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350-501 · Question #176

Refer to the exhibit. ASN 64501 currently reaches the networks under the 10.0.0.0/8 prefix via the North_B router, which is a slow backup link. The administrator of ASN 64502 wants traffic from ASN…

The correct answer is B. Advertise the 10.0.0.0/8 prefix through North_B and specific subnets through North_A. To influence external BGP traffic from ASN 64501 to use the primary link (North_A) for 10.0.0.0/8, the network administrator should advertise more specific routes via the preferred path.

Networking

Question

Refer to the exhibit. ASN 64501 currently reaches the networks under the 10.0.0.0/8 prefix via the North_B router, which is a slow backup link. The administrator of ASN 64502 wants traffic from ASN 64501 to 10.0.0.0/8 to travel via the primary link North_A. Which change to the network configuration accomplishes this task?

Exhibit

350-501 question #176 exhibit

Options

  • ASet a higher local preference between North_A and South_A
  • BAdvertise the 10.0.0.0/8 prefix through North_B and specific subnets through North_A
  • CSet a Lower Weight value for incoming traffic on North_A
  • DSet a lower MED between North_B and South_B

How the community answered

(28 responses)
  • A
    4% (1)
  • B
    82% (23)
  • C
    11% (3)
  • D
    4% (1)

Why each option

To influence external BGP traffic from ASN 64501 to use the primary link (North_A) for 10.0.0.0/8, the network administrator should advertise more specific routes via the preferred path.

ASet a higher local preference between North_A and South_A

Local preference is an AS-internal attribute used to influence outbound traffic from an AS; setting a higher local preference between North_A and South_A would affect ASN 64502's outbound traffic decisions, not ASN 64501's inbound decisions towards 10.0.0.0/8.

BAdvertise the 10.0.0.0/8 prefix through North_B and specific subnets through North_ACorrect

Advertising more specific subnets of 10.0.0.0/8 through North_A, while advertising the less specific 10.0.0.0/8 prefix through North_B, will cause routers in ASN 64501 to prefer the more specific routes via North_A for those subnets, overriding the less specific route via North_B. This leverages the BGP best path selection rule where longer prefix matches are preferred.

CSet a Lower Weight value for incoming traffic on North_A

Weight is a Cisco-proprietary attribute that is local to the router and influences outbound traffic from that specific router; setting a lower weight on North_A would make North_A less preferred for outbound traffic, not influence incoming traffic from ASN 64501.

DSet a lower MED between North_B and South_B

MED (Multi-Exit Discriminator) is an attribute used by an AS to tell an adjacent AS how to enter its network; setting a lower MED on North_B's advertisement to South_B would make the North_B path more preferred, which is the opposite of the desired outcome.

Concept tested: BGP best path selection-longest prefix match

Source: https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-3.html#anc18

Topics

#BGP#BGP Path Selection#Traffic Engineering#Route Specificity

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