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350-501 · Question #155

An engineer is trying to implement BGP in a multihomed architecture. What must the engineer configure to influence inbound path selection?

The correct answer is D. A route map with AS_PATH attribute to control the inbound traffic. To influence inbound BGP path selection in a multihomed architecture, an engineer must configure a route map that modifies the AS_PATH attribute when advertising routes to external neighbors.

Networking

Question

An engineer is trying to implement BGP in a multihomed architecture. What must the engineer configure to influence inbound path selection?

Options

  • AA route map with WEIGHT attribute to control the inbound traffic.
  • BAn offset list to set the metric for routes received from neighboring autonomous systems.
  • CAn access list to identify traffic and enable it on both of the provider-facing interfaces.
  • DA route map with AS_PATH attribute to control the inbound traffic.

How the community answered

(22 responses)
  • A
    5% (1)
  • B
    9% (2)
  • C
    9% (2)
  • D
    77% (17)

Why each option

To influence inbound BGP path selection in a multihomed architecture, an engineer must configure a route map that modifies the AS_PATH attribute when advertising routes to external neighbors.

AA route map with WEIGHT attribute to control the inbound traffic.

The WEIGHT attribute is a Cisco-proprietary attribute that is local to a router and is never advertised to BGP neighbors, so it cannot influence inbound traffic from external autonomous systems.

BAn offset list to set the metric for routes received from neighboring autonomous systems.

Offset lists are typically used to manipulate metrics within Interior Gateway Protocols (IGPs) like OSPF or EIGRP, not BGP attributes to influence inbound path selection from external ASes.

CAn access list to identify traffic and enable it on both of the provider-facing interfaces.

An access list is used for matching traffic or routes but does not, on its own, modify BGP attributes to influence path selection; it would be used in conjunction with a route map to filter or match prefixes before applying an attribute modification.

DA route map with AS_PATH attribute to control the inbound traffic.Correct

The AS_PATH attribute is a globally significant BGP attribute that routers use for loop prevention and path selection. By using a route map to prepend the local Autonomous System (AS) number multiple times to the AS_PATH when advertising routes to a specific upstream provider, the path through that provider appears longer and less desirable, thereby influencing inbound traffic to prefer alternative, shorter paths.

Concept tested: BGP Inbound Path Influence

Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/xe-16/bgp-xe-16-book/bgp-cfg-inbound.html

Topics

#BGP#Path Selection#Multihoming#AS_PATH Prepending

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