routing
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... AS), and lists criteria for such. ASes are the
unit of routing policy in the modern world of exterior routing, and
are specifically applicable to protocols like EGP ...
... ASes are the
unit of routing policy in the modern world of exterior routing, and
are specifically applicable to protocols like EGP (Exterior Gateway ...
... BGP (Border Gateway
Protocol, the current de facto standard for inter-AS routing; see
[BGP-4]), and IDRP ...
... BGP-4]), and IDRP (The OSI Inter-Domain Routing Protocol, which the
Internet is expected to adopt when BGP ...
... need to understand under what circumstances they should make use of
an AS. It is expected that the reader is familiar with routing
protocols and will be someone who configures and operates Internet
networks. Unfortunately, there is a great deal of confusion in how
ASes ...
... ASes should be used today; this memo attempts to clear up some of
this confusion, as well as acting as a simple guide to today's
exterior routing.
...
... AS
appears to other ASes to have a single coherent interior routing
plan and presents a consistent picture of what networks are
...
... or more network operators which has a SINGLE and CLEARLY DEFINED
routing policy.
...
...
Routing policy here is defined as how routing decisions are made in
the Internet today. It is the exchange of routing information ...
... routing decisions are made in
the Internet today. It is the exchange of routing information
between ASes that is subject ...
... between ASes that is subject to routing policies. Consider the case
of two ASes, X and Y exchanging routing information ...
... whether NET1 belongs to ASX or to some other AS which exchanges
routing information with ASX, either directly or indirectly; we just
assume that ASX knows how to direct packets towards NET1. Likewise
ASY knows how to reach NET2.
...
... traffic from NET2 to NET1 to flow between ASX and ASY,
ASX has to announce NET1 to ASY using an exterior routing protocol;
this means that ASX is willing to accept traffic directed to NET1
...
... For traffic to flow, ASY has to accept this routing information and
use it. It is ASY's privilege to either use or disregard the
...
... path as traffic from NET2 to NET1; this is called asymmetrical
routing. Asymmetrical routing is not inherently bad, but can often
cause performance ...
... traffic from NET2 to NET1; this is called asymmetrical
routing. Asymmetrical routing is not inherently bad, but can often
cause performance problems for higher level protocols, such as TCP ...
... TCP,
and should be used with caution and only when necessary. However,
assymetric routing may be a requirement for mobile hosts and
...
... or Autonomous System Number) associated with it; this number is used
in both the exchange of exterior routing information (between
neighboring ASes), and as an identifier ...
... group of prefixes there
are various different routing policies. Without exception, an AS must
have only one routing policy ...
... AS that we list
below; nevertheless, doing so is probably the only way to implement
the desired routing policy.
...
... router implementations use an AS number as a form of tagging to
identify interior as well as exterior routing processes. This tag
does not need to be unique unless routing information ...
... routing processes. This tag
does not need to be unique unless routing information is indeed
exchanged with other ASes. See "IGP ...
...
* Exchange of external routing information
An AS ...
...
An AS must be used for exchanging external routing information
with other ASes through an exterior routing protocol ...
... routing information
with other ASes through an exterior routing protocol. The cur-
rent recommended exterior routing protocol is BGP ...
... ASes through an exterior routing protocol. The cur-
rent recommended exterior routing protocol is BGP, the Border
Gateway Protocol. However, the exchange of external routing
information ...
... routing protocol is BGP, the Border
Gateway Protocol. However, the exchange of external routing
information alone does not constitute the need for an AS. See
"Sample Cases" below.
...
... possible within a given AS, provided all of them conform to the
same routing policy.
* Unique routing policy ...
...
An AS is only needed when you have a routing policy which is
different from that of your border gateway peers. Here routing
policy ...
... routing policy which is
different from that of your border gateway peers. Here routing
policy refers to how the rest of the Internet makes routing
...
... gateway peers. Here routing
policy refers to how the rest of the Internet makes routing
decisions based on information from your AS. See "Sample
...
... lights the clear distinction in the use of the AS number as a
representation of routing policy as opposed to some form of
administrative use.
...
... prefixes. This situa-
tion is rare and should almost never happen. Very few stub sites
require different routing policies than their parents. Because
the AS is the unit of policy, however, this sometimes occurs.
...
... service provider (i.e. more than
one AS with its own routing policy). It does not mean a network
multi-homed running an IGP ...
... create its own AS number. In this case, the site should ensure
that it has the necessary facilities to run appropriate routing
protocols, such as BGP4.
...
... Topology
Routing policy decisions such as geography, AUP (Acceptable Use
Policy) compliance and network topology can influence decisions
...
... consideration of whether or not an AS is needed in terms of
adding additional information for routing policy decisions by
the rest of the Internet. Careful consideration should be taken
...
... AS number application forms have historically made no reference
to routing policy. All too often ASes have been created purely
...
... provider A and provider B have a large presence in a
geographical area (or other routing domain), and many customers are
...
... ASes containing
them. This is, of course, a trade off -- it will not always be
possible to implement all desired routing policies.
...
... direct consequence of the fact that at each point in the Internet
there can be exactly one routing policy for traffic destined to each
prefix ...
... tag does not need to be
globally unique. In practice this information is never seen by
exterior routing protocols. If already running an exterior routing
protocol, it is perfectly reasonable to use your AS number as an IGP ...
... globally unique. In practice this information is never seen by
exterior routing protocols. If already running an exterior routing
protocol, it is perfectly reasonable to use your AS number as an IGP
...
... Kunzinger, C., Editor, "OSI Inter-Domain Routing Protocol (IDRP)", ISO/IEC 10747, Work In Progress ...
... Fuller, V., T. Li, J. Yu, and K. Varadhan, "Classless Inter-Domain Routing (CIDR): an Address Assignment and Aggregation Strategy ...
... Callon, R., "Use of OSI IS-IS for Routing in TCP/IP and Multi- Protocol Environments", RFC 1195prop, Digital Equipment Corporation, December 1990. ...