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This example shows how the community attribute can
be used to dynamically influence the routing decisions of another AS. With the
network illustrated in the figure, the following configuration example shows how
AS3 can advertise route 172.16.65.0/26 to AS1 and dynamically instruct AS1 not
to advertise this route externally. AS3 will assign route 172.16.65.0/26 the
community attribute "no-export" when advertising it to AS1.
Note: The send-community option in the
neighbor router subcommand is used to cause the assigned community to be sent
out.
Note: Click on topology to view command
outputs.
The RTA configuration shows that RTA has defined
a route map SETCOMMUNITY toward neighbor 172.16.20.1 (RTC). Clause 10 of the
route map will match on prefix 172.16.65.0/26 and will set its community
attribute to no-export. The send-community keyword assigned to the neighbor
session is required to enable the community attribute to be sent to the
specified neighbor. Clause 20 of the route map will enable all other networks to
be passed with no change.
Note how the entry has been assigned the
community no-export and instructions that it is not to be advertised to EBGP
peers. RTC will not propagate this entry to its external peer RTD. Note that in
the following RTD BGP table, RTD did not receive an update about 172.16.65.0/26.
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Lab
Activity |
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In
this lab, you will learn how to use the Community
Attribute to dynamically influence the routing decisions
of another AS. |
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