[c-nsp] ISP Dual AS

[Adam Vitkovsky]

Hi Mert,

AFAIK Tier1 ISPs are using backbone designs with several ASNs mainly for
their routing architecture scalability, as well as stability purposes. 
They also tend to use separate ASNs for different services they offer (inet
vs vpn) for manageability reasons
In order to cope with network that encompasses thousands of nodes and spans
across the globe they tend to divide it into several ASNs based on
continents/regions. 
First advantage is the possibility to deploy separate IGP domains (yes you
could accomplish that with confederations or hierarchical mpls) but I guess
this was voted as simplest option back in the late 90's. 
Another advantage is that the separate BGP RRs domains can be scaled down to
"reasonable" number of VPN prefixes compared with a single ASN design. 
However the multi-as designs also suffer from several drawbacks, mostly
increased complexity in service provisioning (inter-as: mpls-te, frr, vpws,
vpls, ....)

adam