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BGP Autonomous System Types, Traffic Flows and Routing Policies
(Page 1 of 3)
When we connect autonomous systems
(ASes) together to form an internetwork, the paths between AS border
routers form the conduit by which messages move from one AS to another.
It is very important that the flow of messages between ASes be carefully
controlled. Depending on circumstances, we may wish to limit or even
prohibit certain types of messages from going to or from a certain AS.
These decisions in turn have a direct impact on BGP route determination.
BGP Traffic Flow and Traffic Types
The flow of messages in an internetwork
is sometimes collectively called traffic. This term presents
a good analogy, because we can consider the matter of traffic flow control
in a BGP internetwork in much the same way we do on the streets of a
city. You have probably in the past seen signs on residential streets
that say No Through Traffic or Local Traffic Only.
These are attempt to control the flow of traffic over those streets.
A more extreme example of this would be a street in the neighborhood
where I used to live, where a barricade was intentionally erected in
the middle to turn a busy through street into a pair of dead-ends. Again,
the goal was traffic control.
These measures highlight a key distinction
between local traffic and through traffic in a neighborhood.
The very same categorization is important in BGP:
BGP Autonomous System Types
- Local Traffic: Traffic carried within
an autonomous system that either originated in that same AS, or
is intended to be delivered within that AS. This is like local traffic
on a street.
- Transit Traffic: Traffic that was generated
outside that AS and is intended to be delivered outside
the AS. This is like what we commonly call through traffic on
the previous topic we discussed the distinction
between internal routers and border (or boundary) routers in an AS.
If we look at the entire BGP internetwork, we can make a similar distinction
between different types of ASes, based on how they are interconnected
in the overall BGP topology. There are two main types of AS:
- Stub AS: This is an AS that is connected
to only one other AS. It is comparable to a cul-de-sac (dead-end street)
in our road analogy; usually, only vehicles coming from or going to
houses on the street will be found on that street.
- Multihomed AS: This is an AS that is connected
to two or more other ASes. It is comparable to a through street in our
road analogy, because it is possible that cars may enter the street
and pass through it without stopping at any of the street's houses.
In the example BGP internetwork in
I have linked border routers in AS #2 to both AS #1 and AS #3. While
traffic from AS #2 can flow both to and from AS #1 and AS #3, it is
possible that traffic from AS #1 may also flow to AS #3 and vice-versa.
AS #2 acts as the through street for these datagrams.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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