From: "Tarko Tikan" <tarko@lanparty.ee>
To: adamv0025@netconsultings.com, nanog@nanog.org
Sent: Friday, June 21, 2019 2:51:20 AM
Subject: Re: few big monolithic PEs vs many small PEs

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