFC1918) IP space.  It's not ideal but I've certainly done plenty of far uglier hack jobs.
[Arie Vayner (avayner)] Not sure why you would need GRE... If you have multiple links, each link would have its own IPs...
What you need are now multiple loopbacks on the LNS (and most likely the LAC), and manage your traffic load sharing by routing the L2TP tunnels using the loopback endpoints.

Thanks for the help.

Regards
Warwick

> 
> Arie
> 
> -----Original Message-----
> From: cisco-nsp-bounces at puck.nether.net 
> [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Warwick Duncan
> Sent: Wednesday, November 28, 2012 11:55
> To: cisco-nsp at puck.nether.net
> Subject: [c-nsp] LAC for PPPoE with multiple links to LNS
> 
> Hi
> 
> We're planning a network of a couple of thousand remote sites connecting with PPP to a central router (we're thinking a Cisco ASR 1002-X).  For most sites we can get ethernet to the central router and can use PPPoE but for several hundred we have to go via a third party's layer 3 network, where the layout could be as follows:
> 
>   Site 1 --                         -- layer 3 upstream -\
>   Site 2 -- Aggregation point (LAC) -- layer 3 upstream -- ASR (LNS)
>   Site 3 --                         -- layer 3 upstream -/
> 
> i.e. we connect a handful of sites (between 2 and 20) with ethernet to a central point at which we buy multiple layer 3 upstream links, all from the same provider, to give the required bandwidth.  For reasons beyond our control, a single link can't provide adequate bandwidth for even a single site.
> 
> The obvious starting point, unless we're missing something, is to put an L2TP LAC at the aggregation point and tunnel the PPP to the central ASR, which we configure as an LNS.  This achieves our goal of having a PPP session between the ASR and each site.
> 
> How do we elegantly aggregate the multiple layer 3 links between LAC and LNS into a single logical bundle?  If it can't be elegant, what's the least ugly way to do it?
> 
> I don't think multilink PPP is applicable in this scenario (it would be between Site N and LAC, had the multiple links been there) but I'd happily be proved wrong.
> 
> It would be nice to guard against equipment failure at the aggregation point.  Is there a way to put up 2 LAC's with the upstream links split between them, yet still provide any single site with the full aggregated bandwidth of all upstream links?
> 
> Lastly, would something like a 3640 be an adequate LAC for, say, 12Mbps upstream (6 x 2Mbps serial links)?  Are there any gotchas with doing MPLS over PPP on an ASR 1002-X?
> 
> Thanks for reading this far...
> 
> Regards
> Warwick
> 
> --
> Warwick Duncan
> Frogfoot Networks ISP
> http://www.frogfoot.com/
> +27.21.448.7225
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--
Warwick Duncan
Frogfoot Networks ISP
http://www.frogfoot.com/
+27.21.448.7225