Which option is used as a loop prevention mechanism to support MPLS VPN customers with multihomed sites?

Question

Options

• ABGP down bit
• Bsham links
• CAS override
• DSOO extended BGP community
• Eallowas-in

• A
  8% (4)
• B
  4% (2)
• C
  11% (6)
• D
  75% (40)
• E
  2% (1)

Explanation

SOO (Site of Origin) extended BGP community is specifically designed to prevent routing loops in MPLS VPN environments where a customer site is multihomed - connected to multiple PE routers. When a PE advertises a route with the SOO value of the originating site, other PE routers connected to that same site will reject the route, preventing it from being re-advertised back into the site it came from.

Why the distractors are wrong:

• A. BGP down bit - used in MPLS TE to prevent sub-optimal routing, not loop prevention in VPNs.
• B. Sham links - an OSPF-over-MPLS VPN feature that prevents backdoor links from being preferred over the VPN backbone; unrelated to multihomed loop prevention.
• C. AS override - replaces a customer AS number so BGP doesn't reject routes as loops; it removes loop protection rather than adding it.
• E. allowas-in - also loosens BGP's loop detection by permitting routes with the local AS in the path; again, the opposite of prevention.

Memory tip: Think of SOO as a "return address stamp" - when a route comes back to the site it originated from, the PE recognizes the stamp and drops it, breaking the loop. SOO = Stop Originating Over and over.

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