Friday, September 18, 2009

OSPF: Link State Advertisement (LSA) - 4

Read more about LSA1,LSA2 and LSA3

Type 4 summary LSA is generated by ABR only when ASBR exists with in the area. It identifies the ASBR and provides route to it. The LSID is set to the ASBR router id. All the traffic destined to an external AS, requires routing table knowledge of the ASBR that originates the external route.
e.g. ASBR sends type 1 router LSA with E bit known as external bit; that bit is the identity of ASBR. When ABR receives this LSA and convert it to LSA4 and flood to the backbone area subsequent ABRs regenerate a type 4 LSA and flood into their areas.

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1 comment:

hanu_blr said...

am bit confused with type4 lsa.
1. las4 is generated by ABR to show route to ASBR for external n/w.

My question: why do u need tyep4 when type3 can do the job. basically to reach any inter areas, ABR is the one which lets you know the route. SO in that case, a default type3 is still ok to reach ASBR too, rt ??