nerdexam
Cisco

352-001 · Question #573

Which native mechanism does OSPF use to prevent loops in MPLS VPNs?

The correct answer is F. PE devices that run OSPF set the DN bit toward the CE router. OSPF uses the DN bit, set by PE routers in LSAs sent to CE routers, to prevent CE routers from re-advertising those VPN routes back into the MPLS backbone and causing loops.

Layer 3 Control Plane

Question

Which native mechanism does OSPF use to prevent loops in MPLS VPNs?

Options

  • ACE devices that run OSPF set the DN bit toward the PE router
  • BPE devices that run OSPF clear the DN bit toward the CE router
  • CCE devices that run OSPF clear the DN bit toward the PE router
  • DCreation of PE to PE OSPF sham link across the MPLS-created super backbone
  • EPE routers verify OSPF domain IDs used by CE OSPF processes
  • FPE devices that run OSPF set the DN bit toward the CE router

How the community answered

(30 responses)
  • B
    3% (1)
  • C
    7% (2)
  • D
    20% (6)
  • E
    10% (3)
  • F
    60% (18)

Why each option

OSPF uses the DN bit, set by PE routers in LSAs sent to CE routers, to prevent CE routers from re-advertising those VPN routes back into the MPLS backbone and causing loops.

ACE devices that run OSPF set the DN bit toward the PE router

CE devices do not set the DN bit - they are the recipients of LSAs carrying the DN bit, not the originators of it in the VPN loop prevention mechanism.

BPE devices that run OSPF clear the DN bit toward the CE router

PE devices clear the DN bit only in specific inter-AS VPN scenarios; in standard intra-AS MPLS VPN loop prevention the PE sets the DN bit toward the CE rather than clearing it.

CCE devices that run OSPF clear the DN bit toward the PE router

CE devices do not clear or set the DN bit in the standard VPN loop prevention mechanism; that responsibility belongs exclusively to the PE router.

DCreation of PE to PE OSPF sham link across the MPLS-created super backbone

OSPF sham links solve a different problem - they prevent backdoor links between CE sites from being preferred over the MPLS backbone path, not loop prevention via LSA bit marking.

EPE routers verify OSPF domain IDs used by CE OSPF processes

Domain IDs control LSA type conversion between OSPF areas at PE-CE boundaries to maintain proper area types, not to prevent routing loops in the MPLS VPN topology.

FPE devices that run OSPF set the DN bit toward the CE routerCorrect

PE routers set the DN bit in Type 3 or Type 5/7 LSAs before sending them to CE routers; when a CE router receives an LSA with the DN bit set it will not redistribute that prefix back toward a PE, breaking the potential routing loop that would otherwise form across the MPLS super backbone.

Concept tested: OSPF DN bit loop prevention in MPLS VPNs

Source: https://www.cisco.com/c/en/us/support/docs/multiprotocol-label-switching-mpls/mpls/13715-dn-bit.html

Topics

#OSPF#MPLS VPN#DN bit#loop prevention

Community Discussion

No community discussion yet for this question.

Sign up to join the discussion
Full 352-001 Practice