300-415 Question #283: Real Exam Question with Answer & Explanation

Question

Options

• Apolicy route-policy BGP-AS-PREPEND sequence 10 action accept set as-path prepend 10, 20 ! default-action accept vpn 10 router ospf route-policy BG-AS-PREPEND in
• Bpolicy route-policy BGP-AS-PREPEND sequence 10 action deny set as-path prepend 10, 20 ! default-action accept vpn 10 router ospf route-policy BG-AS-PREPEND out
• Cpolicy route-policy BGP-AS-PREPEND sequence 10 action accept set as-path prepend 10, 20 ! default-action accept vpn 10 router ospf route-policy BG-AS-PREPEND in
• Dpolicy route-policy BGP-AS-PREPEND sequence 10 action accept set as-path prepend 10, 20 ! default-action accept vpn 10 router ospf route-policy BG-AS-PREPEND out

Explanation

To influence BGP routing traffic flow from LAN to WAN using a data policy, the route-policy applying AS path prepending must be applied outbound on the OSPF routing process within the relevant VPN.

Common mistakes.

• A. Applying the route-policy in on OSPF would influence routes being received by OSPF, not routes being advertised from the LAN to the WAN, which typically involves outgoing policies.
• B. While applying out is correct for influencing outbound traffic, the action deny would block the routes from being advertised or accepted, which defeats the purpose of influencing their path with AS prepending.
• C. Similar to A, applying the route-policy in on OSPF would influence routes being received by OSPF, which is the wrong direction for influencing traffic flow from LAN to WAN.

Concept tested. BGP AS-path prepending with route-policies for traffic engineering

Reference. https://www.cisco.com/c/en/us/td/docs/routers/sdwan/configuration/sdwan-xe-gs-cr-book/policy-reference.html

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