Description of Inter-AS Networks in the RIPE
Routing Registry
Tony Bates
Daniel Karrenberg
Document-ID: ripe-103
Addendum to
Representation of IP Routing Policies
in the RIPE Database (ripe-81)
What is an Inter-AS Network ?
Inter-AS IP networks are those networks which connect multiple auto-
nomous systems (1). An inter-AS network exists for the purpose of
passing traffic and routing information between different autonomous
systems. The most simple example of an inter-AS network is a
point-to-point link, connecting exactly two ASes. Each end of such
a link is connected to an interface of router living in each of the
autonomous systems. More complex examples are broadcast type net-
works with multiple interfaces connecting multiple ASes with the
possibility of more than one connection per AS.
_________________________
(1) Inter-AS networks are currently called FIXes, IXFs,
DMZs, NAPs, GIX and other names.
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Which additional information is needed?
Consider the following example of three routers 1, 2 and 3 with
interfaces a through f connected by two inter-AS networks X and Y:
X Y
a1b --- c2d --- e3f
Suppose that network X is registered in the routing registry as part
of AS1 and net Y as part of AS3. If traffic passes from left to
right prtraceroute will report the following sequence of interfaces
and ASes:
a in AS1
c in AS1
e in AS3
The traceroute algorithm enumerates only the receiving interfaces on
the way to the destination. In the example this leads to the pas-
sage of AS2 going unnoticed. This is confusing to the user and will
also generate exceptions when the path found is checked against the
routing registry.
For operational monitoring tools such as prtraceroute it is neces-
sary to know which interface on an inter-AS network belongs to which
AS. If AS information is not known about interfaces on an inter-AS
network, tools like prtraceroute cannot determine correctly which
ASes are being traversed.
Routing Registry Format
All interfaces on inter-AS networks will be described in a new ias-
int attribute of the corresponding network object in the RIPE data-
base. The ias-int attribute has the following syntax:
ias-int: <interface-address> <autonomous-system>
The <interface-address> must be an address within the corresponding
intenum and <autonomous-system> must be of the form AS<number>
referring to an aut-num object in the database.
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For example:
inetnum: 193.193.193.0
netname: INTER-AS-EXAMPLE
descr: This is a hypothetical inter-as network.
descr: It might be called a NAP, FIX, GIX, IXF, DMZ,
descr: Mehrfachdienstanbieterkommunikationseinrichtung or ...
country: DE
admin-c: Werner Mueller
tech-c: Paul Schmitz
tech-c: Hans Meier
changed: ripe-dbm@ripe.net 920714
aut-sys: AS4711
ias-int: 193.193.193.1 AS123
ias-int: 193.193.193.3 AS4711
ias-int: 193.193.193.9 AS789
source: RIPE
Note that the interface 193.193.193.3 is described although it is in
the same AS as the network. This is recommended practice.
The update procedure for the ias-int attribute will be the normal
update procedure for network objects. The attribute does not need
to be guarded because it does not influence routing policy of opera-
tional traffic.
In which AS does an Inter-AS Network belong?
Only one AS announces an inter-AS network externally. The other
ASes connected to the inter-AS network will probably carry this net-
work in their internal routing for redundancy but will not announce
it to other ASes.
In exceptional cases more than one AS may need to originate external
routing information about the inter-AS network, This kind of routing
setup cannot be described within the framework of ripe-81 and is
generally discouraged. Tools using a ripe-81 type registry could
take heuristic hints from the ias-int attributes when they encounter
such situations.
ripe-103.txt December 3, 1993