Hi Venkat,
both type 3 and type 4 are generated by Area Border routers. Type 3
describes routes to networks, also used for aggregating routes. Type 4
describes routes to ASBR. Summary Links (type 3) are identified by the IP
network numbers of the destinations they are pointing at. ASBR Summary
By cisco default type 3 and 4 are flooded into the stub area
If you configure the area as stub no-summary, then the summary
routes are not flooded into the stub. The stub only needs to know its
internal area routes, and can access everything else via the default
route 0.0.0.0.
The type
Type 3 LSA are advertised by all ABR. Type 4 and 5 LSA are advertised by
ASBR. Type 4 is used to identify the ASBR and Type 5 is used to get
information from the ASBR.
Stub area blocks Type 4 and Type 5 LSA from propagating into that area. Also
Type 4 and Type 5 are not allowed to propagate
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