AS
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... register and utilize an
Autonomous System (AS), and lists criteria for such. ASes are the
unit of routing policy ...
... EGP]), BGP (Border Gateway
Protocol, the current de facto standard for inter-AS routing; see
[BGP-4 ...
... service providers who
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
...
... under a single technical administration, using an interior gateway
protocol and common metrics to route packets within the AS, and
using an exterior gateway protocol to route ...
... ASes.
Since this classic definition was developed, it has become common
for a single AS to use several interior gateway protocols and
sometimes several sets of metrics within an AS ...
... single AS to use several interior gateway protocols and
sometimes several sets of metrics within an AS. The use of the
term Autonomous System here stresses the fact that, even when
...
... Autonomous System here stresses the fact that, even when
multiple IGPs and metrics are used, the administration of an AS
appears to other ASes to have a single coherent interior routing ...
... ASX knows how to reach a prefix called NET1. It does not matter
whether NET1 belongs to ASX or to some other AS which exchanges
routing information with ASX, either directly or indirectly; we just
...
...
An AS has a globally unique number (sometimes referred to as an ASN,
or Autonomous System Number ...
...
In routing terms, an AS will normally use one or more interior
gateway protocols (IGPs) when exchanging reachability ...
...
The term AS is often confused or even misused as a convenient way of
grouping together a set of prefixes which belong under the same
...
... prefixes there
are various different routing policies. Without exception, an AS must
have only one routing policy.
...
...
It is essential that careful consideration and coordination be
applied during the creation of an AS. Using an AS merely for the sake
of having an AS ...
... It is essential that careful consideration and coordination be
applied during the creation of an AS. Using an AS merely for the sake
of having an AS is to be avoided, as is the worst-case scenario of
...
... AS. Using an AS merely for the sake
of having an AS is to be avoided, as is the worst-case scenario of
one AS per classful network ...
... of having an AS is to be avoided, as is the worst-case scenario of
one AS per classful network (the IDEAL situation is to have one
prefix ...
... prefix, containing many longer prefixes, per AS). This may mean that
some re-engineering may be required in order to apply the criteria
and guidelines for creation and allocation of an AS ...
... AS). This may mean that
some re-engineering may be required in order to apply the criteria
and guidelines for creation and allocation of an AS that we list
below; nevertheless, doing so is probably the only way to implement
the desired routing policy ...
...
If you are currently engineering an AS, careful thought should be
taken to register appropriately sized CIDR ...
... registration authority in order to minimize the number of advertised
prefixes from your AS. In the perfect world that number can, and
should, be as low as one.
...
...
Some router implementations use an AS number as a form of tagging to
identify interior as well as exterior routing processes. This tag ...
... Criteria for the decision -- do I need an AS? ...
... routing information
An AS must be used for exchanging external routing information
with other ASes ...
... 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.
...
... As a general rule, one should try to place as many prefixes as
possible within a given AS, provided all of them conform to the
same routing policy.
...
... routing policy
An AS is only needed when you have a routing policy which is
different from that of your border gateway ...
... Internet makes routing
decisions based on information from your AS. See "Sample
Cases" below to see exactly when this criteria will apply.
...
... AS is not needed; the prefix should be placed in an
AS of the provider. The site's prefix has exactly the same rout-
...
...
This idea may at first seem slightly alien to some, but it high-
lights the clear distinction in the use of the AS number as a
representation of routing policy as opposed to some form of
...
... provider, or the rest of the site. In such an instance, a sepa-
rate AS must be created for the affected prefixes. This situa-
...
... require different routing policies than their parents. Because
the AS is the unit of policy, however, this sometimes occurs.
* Single-homed ...
... which connects to more than one service provider (i.e. more than
one AS with its own routing policy). It does not mean a network
...
... IGP for the purposes of resilience.
An AS is required; the site's prefixes should be part of a
single AS ...
... AS is required; the site's prefixes should be part of a
single AS, distinct from the ASes of its service providers.
...
... network operator should
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.
...
... Policy) compliance and network topology can influence decisions
of AS creation. However, all too often these are done without
consideration of whether or not an AS is needed in terms of
...
... of AS creation. However, all too often these are done without
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
when basing AS creation on these type of criteria.
* Transition / "future-proofing"
...
... has plans to connect to another at some point in the future.
This is not enough of a reason to create an AS before you really
need it. The AS number space is finite and the limited amount
...
... create an AS before you really
need it. The AS number space is finite and the limited amount
of re-engineering needed when you connect to another service
provider should be considered as a natural step in transition.
...
... * History
AS number application forms have historically made no reference
to routing policy. All too often ASes ...
... the Internet. The document should be used as a reference from
future application forms to show clearly when an AS is needed.
...
... multi-homed between them, it makes sense for all of those customers
to be placed within the same AS. However, it is noted that case
should only be looked at if practical to do so and fully coordinated
between customers ...
...
2) Sites should not be forced to place themselves in a separate AS
just so that someone else (externally) can make AS-based policy
...
... 2) Sites should not be forced to place themselves in a separate AS
just so that someone else (externally) can make AS-based policy
decisions. Nevertheless, it may occasionally be necessary to split
up an AS ...
... AS-based policy
decisions. Nevertheless, it may occasionally be necessary to split
up an AS or a prefix into two ASes for policy reasons. Those making
...
... ASes for policy reasons. Those making
external policy may request the network operators make such AS
changes, but the final decision is up to those network operators
...
...
Generally, a prefix can should belong to only one AS. This is a
direct consequence of the fact that at each point in the Internet
...
... neighbor peering
between two ASes, a conscious decision should be made as to which AS
this prefix actually resides in.
...
... aggregation it should be noted that a prefix
may be represented as residing in more than one AS, however, this is
very much the exception rather than the rule. This happens when
aggregating using the AS_SET ...
... AS, however, this is
very much the exception rather than the rule. This happens when
aggregating using the AS_SET attribute in BGP, wherein the concept of
origin is lost. In some cases the origin AS ...
... AS_SET attribute in BGP, wherein the concept of
origin is lost. In some cases the origin AS is lost altogether if
there is a less specific aggregate announcement setting the
ATOMIC_AGGREGATE attribute.
...
... exterior routing protocols. If already running an exterior routing
protocol, it is perfectly reasonable to use your AS number as an IGP
tag ...
... tag; if you do not, choosing from the private use range is also
acceptable (see "Reserved AS Numbers"). Merely running an IGP is not
grounds for registration ...
... AS Space exhaustion ...
...
The AS number space is a finite amount of address space. It is
currently defined as a 16 bit ...
... 16 bit integer and hence limited to 65535
unique AS numbers. At the time of writing some 5,100 ASes have been
allocated and a little under 600 ASes ...
... Internet. It is clear that this growth needs to be continually
monitored. However, if the criteria applied above are adhered to,
then there is no immediate danger of AS space exhaustion. It is
expected that IDRP will be deployed before this becomes an issue.
...
... Reserved AS Numbers ...
... Internet Assigned Numbers Authority (IANA) has reserved the
following block of AS numbers for private use (not to be advertised
on the global Internet ...
...
AS number to owner mappings are public knowledge (in WHOIS), and
attempting to change that would serve only to confuse those people
...