Exam 300-410 topic 1 question 346 discussion

AS override: this can be configured on the PE routers, the AS number will be replaced with the AS number from the service provider. Allow-AS in: this can be configured on the CE routers which tells them to accept prefixes with their own AS number in the AS path. https://networklessons.com/bgp/mpls-layer-3-vpn-bgp-override https://content.cisco.com/chapter.sjs?uri=/searchable/chapter/content/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/xe-3s/irg-xe-3s-book/bgp-as-override-split-horizon.html.xml

AS-Override: Purpose: This is used to replace occurrences of the AS number of the provider edge (PE) router with the AS number of the remote AS, typically in the context of BGP VPNs. Typical Use: This is often used by service providers in a BGP/MPLS VPN to prevent loops in the customer's network when the customer is using the same AS number at multiple sites.

here is detailed explanation of neighbor as-override command https://www.n-study.com/en/bgp-detail/bgp-neighbor-as-override/

D is correct

D is right, you will mantain your ASN in the CE routers. The PE router will put the provider AS so the CE routers will not see the ASN of the other CE that is sending the routes to the PE and then the PE sending these routes to the other CEs routers.

D is correct

yes, correct option "D"

While former is used by PE to modify the AS Number in AS Path so that prefix is not dropped, the latter is implemented in CE device to introduce an exception in BGP AS path loop prevention mechanism. Hub should be seen as a provider edge, and Spoke as a customer edge. "If the customer requires to keep minimal configuration at CE side and let the provider perform the BGP routing control, the best approach will be to use “As-Override“." Hence, the Hub is provider edge, it should be configured with AS-Override

We need both commands to prevent loops

Well after reading more I believe D is correct. Since the answers are only on the provider end.

1. AS Override : Its feature allows a provider edge (PE) router to change private autonomous system used by customer edge (CE) device on an external BGP session running on a VPN routing and forwarding access link. The private AS number is changed to PEAS number. 2. Allowas In : This feature allows for routes to be received and processed even if router detects its own ASN in AS-Path. A router discards BGP network prefixes if it sees its ASN in AS-Path as a loop prevention mechanism. "https://www.geeksforgeeks.org/difference-between-as-override-and-allowas-in/#:~:text=2.,as%20a%20loop%20prevention%20mechanism."