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The Large ISP: An Introduction1 of 5
By large ISP we mean an ISP with a physically large network, serving many external customers and providing a broad range of network and hosting services. We have in mind ISPs with national up to global reach, including the so-called 'Tier 1' ISPs.
Before considering what a Next Generation ISP might look like, we need a reasonably simple model of a current ISP where we have:
- a number of geographically separated "Points-of-Presence" (PoPs) connected by the ISP's "Core Network". The Core Network will use a variety of routing protocols-notably OSPF or IS-IS, and iBGP-which may be overlaid over MPLS and/or Layer 2 networks.
at any given PoP, there may be:
- "border" connections to other ISPs: peers and (except at Tier 1) transit providers-either directly or via Internet Exchange Points (IXPs). These connections all use eBGP.
a local Data Centre, where there may be some quantity of:
- ISP internal services, including: internal management and monitoring systems (OSS), DNS, email servers, etc.
- customer equipment or equipment provided for customer use.
- local Content Distribution Network (CDN) equipment: content cacheing. (Connections to external CDNs are, essentially, peering connections.)
- customer connections: either simple (Default Route) connections or Transit Customer connections (using eBGP). Transit customers may be other ISPs or multi-homed end-customers.
Within the PoP the Site Network will connect things locally and to the Core Network. That Site Network will use a variety of routing protocols and lower layer networks.
Not explicit in this model of an ISP are:
- network management and monitoring: the equipment for these will be distributed across the PoPs and connected by some internal network within a PoP, and across the Core Network between PoPs and to one or more Network Operation Centres. There may be some entirely separate way of reaching some PoPs, for disaster recovery.
- network services: services such as VPNs will overlay the connections and networks shown. General Internet Access is also provided over the connections and networks shown.
- the infrastructure for the PoPs: the buildings, their security, the reliable supply of electricity and cooling, etc.
- the network infrastructure between PoPs: from the fibre upwards.
- geography: the ISP's PoPs may be widely geographically spread, so the network between those PoPs may have significant latency and be less reliable than the network within a PoP.
- interconnection with other networks: which is a quite different from connections within a network.