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300-415 · Question #283

QUESTION 329 Which data policy configuration influences BGP routing traffic flow from LAN to WAN?

The correct answer is D. policy 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. 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.

Policies

Question

QUESTION 329 Which data policy configuration influences BGP routing traffic flow from LAN to WAN?

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

How the community answered

(26 responses)
  • A
    4% (1)
  • B
    8% (2)
  • C
    12% (3)
  • D
    77% (20)

Why each option

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.

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

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.

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

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.

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

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.

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 outCorrect

This configuration defines a route-policy that prepends AS numbers to the BGP AS-path, making routes less preferred. Applying this policy `out` on the OSPF router within `vpn 10` influences the BGP routing traffic flow from LAN to WAN because it modifies routes before they are redistributed into BGP or advertised out of the local OSPF domain towards the WAN, thereby influencing path selection by external BGP peers. The `action accept` ensures the routes are allowed, and the `set as-path prepend` modifies their attributes.

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

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

Topics

#BGP Traffic Engineering#AS-Path Prepend#SD-WAN Control Policy#Outbound Policy

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