1. Network Architecture and Design
Advanced IP Routing
Open Shortest Path First (OSPF) Protocol
Reference
D. E. Comer, Internetworking with TCP/IP, ISBN , 4th Ed., Vol. 1, Ch. 15.
Network Architecture and Design

2. Network Architecture and Design
OSPF
OSPF is the Open Shortest Path First protocol.
That means it is an “Open” version of the “Shortest Path First” algorithm, it does not mean it tries to open the shortest path first.
Network Architecture and Design

3. Network Architecture and Design
OSPF
OSPF version 2 is defined in RFC 2328.
(Version 1 is no longer in use.)
Network Architecture and Design

4. Network Architecture and Design
OSPF
OSPF is an update-based, link-state, open routing protocol.
These attributes make OSPF the most commonly used IGP in use today.
Network Architecture and Design

5. Network Architecture and Design
OSPF
OSPF sends its entire routing table upon startup, then sends a keep alive every 10 seconds. If a topology change occurs, only the changes are sent.
This is much less bandwidth intensive and much faster than RIP.
Network Architecture and Design

6. OSPF - Link State Protocol
OSPF is a “link state” protocol.
This means that each router keeps a database of all the links in its “area”, and calculates the “shortest path” to each destination network from that database.
Network Architecture and Design

7. OSPF - Dijkstra Algorithm
In a nutshell, the Dijkstra Algorithm has each router imagine itself as the root of a tree, and calculates each successive link as a branch in the tree.
Network Architecture and Design

8. Network Architecture and Design
Dijkstra
See ospf.pdf
Slides 5 to 16.
Network Architecture and Design

9. Network Architecture and Design
OSPF - Flooding
In each area, every time a link changes state, every router is “flooded” with Link State Advertisements (LSAs) describing the change.
Every router must run the Dijkstra algorithm to re-calculate every route in the area.
Network Architecture and Design

10. When a Link Changes State
Network Architecture and Design

11. Network Architecture and Design
OSPF - LSAs
Different types of LSAs.
Examples
Router link (LSA type 1)
Network link (LSA type 2)
Network summary (LSA type 3)
External (LSA type 5)
…..
Network Architecture and Design

12. Location of Different LSAs
Network Architecture and Design

13. Network Architecture and Design
OSPF - Link Cost
Each link is assigned a cost. By default this cost is 100,000,000 / (speed of link in bps). So, the default cost for a FDDI link is 1, 10BaseT is 10, and a T1 is 64.
Unfortunately, this does not scale well with today’s technology.
Network Architecture and Design

14. Network Architecture and Design
OSPF - Link Cost
Fortunately, the cost of a link can be set manually. In cisco, this is done under each interface:
interface serial 0
ip ospf cost 10
Network Architecture and Design

15. Network Architecture and Design
OSPF – Link Cost
A “total” cost is then calculated for each destination prefix. Each prefix is installed into the routing table with a next hop relating to the lowest cost path.
Network Architecture and Design

16. Network Architecture and Design
OSPF - Convergence
When a link changes state, the LSA flood and recalculation happen in a very short time, usually seconds.
Because a link change is explicitly stated, there are very few routing loops (and for very short periods) in OSPF.
Network Architecture and Design

17. Network Architecture and Design
OSPF - Convergence
This may seem CPU and memory intensive, and it does take a great deal more memory and CPU than RIP, but it also allows OSPF to converge routes in seconds even over large and complex networks.
Network Architecture and Design

18. Network Architecture and Design
OSPF - Areas
To help conserve CPU and RAM, and to limit LSA floods, areas were introduced.
Each router need only know about the links in its area, and the link back to Area 0 (zero).
Network Architecture and Design

19. Network Architecture and Design
OSPF - Areas
Areas are defined as a 32-bit number, either straight decimal (e.g ) or as a “dotted decimal” (e.g ).
Network Architecture and Design

20. Network Architecture and Design
OSPF - Area 0
Every area must be directly connected to “Area 0”.
Area 0
Area 1
Area 3
Area 2
Network Architecture and Design

21. Network Architecture and Design
OSPF - Area 0
The routers in Area 0 contain the aggregated prefixes for every area.
Area 0 is sometimes called the “Backbone” area because all inter-area traffic must traverse Area 0.
Network Architecture and Design

22. Network Architecture and Design
OSPF - Area 0
Area 0 places an extreme burden on a network designer using OSPF.
Many networks “grow” in non-elegant ways, making a truly hierarchical network difficult or even impossible.
Network Architecture and Design

23. Network Architecture and Design
OSPF - ABRs
Routers with an interface in Area 0 and an interface in a non-zero area are called “Area Border Routers” or ABRs.
ABRs aggregate the prefixes for a non-zero area and inject the aggregated prefixes into Area 0.
Network Architecture and Design

24. Different Types of Routers
Network Architecture and Design

25. Location of Different Routers
Network Architecture and Design

26. Network Architecture and Design
OSPF - Neighbors
OSPF uses neighbor relationships to send routing updates.
If a neighbor relationship cannot be achieved, no routing updates will pass.
Network Architecture and Design

27. Routing Protocol Packets
Network Architecture and Design

28. Network Architecture and Design
OSPF - DR
On broadcast media (e.g. Ethernet), OSPF elects a Designated Router (DR) and a Backup Designated Router (BDR).
Network Architecture and Design

29. Network Architecture and Design
OSPF - DR
When updates are sent, each router on the LAN sends the updates to the DR (and the BDR), which sends one copy to each router.
This is much better than each router sending a copy of each update to each other router.
Network Architecture and Design

30. Network Architecture and Design
OSPF - BDR
If the DR is disabled or otherwise does not respond to queries, the BDR takes over.
Network Architecture and Design

31. Network Architecture and Design
DR and BDR
Hellos elect DR and BDR to represent segment
Each router then forms adjacency with DR and BDR
Network Architecture and Design

32. Network Architecture and Design
OSPF - Virtual Links
A tunnel, or “virtual link”, can be used when direct physical connectivity cannot be achieved.
This is not the preferred method.
Network Architecture and Design

33. Network Architecture and Design
OSPF - Multicast
OSPF uses multicast ( x) to propagate its routing updates, not broadcast. This reduces the CPU requirement on other hosts on the LAN as they do not have to process the multicast packet if they are not part of the multicast group.
Network Architecture and Design

34. Third Intermediate Report
OSPF
I. Stergiou
BGP
A. Sgora
Deadline: 25/02/03
Network Architecture and Design

35. Third Intermediate Report
Structure
Overview of examined technology
Focus on open research points
Related to open points works - State of the art behind open points
Your own interests - Ideas
Conclusions
References
Network Architecture and Design

36. Third Intermediate Report
Report (soft and hard copy)
Use of Greek language
A related presentation (about thirty minutes).
Network Architecture and Design

37. Network Architecture and Design
End of Sixth Lecture
Network Architecture and Design