OSPF Basics

What Is OSPF?
• Open Shortest Path First
• Link State Protocol using the Shortest Path First algorithm (Dijkstra) to calculate loop-free routes
• Used purely within the TCP/IP environment
• Designed to respond quickly to topology changes but using minimal protocol traffic
• Used in both Enterprise and Service Provider Environment
• Uses IP protocol 89
• Metric is cost, based on interface bandwidth by default (10^8 / BW in bps)
• Sends partial route updates only when there are changes
• Send hello packets every 10 sec with dead timer of 40 sec over Point to Point & Broadcast networks
• Send hello packets every 30 sec with dead timer of 120 sec over NBMA networks
• Uses multicast address 224.0.0.5 (ALL SPF Routers)
• Uses multicast address 224.0.0.6 (ALL DR Routers)

Different Types of OSFP LSAs1. Router Link State Advertisement (Type 1 LSA)
2. Network Link State Advertisement (Type 2 LSA)
3. Summary Link State Advertisement (Type 3 and Type 4 LSA)
4. External Link State Advertisement (Type 5 LSA)

Different types of OSPF Packet
1. Hello
2. Database description
3. Link State Request
4. Link State Update
5. Link State Acknowledgement

Different Types of OSPF Areas
Regular Area: ABRs forward all LSAs from backbone
• Summary LSA (summarized/non-summarized) from other areas injected
• External links injected

Stub Area: A stub area is an area with single exit point (if you need multiple exit points then configure it as NSSA) into which External LSA are not flooded
• Summary LSAs from other areas injected
• LSA type 5 not injected
• Default LSA injected into area as Summary LSA
• Define all routers in area as stub
• External link flaps will not be injected
Consolidates specific external links— default 0.0.0.0
Used in networks with a lot of LSA Type 5

Totally Stubby Area
A Totally Stubby Area Forwards Default Link 0.0.0.0
The ABR will block not only the AS External LSAs bit also all Summary LSAs, except a single Type 3 LSA to advertise the default route.

Not So Stubby Areas (NSSA)
Benefits of stub area, but ASBR is allowed

New type external LSA (type 7)
• Type 7 LSAs flooded throughout the area
• No type 5 external LSAs in the area
• Type 7 LSAs will be converted into type 5 LSAs when flooded into area 0 by ABRs
Filtering and summaries allowed at ABRs

Areas are used to make OSPF Scale
• OSPF uses a 2 level hierarchical model
• One SPF per area, flooding done per area
• Regular, Stub, Totally Stubby and NSSA Area Types
• A router has a separate LS database for each area to which it belongs
• All routers belonging to the same area should have identical databases
• SPF calculation is performed independently for each area
• LSA flooding is bounded by area
• If any link flaps in the area it takes nlogn calculations where n is the number of the links in the area