[AusNOG] Spreading the load of ISP customers at Layer2

[Damian Ivereigh]

Hi guys,

We have built all our ISP infrastructure based on the NBN style doubled 
tagging of services - in other words each subscriber circuit comes 
through on it's own ctag. This makes separating everything really easy 
because we pipe each vlan through to different BNG's. However we are now 
presented with a wholesaler who does not separate each circuit, but 
instead just bridges them all together into a single circuit. We can 
distinguish each circuit only by inspecting the DHCP Option82 so that we 
can allocate the right IP address, which is fine, but it is hard to 
allocate them to use a particular BNG to send and receive traffic.

By the way I am not talking dynamic load balancing just having multiple 
BNG with a subsection of the customers on each one - load sharing?

Until now with double tagging, we can reuse the same gateway IP address 
(i.e. the side facing the customer) on all the BNG and because each BNG 
only sees it's circuits, it will only respond to arps that it should do 
on the vlans assigned to it. However with all the customers on the same 
circuit it is impossible for multiple BNG to have the same IP address 
without creating all sorts of duplicate arps etc. We could turn off arp 
on all but one of the BNG and then put up with the asymmetric routing 
(makes reverse path filtering impossible) - i.e. send all upload traffic 
through a single BNG, but download comes from different ones (according 
to what BNG they are allocated to).

I have come up with another hack by using essentially using arp spoofing 
where we get a separate box to respond to the arp requests based on what 
the source IP is, but I can't help wondering how others have handled 
this. The wholesaler tells me there are other ISPs with 5000+ services 
on the single circuit (feels like a recipe for a broadcast storm to me).

Oh and no we don't want to use PPPoE :-)

Ideas anyone?

Damian

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