Read more about LSAs
External LSA describes the router that are outside the OSPF domain. Type 5 LSAs are originated by ASBR and flooded throughout the entire AS. THe LSID is the external network number therefore the flooding scope and depending upon the number of the external network. The default lack of summarization can also be a major issue with external LSA. Therefore try to summarize the block of pool at ASBR to reduce the flooding problems.
Forwarding address plays a vital role in LSA5