Type 7 LSA's: Flooded, or Unicast to ABR?

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Oct 26th, 2009
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When an ASBR in a not-so-stubby area creates a Type 7 LSA, does it unicast it to the nearest ABR; or is it flooded throughout the NSSA?


It doesn't seem to make sense to flood it throughout an area that isn't supposed to have external routing info in the first place; but I can't find a spec that says it's targeted straight to the ABR.


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Giuseppe Larosa Mon, 10/26/2009 - 00:42
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Hello Stuey,

it is flooded on the NSSA area, you should see O N2 (default) or O N1 routes in a router internal to the NSSA area.


all routers inside an area need to have the same link-state DB.


Depending on the settings of the P bit (propagate bit) the ABR (0,NSSA) can translate the LSA type 7 to a standard LSA type 5 to be sent in area 0 and all standard areas.


if two ABRs exist only one translate the type 7 to type 5.


this is the only way in OSPF to have external routes only in one area (when P=0).

And it is the only case in which an OSPF external route can be blocked at area boundary.

But for being able to do this a new LSA type 7 has been introduced.


see

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094a88.shtml#type7


Hope to help

Giuseppe


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