View source for Default-free zone

{{Short description|Network zones not needing default routing rules}}
{{Use American English|date=January 2019}}
In [[Internet]] [[routing]], the '''default-free zone''' ('''DFZ''') is the collection of all Internet [[Autonomous system (Internet)|autonomous system]]s (AS) that do not require a [[default route]] to route a packet to any destination. Conceptually, DFZ routers have a "complete" [[Border Gateway Protocol]] table, sometimes referred to as the ''Internet routing table'', ''global routing table'' or ''global BGP table''. However, internet routing changes rapidly and the widespread use of [[route filtering]] ensures that no router has a complete view of all routes. Any routing table created would look different from the perspective of different routers, even if a stable view could be achieved.
== Highly connected autonomous systems and routers ==
The Weekly Routing Reports<ref>{{cite web
| url=http://thyme.apnic.net/
| title=BGP Routing Table Analysis
| date=September 2014
| accessdate=2015-06-13}}</ref> used by the ISP community come from the [[Asia-Pacific Network Information Centre]] (APNIC) router in [[Tokyo]], which is a well-connected router that has as good a view of the Internet as any other single router. For serious routing research, however, routing information will be captured at multiple well-connected sites, including high-traffic ISPs (see the "skitter core") below.
On May 12, 2014, there were 494,105 routes seen by the APNIC router. These came from 46,795 autonomous systems, of which only 172 were transit-only and 35787 were stub/origin-only. 6087 autonomous systems provided some level of transit.<ref>{{cite web
| url=http://thyme.apnic.net/ap-data/2014/05/12/0400/mail-global
| title=Routing Table Report   04:00 +10GMT Mon 12 May, 2014
| date=2014-05-12
| accessdate=2015-06-13}}</ref>
== The idea of an "Internet core" ==
The term "default-free zone" is sometimes confused with an "Internet core" or [[Internet backbone]], but there has been no true "core" since before the [[Border Gateway Protocol]] (BGP) was introduced. In pre-BGP days, when the [[Exterior Gateway Protocol]] (EGP) was the exterior routing protocol, it indeed could be assumed there was a single Internet core.
That concept, however, has been obsolete for a long time. At best, today's definition of the Internet core is statistical, with the "skitter core" being some number of AS with the greatest traffic according to the [[Cooperative Association for Internet Data Analysis|CAIDA]] measurements, previously made with its measuring tool called "skitter".<ref>{{cite web
| url=http://www.caida.org/research/topology/as_core_network/
| title=IPv4 and IPv6 AS Core: Visualizing IPv4 and IPv6 Internet Topology at a Macroscopic Scale
| date=2014-06-23
| publisher=Center for Applied Internet Data Analysis
| accessdate=2015-06-13}}</ref>  The CAIDA measurements are constantly updated.
== Information at Internet exchange points ==
Large [[Internet Exchange Point]]s (IXP)—in that they typically include full routes as seen by multiple ISPs, as well as customer routes, in their exchange fabric—are extremely good places to assess global Internet routing.<ref>{{cite web
| url=https://www.eecs.umich.edu/techreports/cse/98/CSE-TR-368-98.pdf
| title=Origins of Internet Routing Instability
| first1=Craig
| last1=Labovitz
| first2=G. Robert
| last2=Malan
| first3=Farnam
| last3=Jahanian
| date=1998
| accessdate=2015-06-13}}</ref>
Before the current commercial Internet evolved, the [[National Science Foundation Network|NSFNET]], which interconnected five US government funded supercomputer centers, could have been considered the high-speed Internet core. Four IXPs supported NSFNET, but these IXPs evolved into a model where commercial traffic could meet there. While it is slightly difficult to point to a precise endpoint, NSF funding for transmission ceased by 1998.
== Customer, non-ISP participation in the DFZ ==
It is common practice, in a [[multihoming|multihomed]] but [[stub network|stub]] (i.e., non-transit) autonomous system,{{Ref RFC|1930}} for the BGP-speaking router(s) to take "full routes" from the various ISPs to which the AS is multihomed.  Especially if there is more than one router connected to the same ISP, a common practice, it will receive ''more'' routes than are in the DFZ.  This is because when there are two routers connected to a major ISP such as [[Sprint Nextel|Sprint]], [[France Telecom]] or [[Qwest]], that provider has a number of customer AS connected to it. The optimal route to those customer AS are important to the ISP itself, but also tells one customer AS which specific router has the best path to the other customer. The "full routes", or properly "full routes plus customer routes", coming to a customer router makes that customer router part of the DFZ, but certainly not part of the "skitter core".
== See also ==
* [[Multihoming]]
* [[IP transit]]
* [[Peering]]
* [[Route filtering]]
* [[512K Day]]
== References ==
<references/>
[[Category:Internet Standards]]
[[Category:Routing]]