nerdexam
Cisco

300-420 · Question #108

An engineer is tasked with designing a dual BGP peering solution with a service provider. The design must meet these conditions: - The routers will not learn any prefix with a subnet mask greater…

The correct answer is D. Use an IP prefix list to block the desired networks and apply the IP prefix list to BGP neighbors inbound. To prevent learning BGP prefixes with subnet masks greater than /24, an engineer should use an IP prefix list applied inbound to BGP neighbors.

Advanced Addressing and Routing Solutions

Question

An engineer is tasked with designing a dual BGP peering solution with a service provider. The design must meet these conditions:

  • The routers will not learn any prefix with a subnet mask greater than

/24.

  • The routers will determine the routes to include in the routing table

based on the length of the mask alone.

  • The routers will make this selection regardless of the service

provider configuration. Which solution should the engineer include in the design?

Options

  • AUse a route map and access list to block the desired networks, and apply the route map to BGP neighbors
  • BUse a route map and prefix list to block the desired networks, and apply the route map to BGP neighbors
  • CUse an IP prefix list to block the desired networks and apply the IP prefix list to BGP neighbors outbound.
  • DUse an IP prefix list to block the desired networks and apply the IP prefix list to BGP neighbors inbound.

How the community answered

(49 responses)
  • A
    4% (2)
  • B
    16% (8)
  • C
    8% (4)
  • D
    71% (35)

Why each option

To prevent learning BGP prefixes with subnet masks greater than /24, an engineer should use an IP prefix list applied inbound to BGP neighbors.

AUse a route map and access list to block the desired networks, and apply the route map to BGP neighbors

Route maps and access lists are less specific for mask length filtering than prefix lists and can be more complex for this particular requirement.

BUse a route map and prefix list to block the desired networks, and apply the route map to BGP neighbors

Applying a route map with a prefix list to block networks is technically possible, but the condition specifies filtering based on mask length, and applying it outbound would filter advertised routes, not learned routes.

CUse an IP prefix list to block the desired networks and apply the IP prefix list to BGP neighbors outbound.

Applying the IP prefix list outbound would filter prefixes that the local router advertises to the service provider, not the prefixes it learns from the service provider.

DUse an IP prefix list to block the desired networks and apply the IP prefix list to BGP neighbors inbound.Correct

An IP prefix list can precisely filter incoming BGP routes based on both the network prefix and the mask length. Applying it inbound ensures that prefixes with mask lengths greater than /24 (e.g., /25, /26) are rejected before being added to the local routing table, regardless of the service provider's advertisements.

Concept tested: BGP route filtering, IP prefix lists, inbound policy

Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/xe-3s/asr1000/b_bgp_xe_3s_asr1000_chapter_0100.html

Topics

#BGP Filtering#Prefix Lists#Route Manipulation#Service Provider Peering

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