OSPF Design in Light of LSA Types
OSPF’s main design trade-offs consist of choosing links for particular areas, with the goal of
speeding convergence, reducing memory and computing resources, and keeping routing tables
small through route summarization. For instance, by using a larger number of areas, and the
implied conversion of dense types 1 and 2 LSAs into sparser type 3 LSAs, the OSPF LSDBs can
be made smaller. Also, link flaps in one area require SPF calculations only in that area, due to the
partial calculation feature. Additionally, ABRs and ASBRs can be configured to summarize routes,
reducing the number of type 3 LSAs introduced into other areas as well. (Route summarization is
covered in Chapter 10, “IGP Route Summarization, Route Redistribution, and Default Routes.”)
The OSPF design goals to reduce convergence time, reduce overhead processing, and improve
network stability can be reached using the core OSPF protocols and features covered so far.
Another key OSPF design tool, stubby areas, will be covered next.