CCIE Routing & Switching Question of the Week: OSPF Process

Refer to the exhibit below. R6 (in standard area 1) is redistributing routes learned from BGP into the Open Shortest Path First (OSPF) process. Which three OSPF LSA types will R3 advertise into the area 0? (Choose three.)

A. Type 1 — Router LSAs
B. Type 2 — Network LSAs
C. Type 3 — Summary LSAs
D. Type 4 — ASBR Summary LSAs
E. Type 5 — AS external LSAs
F. Type 7 — NSSA external LSAs

Reveal Answer

Answer: C, D and E.

The area boarder router (ABR — R3) will convert Router LSA (LSA type 1) into Summary LSAs (LSA type 3) when passing them into another area, this is normal behavior and is not triggered from the external routes from R6. The External LSAs (LSA type 5) will be passed through the ABR into Area 0, without changing who the advertising router ID was within the LSA. The issue is that the router ID of the ASBR (Autonomous System Boundary Router – R6) is not known in any other area than the one it is attached to, so the ABR then generates an LSA type 4, which is a ASBR summary LSA, that indicates that the ABR has a path to the ASBR router ID that is listed in the LSA type 5.

CCIE Routing & Switching Question of the Week Series

CCIE Routing & Switching Question of the Week: Inbound BGP
CCIE Routing & Switching Question of the Week: IS-IS
CCIE Routing & Switching Question of the Week: Outbound BGP
CCIE Routing & Switching Question of the Week: CBWFQ
CCIE Routing & Switching Question of the Week: Query Response Time
CCIE Routing & Switching Question of the Week: Type Length Value
CCIE Routing & Switching Question of the Week: OSPF Process

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