Which two BGP attributes prevent loops in a route reflector environment? (Choose two.)

Question

Options

• Acluster ID
• Blocal preference
• Corigin
• Doriginator ID
• EAS_PATH

• A
  81% (30)
• B
  5% (2)
• C
  3% (1)
• E
  11% (4)

Explanation

In a route reflector (RR) environment, two attributes work together to prevent routing loops. Originator ID (option D) is set by the RR to the Router ID of the originating iBGP peer - if a route arrives back at its originator, that router discards it. Cluster ID (option A) identifies the RR cluster; each RR appends its cluster ID to the Cluster List, and any router that sees its own cluster ID in that list drops the route, preventing loops between multiple RRs or hierarchical RR setups.

Distractors:

• Local preference (B) influences path selection within an AS but has no loop-detection mechanism.
• Origin (C) indicates how a route entered BGP (IGP, EGP, or incomplete) - it affects preference, not loop prevention.
• AS_PATH (E) prevents loops in eBGP environments (a router drops routes containing its own ASN), but iBGP routes inside an RR cluster don't modify AS_PATH, so it offers no protection there.

Memory tip: Think "RR clusters need two guards at the door" - the Originator ID checks who sent it, and the Cluster List checks where it's been. Both are RR-specific constructs invented solely for this purpose.

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