[cisco-bba] IP DSLAM ( Fully Distributed BRAS)

Vusi Ndebele vusi at webdevuk.com
Fri Aug 5 16:02:57 EDT 2005



I wanted to get some views and experiences of the much touted 'IP DSLAM'. As
far as I can tell, many of the DSLAMs on the market right now are more
Ethernet DSLAMS effectively bridging with a bit of fancy snooping etc for
multicast. I call an IP DSLAM a box that has encapsulates subscriber traffic
into IP packets on the trunk interface. Products on the market include the
Alcatel ISAM, Huawei MA5600, Lucent Stinger etc with their relevant 'BRAS/
IP Services' module. 

Up to now, I can't find many examples of large deployments of fully
distributed BRAS within the DSLAM. In an IP world, surely this is the
realisation of a network engineer's dream but few seem to have implemented
it. The industry seems divided on this. Some vendors believe the Metro POP
is as far out as you should push the BRAS.


I'm looking for some real world telco/clec  views fro people who have either
evaluated or implemented this entirely distributed BRAS architecture to
support single, double or triple play services. We are still running PPPoE
on subscriber CPE and though getting rid of PPP and L2TP altogether in favor
of a DHCP addressing mechanism is a sweet dream, it's a few years away at


In my scenario, I'd be looking for the 'BRAS Card' to be terminating PPP
sessions and performing L2TP LAC functionality. Next stage would be to
remove L2TP and terminate PPP sessions at the DSLAM allowing local routing,
VPN's, Voice breakout etc., The final Phase would be to wave goodbye to PPP
and address all devices with DHCP using an IP Helper on the BRAS card. (This
PPP issue warrants a thread of its own)


I'd like to hear pro's and cons, a few of mine are listed below.




1.	IP Backhaul ( Easier to diagnose IP than PPPoE, Easier and cheaper
to create resilience and to load balance over multiple links)
2.	Better abilty to schedule based on IP CoS/ QoS requirements at first
congestion point (middle mile)
3.	Potentially better network availability vs huge centralized BRAS at
POP ( less customers affected by BRAS outage)



1.                   Complex address management

2.                   More devices in dynamic routing domain


Please let me know your views





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