network cisco ccna gns3 certification arteq

network cisco ccna gns3 certification arteq
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Sunday, November 25, 2012

area types and lsa's...

all area types will have type 1 and 2 lsa's

bb and standard will  also have 3,4,5 but no type 7 (nssa)

stub will have 1,2 and 3 while totally stubby can have only 1 and 2 (no summary)

nssa will have 1,2 and 3 and also type 7; totally nssa is similar to totally stubby (can you say totally) in that type 3 summary is not allowed but type 7 (nssa) still is... only the nssa types support type 7...

the overwhelming difference with the stubs is their treatment of abr summary and external routes... the stub types never support type 4 (asbr summary) or 5 (external)...

nssa supports external routes (type 5) in that they are interpreted as type 7 on the way in and out of the nssa area by the nssa abr...

don't believe me; here is how cisco puts it:

The OSPF not-so-stubby area (NSSA) feature is described by RFC 1587 and is first introduced in Cisco IOS® Software release 11.2. It is a non-proprietary extension of the existing stub area feature that allows the injection of external routes in a limited fashion into the stub area.

Redistribution into an NSSA area creates a special type of link-state advertisement (LSA) known as type 7, which can only exist in an NSSA area. An NSSA autonomous system boundary router (ASBR) generates this LSA and an NSSA area border router (ABR) translates it into a type 5 LSA, which gets propagated into the OSPF domain.

and further:

This is a type 7 LSA that is generated by an NSSA ASBR. Type 5 LSAs are not allowed in NSSA areas, so the NSSA ASBR generates a type 7 LSA instead, which remains within the NSSA. This type 7 LSA gets translated back into a type 5 by the NSSA ABR.

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