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Why loopback port should be placed in STUB or NSSA area on ABR?

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TitleWhy loopback port should be placed in STUB or NSSA area on ABR?
Question
Why loopback port should be placed in STUB or NSSA area on ABR?
 
Environment
EXOS all
Answer
OSPF allows certain areas to be configured as stub or NSSA areas. They are connected to only one other area. The area that connects to a stub or NSSA area can be the backbone area. External route information is not distributed into stub or NSSA areas but ABR generate a default route so that other routers can communicate without knowing external route.
In a network with a redundancy, two routers are configured as ABRs and both generate a default route to others. If one of the ABRs faces a trouble connecting backbone area, it should stop generate default route so the traffic can detour to the redundant router. It means it loses its role of ABR.
But it will remain as a ABR if a loopback is placed in the backbone area even it loses connection to the backbone area. The traffic destined to external routes (or inter-area route with no-summary option) will be isolated on the router and produce a connectivity problem.

 
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