What do routers on the network use to avoid routing loops when OSPF is running as the PE-CE routing protocol on a service provider network?

Question

Options

• Athe AS-Override feature
• Bthe DN bit with type 3, 5, or 7 LSA
• Cthe domain tag for type 2 LSA
• Dsham links to create a super backbone over the service provider network

• A
  8% (2)
• B
  67% (16)
• C
  4% (1)
• D
  21% (5)

Explanation

Option B is correct because in MPLS VPN environments running OSPF as the PE-CE protocol, PE routers set the DN (Down) bit in Type 3 (summary), Type 5 (external), and Type 7 (NSSA external) LSAs when redistributing routes from MP-BGP back into OSPF toward the CE. When another PE router receives an LSA with the DN bit set, it will not redistribute that route back into MP-BGP, breaking the potential loop between the OSPF and BGP domains.

Why the distractors are wrong:

• A (AS-Override) is a BGP PE-CE feature, used to strip and replace a CE's AS number in the BGP AS_PATH so the CE doesn't reject its own routes - it has no role in OSPF loop prevention.
• C (domain tag for Type 2 LSA) is doubly wrong: domain tags apply to Type 5/7 LSAs, and Type 2 LSAs (network LSAs from a DR) are not the mechanism for VPN loop prevention.
• D (sham links) solve a different problem entirely - they prevent a backdoor link between sites from being preferred over the MPLS backbone by making OSPF "see" an intra-area path through the provider network.

Memory tip: Think "DN = Down, Not redistributed" - the DN bit marks a route as having come down from the provider backbone, signaling PE routers to never send it back up into BGP.

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