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300-420 · Question #241

Refer to the exhibit. A network engineer with an employee ID: 4433:77:420 must design a BGP solution based on these conditions: - Traffic sessions occur between the branches and the data center…

The correct answer is B. Use a prefix list with the 10.10.10.0/24 subnet for ORF on R2. To filter Branch A's prefixes (10.10.10.0/24) from reaching R4 (Branch B) and conserve R4's resources, R4 should send an Outbound Route Filtering (ORF) prefix list to R2 requesting that 10.10.10.0/24 not be advertised to it.

Advanced Addressing and Routing Solutions

Question

Refer to the exhibit. A network engineer with an employee ID: 4433:77:420 must design a BGP solution based on these conditions:

  • Traffic sessions occur between the branches and the data center.
  • Branch B has limited resources to process routing updates.
  • HQ must filter out all prefixes from branch A to R4.

Which outbound route filtering (ORF) solution must the engineer choose?

Exhibit

300-420 question #241 exhibit

Options

  • AUse a prefix list with the 10.10.10.0/24 subnet for ORF on R5.
  • BUse a prefix list with the 10.10.10.0/24 subnet for ORF on R2
  • CUse a prefix list with the 192.168.10.0/24 subnet for ORF on R2.
  • DUse a prefix list with the 192.168.10.0/24 subnet for ORF on R4.

How the community answered

(22 responses)
  • A
    9% (2)
  • B
    82% (18)
  • C
    5% (1)
  • D
    5% (1)

Why each option

To filter Branch A's prefixes (10.10.10.0/24) from reaching R4 (Branch B) and conserve R4's resources, R4 should send an Outbound Route Filtering (ORF) prefix list to R2 requesting that 10.10.10.0/24 not be advertised to it.

AUse a prefix list with the 10.10.10.0/24 subnet for ORF on R5.

Applying ORF on R5 would affect what R5 advertises to R2, not what R2 advertises to R4.

BUse a prefix list with the 10.10.10.0/24 subnet for ORF on R2Correct

Outbound Route Filtering (ORF) allows a BGP speaker (R4 in Branch B) to tell its peer (R2 in HQ) which prefixes it is not interested in receiving, thereby reducing the routing updates sent to the resource-limited router. R4 would send an ORF capability and a prefix list specifying 10.10.10.0/24 to R2, and R2 would then apply this filter to its outbound advertisements to R4.

CUse a prefix list with the 192.168.10.0/24 subnet for ORF on R2.

The requirement is to filter prefixes from Branch A (10.10.10.0/24), not Branch B (192.168.10.0/24).

DUse a prefix list with the 192.168.10.0/24 subnet for ORF on R4.

Applying an outbound filter on R4 for 192.168.10.0/24 (Branch B's own prefix) would prevent R4 from advertising its own prefixes, and applying ORF on R4 would mean R4 sends a request to its peer, not applies a local outbound filter for incoming routes.

Concept tested: BGP Outbound Route Filtering (ORF)

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

Topics

#BGP#Route Filtering#Outbound Route Filtering (ORF)#Network Design

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