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350-501 · Question #615

Refer to the exhibit. CE1 and CE2 reside in AS 65516 and access the internet through PE1, which resides in AS 65517. The PE-CE routing protocol is BGP, and CE1 and CE2 have an established iBGP…

The correct answer is A. Apply a route map on CE1 in the outbound direction to PE1 that sets the MED to a lower value. To make CE1 the preferred entry point for traffic from PE1 into AS 65516, CE1 must influence PE1's inbound routing decision. This is best achieved by setting a lower BGP Multi-Exit Discriminator (MED) value on routes advertised from CE1 to PE1.

Networking

Question

Refer to the exhibit. CE1 and CE2 reside in AS 65516 and access the internet through PE1, which resides in AS 65517. The PE-CE routing protocol is BGP, and CE1 and CE2 have an established iBGP peering. A network engineer must implement a route map on CE1 so that it becomes the preferred entry point for traffic from PE1. Which action must the engineer take to meet the requirement?

Exhibit

350-501 question #615 exhibit

Options

  • AApply a route map on CE1 in the outbound direction to PE1 that sets the MED to a lower value
  • BApply a route map on CE1 in the inbound direction from PE1 that sets the local preference to a
  • CApply a route map on CE1 in the inbound direction from PE1 that sets the weight to a value
  • DApply a route map on CE1 in the outbound direction to PE1 that prepends AS 65516 twice to the

How the community answered

(57 responses)
  • A
    77% (44)
  • B
    4% (2)
  • C
    14% (8)
  • D
    5% (3)

Why each option

To make CE1 the preferred entry point for traffic from PE1 into AS 65516, CE1 must influence PE1's inbound routing decision. This is best achieved by setting a lower BGP Multi-Exit Discriminator (MED) value on routes advertised from CE1 to PE1.

AApply a route map on CE1 in the outbound direction to PE1 that sets the MED to a lower valueCorrect

The BGP Multi-Exit Discriminator (MED) attribute is used to influence the path that *inbound* traffic takes into an Autonomous System (AS) from a neighboring AS. By configuring a route map on CE1 in the outbound direction to PE1 that sets a lower MED, CE1 signals to PE1 that its path is more desirable for reaching networks within AS 65516, effectively making CE1 the preferred entry point for traffic coming from PE1.

BApply a route map on CE1 in the inbound direction from PE1 that sets the local preference to a

Local preference is a BGP attribute used to influence a router's *outbound* traffic path selection within its own AS, not the path selection of inbound traffic from a neighboring AS. Setting it inbound on CE1 from PE1 would only affect CE1's decision to send traffic out via PE1.

CApply a route map on CE1 in the inbound direction from PE1 that sets the weight to a value

Weight is a Cisco-proprietary BGP attribute that is local to the router and primarily influences the router's *outbound* path selection. Applying it inbound on CE1 would only influence CE1's perspective of routes from PE1 for its own outbound traffic, not PE1's decision for inbound traffic.

DApply a route map on CE1 in the outbound direction to PE1 that prepends AS 65516 twice to the

AS-path prepending is a technique used to make a path *less* desirable for inbound traffic by artificially lengthening the AS path. The requirement is to make CE1 the *preferred* entry point, so prepending AS 65516 would have the opposite, undesired effect.

Concept tested: BGP MED for inbound traffic control

Source: https://www.cisco.com/c/en/us/td/docs/ios/iproute_bgp/configuration/guide/irg_bgp_psg7435_VTD_Component_Guide.html#wp1074312

Topics

#BGP#MED#Route Map#Traffic Engineering

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