AS_path information is manipulated to
affect interdomain routing behavior. Because BGP prefers a shorter
path over a longer one, system operators are tempted to change the
path information by including dummy AS path numbers that would
increase the path length and influence the traffic trajectory one
way or the other. Cisco's implementation enables a user to insert AS
numbers at the beginning of an AS_path to make the path length
longer. The following example shows how this feature can be used.
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Lab
Activity |
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In
this lab, you will learn how to filter BGP updates based on the
AS_path attribute. |
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Because BGP prefers a shorter path
over a longer one, system operators can add to the AS path length by
prepending extra path entries and influence the preferred route used
by other ASs. The following example shows how this feature can be
used.
In Figure
, AS50 is connected to two providers, AS200 and AS100. AS100 is
directly connected to an Internet Network Access Poin t (NAP), while
AS200 has to go through an extra hop via AS300 to reach the
Internet. Figure
shows the AS path of prefix 192.213.1.0/24 as it traverses the ASs
in its way to the NAP. When the 192.213.1.0/24 prefix reaches the
NAP via AS300, it would have an AS_path of 300 200 50. If the same
prefix reaches the NAP via AS100, it would have an AS_path of 100
50, which is shorter. ASs upstream from the NAP would prefer the
shorter AS_path length and would direct their traffic toward AS100
at all times for destination 192.213.1.0/24.
AS50's network administrator is not
too happy about this behavior because they prefer for Internet
traffic to come in via their higher bandwidth T3 link to AS200,
rather than through the slower link to AS100. AS50's network
administrator can resolve this by manipulating the AS_path
information, inserting extra AS hops when sending routing updates to
AS100. One common practice is for AS50 to repeat its AS number as
many times as necessary to tip the balance and make the path via
AS200 become shorter.
In Figure
, AS50 will insert two AS numbers 50 50 at the beginning of the
AS_path of prefix 192.213.1.0/24. When the prefix 192.213.1.0/24
reaches the NAP via AS100, it would have the AS_path 100 50 50 50,
which is longer than the AS_path 300 200 50 via AS300. ASs upstream
of the NAP would prefer the shortest path and would direct the
traffic toward AS300 for destination 192.213.1.0/24.
The bogus number should always be a
duplicate of the AS announcing the route or the neighbor the route
is learned from (in case an AS is increasing the path length for
incoming updates). Adding any other number is misleading and could
potentially lead to routing loops. Note the insertion point in
Figure .
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