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.
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
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)- A4% (1)
- B82% (23)
- C11% (3)
- D4% (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.
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.
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.
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.
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
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