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.
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
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)- A9% (2)
- B82% (18)
- C5% (1)
- D5% (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.
Applying ORF on R5 would affect what R5 advertises to R2, not what R2 advertises to R4.
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.
The requirement is to filter prefixes from Branch A (10.10.10.0/24), not Branch B (192.168.10.0/24).
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
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