Click the Exhibit button.
AS 100 is originating and sending EBGP routes to AS 200. AS 200 is acting as a transit provider and sending
those routers to AS 300. AS 200 is using the confederation topology shown in the exhibit.
What will the AS path of AS 100’s routes be when they are received on the AS 300 router?
A.
200 100
B.
200 (65200 65100) 100
C.
(65200 65100) 100
D.
({65200 65100}) 100
Explanation:
To avoid routing loops, a sub-AS uses a confederation sequence, which operates like an AS path but uses only
the privately assigned sub-AS numbers.
The following applies to BGP Confederations
https://jncie.files.wordpress.com/2008/09/350010_differences-between-bgp-route-reflectors-andconfederations.pdf
https://www.juniper.net/documentation/en_US/junos/topics/topic-map/bgp-confederations.html
routing-options {
autonomous-system local-member-SUBAS-NUMBER;
confederation AS-CONF-ID members ALL-member-SUBAS-N;
}
protocols {
bgp {
group group-name {
peer-as autonomous-system;
RFC3065 section 6:
A member of a BGP confederation will use its AS Confederation ID in
all transactions with peers that are not members of its
confederation. This confederation identifier is considered to be the
“externally visible” AS number and this number is used in OPEN
messages and advertised in the AS_PATH attribute.
A member of a BGP confederation will use its Member AS Number in all
transactions with peers that are members of the same confederation as
the given router.
A BGP speaker receiving an AS_PATH attribute containing an autonomous
system matching its own confederation shall treat the path in the
same fashion as if it had received a path containing its own AS
number.
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The correct answer is A.
Take notice that sub-AS numbers are taken from the private AS numbers between 64512 and 65535.
The AS path received by other ASs shows only the globally assigned AS number. It does not include the confederation sequence or the privately assigned sub-AS numbers. The sub-AS numbers are removed when the route is advertised out of the confederation AS.
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