After some changes in the routing policy, it is noticed that the router in AS 45123 is being used as a transit AS router for several service provides .
Which configuration ensures that the branch router in AS 45123 advertises only the local networks to all SP neighbors?
A . ip as-path access-list 1 permit ^45123$
!
router bgp 45123
neighbor SP-Neighbors filter-list 1 out
B . ip as-path access-list 1 permit ^45123
!
router bgp 45123
neighbor SP-Neighbors filter-list 1 out
C . ip as-path access-list 1 permit ^$
!
router bgp 45123
neighbor SP-Neighbors filter-list 1 out
D . ip as-path access-list 1 permit .*
!
router bgp 45123
neighbor SP-Neighbors filter-list 1 out
Answer: C
Explanation:
By default BGP advertises all prefixes to external BGP neighbors. This means that if you are multi-homed (connected to two or more ISPs) then you might become a transit AS. For example, ISP 2 in AS 200 can send traffic to your router in AS 100 to reach ISP 3 in AS 300 because you advertised prefixes in ISP 3 to ISP 2.
This is what will be seen in the BGP routing table of ISP1:
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