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 providers.
Which configuration ensures that the branch router in AS 45123 advertises only the local networks to all SP neighbors?
A.
B.
C.
D.
The correct configuration to ensure that the branch router in AS 45123 advertises only the local networks to all SP (Service Provider) neighbors is C. The regular expression ^$ matches any route that has an empty AS path attribute, which indicates that the route is locally originated. By using this AS-path access-list, you can filter the routes sent to the Service Provider neighbors, ensuring that only locally originated routes are advertised and preventing the router from being used as a transit AS router.
the regular expression (^$) matches any route that has an empty AS path attribute (that is, no character from start to end). Only locally originated routes have an empty AS path attribute; hence this regular expression is used when matching local routes. This type of filter is used by multihomed customers to send only their address space to their service providers, to prevent them from becoming a transit AS.
C is correct. -> https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/26634-bgp-toc.html#asregexp --> ^$ : This expression indicates origination from this AS.
It is C. The ^$ regex statement matches an empty AS Path. Only locally originated routes will have an empty AS Path.
C. Specify only the routes generated by your own AS.
C is correct
C is correct.
C is correct
The correct answer is A ^$ ^ = begins with, $ = ends with. What BGP routes have no ASN!
@examShark, Thank you for your contribution, but here you are wrong. Paths originating locally can be matched by ^$ and filter out to ISPs. So answer C is the correct one.
While a newly sourced route is still within the AS in which it was created, the AS path is empty. When the AS has a requirement to filter out all but the routes that are local to itself before sending them to a neighboring AS, the AS will permit sending of the routes with the empty AS path and will deny all others. Answer C
It's B, locally orginitated - started with ASN of enterpise and ends on this ASN. Other uses match characterf at the end of matching string -$ Correct me