Exam 400-101 topic 1 question 317 discussion

Actual exam question from Cisco's 400-101

Question #: 317
Topic #: 1

According to RFC 4577, OSPF for BGP/MPLS IP VPNs, when must the down bit be set?
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A. when an OSPF route is distributed from the PE to the CE, for Type 3 LSAs
B. when an OSPF route is distributed from the PE to the CE, for Type 5 LSAs
C. when an OSPF route is distributed from the PE to the CE, for Type 3 and Type 5 LSAs
D. when an OSPF route is distributed from the PE to the CE, for all types of LSAs

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Suggested Answer: C 🗳️
If an OSPF route is advertised from a PE router into an OSPF area, the Down bit (DN) is set. Another PE router in the same area does not redistribute this route into iBGP of the MPLS VPN network if down is set.
RFC 4577 says:
"When a type 3 LSA is sent from a PE router to a CE router, the DN bit in the LSA Options field MUST be set. This is used to ensure that if any CE router sends this type 3 LSA to a PE router, the PE router will not redistribute it further. When a PE router needs to distribute to a CE router a route that comes from a site outside the latter's OSPF domain, the PE router presents itself as an ASBR (Autonomous System Border Router), and distributes the route in a type 5 LSA. The
DN bit [OSPF-DN] MUST be set in these LSAs to ensure that they will be ignored by any other PE routers that receive them."
For more information about Down bit according to RFC 4577 please read more herE. http://tools.ietf.org/html/rfc4577#section-4.2.5.1.

by [deleted] at April 3, 2025, 6:29 p.m.

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