Type 4 LSA's

What are they all about?

Description in book,routers searching for a path to an external network use them to determine the next best hop.
Now i for one dont get this! Since all external routes will be advertised into the network with type 5 LSA's when will the information about type 4 be used?

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Comments

darkuser

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fipr_c/ipcprt2/1cfospf.htm

EdTheLad

Nothing in that doc about the need of type 4 LSA's.

EdTheLad

Ok,maybe i have an idea why there are type 4 LSA's.
If an external route is being advertised into an AS with a type 2 metric by two ASBR's the only way to know which route is least cost within the AS is by knowing the least cost path to the ASBR which is learned via type 4 LSA's.
Am i right?

dmaftei

RFC 2328 wrote:

Type 4 summary-LSAs describe routes to AS boundary routers.

http://www.ietf.org/rfc/rfc2328.txt?number=2328