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
(see Figures
and ).
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 can influence the preferred route
used by other autonomous systems. The following example shows how
this feature can be used.
In Figure ,
AS 50 is connected to two providers, AS 200 and AS 100. AS 100 is
directly connected to an Internet network access point (NAP), while
AS 200 must go through an extra hop via AS 300 to reach the
Internet. Figure
shows the AS path of prefix 192.213.1.0/24 as it traverses the
autonomous systems on its way to the NAP. When the 192.213.1.0/24
prefix reaches the NAP via AS 300, it has an AS_Path of 300 200 50.
If the same prefix reaches the NAP via AS 100, it has an AS_Path of
100 50, which is shorter. Autonomous systems upstream from the NAP
prefer the shorter AS_Path length and direct their traffic toward AS
100 at all times for destination 192.213.1.0/24.
AS 50's network administrator is not
too happy about this behavior because she prefers Internet traffic
to come in via the higher bandwidth T3 link to AS 200, rather than
through the slower link to AS 100. AS 50's network administrator can
resolve this by manipulating the AS_Path information, inserting
extra AS hops when sending routing updates to AS 100. One common
practice is for AS 50 to repeat its AS number as many times as
necessary to tip the balance and make the path via AS 200 appear to
become the shorter route.
In Figure ,
AS 50 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 AS 100, it has the AS_Path 100 50 50 50, which
is longer than the AS_Path 300 200 50 via AS 300. Autonomous systems
upstream of the NAP prefer the shortest path and thus direct the
traffic toward AS 300 for destination 192.213.1.0/24.
The bogus number should always be a
duplicate of the AS announcing the route or the neighbor that 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.
To configure the prepending of dummy
AS numbers, a route map is used in a BGP neighbor
statement:
RTX(config)#router bgp 100
RTX(config-router)#neighbor 172.16.20.2 remote-as 300
RTX(config-router)#neighbor 172.16.20.2 route-map AddASnumbers
out
RTX(config-router)#exit
RTX(config)#route-map AddASnumbers permit
RTX(config-route-map)#set as-path prepend 100 100
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