Re: few big monolithic PEs vs many small PEs

[Tarko Tikan <tarko () lanparty ee>]

hey,

> So what is the primary goal of us using the aggregation/access layer? It's to achieve better utilization of the 
> expensive router ports right? (hence called aggregation)

I'm in the eyeball business so saving router ports is not a primary concern.

Aggregation exists to aggregate downstream access devices like DSLAMs, 
OLTs etc. First of all they have interfaces that are not available in 
your typical PEs. Secondly they are physically located further 
downstream, closer to the customers. It is not economical or even 
physically possible to have an MPLS device next to every DSLAM, hence 
the aggregation.

Eyeball network topologies are very much driven by fiber layout that 
might have been built 10+ years ago following TDM network best practices 
(rings).

Ideally (and if your market situation and finances allow this) you want 
your access device (or in PON case, perhaps even a OLT linecard) to be 
only SPOF. If you now uplink this access device to a PE, PE linecard 
becomes a SPOF for many, let's say 40 as this is a typical port count, 
access devices.

If you don't want this to happen you can use second fiber pair for 
second uplink but you typically don't have fiber to second aggregation 
site. So your only option is to build on same fiber (so thats a SPOF 
too) to the same site. If you now uplink to same PE, you will still 
loose both uplinks during software upgrades.

Two devices will help with that making aggregation upgrades invisible 
for customers thus improving customer satisfaction. Again, it very much 
depends on market, in here the customers get nosy if they have more than 
one or two planned maintenances in a year (and this is not for some 
premium L3VPN service but just internet).

--
tarko