Hi guys! one short question.. just a little doubt that's spinning in my head:
Why LSA 5 does not follow LSA 7 rules regarding the Forwarding Address? I mean, in LSA 7, there's always an IP Address in the Forwarding Address field, and it's the highest IP among the OSPF enabled loopbacks, and if none, highest OSPF enabled Interface.
But, LSA 5 gets an Forwarding Address of 0.0.0.0 if the External route resolves to a non-OSPF interface, so then it needs an LSA 4 so ABRs can show the world how to get to this route. How come it cant get a Forward address just like a LSA 7 (and translated LSA5)? and then removing the need of a LSA 4?
What am I missing :S?
The quotation from the RFC 2328 you have quoted is of course correct but it only describes a sanity check after the LSA5 was even considered valid for examination. Also, the "routing table" is not the routing table in the usual sense, rather, it is a routing table as defined by the RFC 2328 section 11 for the needs of OSPF.
The section 16.2 says about the LSA4 processing:
Next, look up the routing table entry for the destination N. (If N is an AS boundary router, look up the "router" routing table entry associated with Area A). If no entry exists for N or if the entry's path type is "type 1 external" or "type 2 external", then install the inter-area path to N, with associated area Area A, cost IAC, next hop equal to the list of next hops to router BR [border router], and Advertising router equal to BR.
In other words, this step installs information from the LSA4 about the ASBR (note - not its IP address but rather its RID because the "destination" in LSA4 is always the ASBR's RID!) into the OSPF routing table. And further, the section 16.4 step 3 says:
Look up the routing table entries (potentially one per attached area) for the AS boundary router (ASBR) that originated the LSA. If no entries exist for router ASBR (i.e., ASBR is unreachable), do nothing with this LSA and consider the next in the list.
Note that what you have quoted is an excerpt from the same step but it takes place only after this test. Here, you have it: the ASBR (its RID) must be present in the OSPF routing table, either by LSA1 if it is in the same area, or by means of LSA4 as described earlier. If it is not, no matter how valid the LSA5 data is, it will be ignored.