OSPF networking protocol basics

1. Open Shortest Path First Protocol
(OSPF)
Speaker:
Martín Bratina. martin.bratina@att.com
Date 22/03/2013

2. Agenda
• What is OSPF?
• Why is OSPF needed?
• How OSPF works?
• LAB
2 © 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.

3. What is OSPF?
• Open Shortest Path First
• Internal routing protocol (IGP)
• Open Standard. RFC 2328
• Link State Protocol
• Designed for IP networks
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 3 AT&T Intellectual Property.

4. Why is OSPF needed?
• Open standard
• Link State Protocol
• Scalability
• Fast convergence
• Supports authentication/security
• Supports VLSM (Variable Length Subnet Masks)
• Support IPv6 (OSPFv3)
• Support load balancing
• Consumes low bandwidth
• Uses a hierarchical structure
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 4 AT&T Intellectual Property.

5. How OSPF works?
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 5 AT&T Intellectual Property.

6. How OSPF works?
• Uses a Link State Logic.
• Neighbor discovery
• Topology database exchange
• Route computation
• Divides routers into areas for route optimization and
segmentation
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 6 AT&T Intellectual Property.

7. Neighbor Discovery
• Neighbor discovery
• Each router discover its neighbors routers sending hello messages
• Routers from adjacencies with its neighbors
• Neighbor adjacency requirements
• Interfaces in the same subnet
• Different router ID between neighbors
• Routers in the same area
• Same authentication parameters
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 7 AT&T Intellectual Property.

8. Topology Database Exchange
• Topology Database Exchange
• Each router generates its own network topology information (LSA
Message)
• Each router flood its LSA message to its neighbors
• Each router stores a LSA copy on its internal OSPF topology database
(LSDB)
• Each router has the same LDSB
• Link State Advertisement (LSA) Message information
• Router ID of the LSA
• List of the router interfaces, IP address, subnet and mask
• List of routers reachable (neighbors) on each interface
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 8 AT&T Intellectual Property.

9. Link State Advertisements(LSAs)
LS Age
Options LS Type
Link State ID
Advertising Router
LS Sequence Number
LS Checksum
Length
0 16
LSA Header
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10. Route Computation
• Route Computation
• Each router analyzes its topology database and executes Shortest Path
First (SPF) algorithm
• Router choses the shortest (best) route to each destination from its
perspective (SPF Tree)
• Router generates entries in the routing table for those selected
destinations
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 10 AT&T Intellectual Property.

11. SPF Tree
BEFORE run SPF AFTER run SPF
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 11 AT&T Intellectual Property.

12. Communication between OSPF Routers
• OSPF packets encapsulated in IP packets
• IP protocol # 89
• Standard 24 byte header
• OSPF packet type field
• OSPF router ID of sender
• Packet checksum
• Authentication fields
• OSPF Area ID
• Uses Multicast IP address 224.0.0.5 for all OSPF routers
• DR router use Multicast IP address 224.0.0.6
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 12 AT&T Intellectual Property.

13. The Hello Protocol
• Maintains neighbor relationships
• Elects Designated Router (DR) in multi-access networks
• The Hello Packet
• Hello packets sent out every 10 seconds
• RouterDead Interval (default 40 seconds)
• Network Mask
• List of neighbors
• Ensures that link is bidirectional
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 13 AT&T Intellectual Property.

14. The Hello Protocol
Hello Hello
Hello Hello
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15. Designated Router (DR)
• One per multi access network
• Generates Network Link Advertisements (LSA type 2)
• All routers form adjacency to it
• Assist in DB synchronization
• Elected by priority. It tie, it is elected by the highest router ID
Adjacencies WITHOUT DR
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 15 AT&T Intellectual Property.
Adjacencies WITH DR
DR

16. Database Synchronization
• Crucial to ensure correct and loop free routing
• Must be done before 2 neighbors start
communication
• Whenever new LSAs are introduced
• uses reliable flooding
• Each router sends LSA headers (small LSA’s) to its
neighbor when connection comes up
• Requests only those LSAs which are recent
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 16 AT&T Intellectual Property.

17. Database Exchange
• Neighboring routers first exchange hellos
• A database description packet establishes the
sequence number
• The other router sends LSA headers
• Sequence number incremented for every pair of
database description packets
• implicit acknowledgement for the previous pair
• After examining LSA headers explicit request sent
for complete LSA
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 17 AT&T Intellectual Property.

18. Reliable Flooding
• Starts when a router wants to update self-originated
LSAs
• Neighbor installs more recent LSAs into its
database
• Floods out on all interfaces except the one on
which it arrived
• Reliability-retransmissions until acks received
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 18 AT&T Intellectual Property.

19. Reliable Flooding (cont..)
10.1.1.1 10.1.1.2 10.1.1.4 10.1.1.6
10.1.1.3 10.1.1.5
Time T1
u
u
u
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 19 AT&T Intellectual Property.

20. Reliable Flooding (cont..)
10.1.1.1 10.1.1.2 10.1.1.4 10.1.1.6
u u
10.1.1.3 10.1.1.5
Time T2
u
u
u
20 © 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.

21. Reliable Flooding (cont..)
10.1.1.1 10.1.1.2 10.1.1.4 10.1.1.6
10.1.1.3 10.1.1.5
Time T3
u
u
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 21 AT&T Intellectual Property.

22. Reliable Flooding (cont..)
10.1.1.1 10.1.1.2 10.1.1.4 10.1.1.6
10.1.1.3 10.1.1.5
Time T3+
ack ack ack ack
ack
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 22 AT&T Intellectual Property.

23. Routing Calculations
• Link costs configurable by administrator
• Smaller values for more preferred links
• Different costs for each link direction possible
• Dijkstra’s shortest path algorithm
• incrementally calculates tree of shortest paths
• each link in the network examined once
• computes multiple shortest paths (equal-cost multipath)

24. LAB
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25. OSPF Areas
• Two-level hierarchical routing scheme through the use of
areas
• Areas identified by 32-bit id
• Each area has its own link state database which is a
collection of network-LSAs and router-LSAs
• Area’s topology hidden from all other areas
• All Areas connected through Backbone Area (Area 0)
• Interconnection of areas through area border routers
(ABRs)
• ABR leaks IP addressing information to other areas through
summary LSAs

26. Sample Area Configuration
AREA 0
Backbone
AREA 1
AREA 35
ASBR
Internet
ABR
ABR

27. OSPF Areas (Cont…)
• Reduction in link state databases of an area
• Reduction in amount of flooding traffic needed for
synchronization
• Reduction in the cost of the shortest path
calculations
• Increased robustness
• Routing protection
• Hidden prefixes

28. Incorporating external routing information
• Special routers called AS boundary routers at the
edge of OSPF domain
• ASBRs originate AS-External LSAs
• only routes for which the choice of an ASBR makes
sense are imported
• otherwise default routes are used

29. Interaction with areas
• Inter Area communication
• LSA type 1. Router LSA. All routers in an area
• LSA type 2. Network LSA. DR generates it.
• Intra Area communication
• LSA type 3. Summary ABR LSA.
• LSA type 5. Summary ASBR LSA.

30. OSPF Area Types
• Restrict the amount of external routing
information within an area
• Used when resources especially router memory is
very limited
• Many types of restricted areas
• Stub Areas
• Others: not covered on this training

31. OSPF Area Types (cont…)
• Stub Areas
• Don’t support ASBRs and hence no AS-External-LSAs
• Routing to external destinations based on default routes
originated by the area’s border routers
• Summary LSAs also made optional
• Must lie on the edge of OSPF routing domain
• Inter-area routing may also be based on default routes
• Improved scaling
• But not preferred due to the possibility of suboptimal
routes

32. LAB
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 32 AT&T Intellectual Property.

33. OSPF configuration
Normal area configuration
R1#configure terminal
R1(config)#router ospf 1
R1(config-router)# router-id 150.1.1.1
R1(config-router)# network 10.1.1.0 0.0.0.255 area 0
Stub area configuration
R1#configure terminal
R1(config)#router ospf 1
R1(config-router)# router-id 150.1.1.1
R1(config-router)# network 10.1.1.0 0.0.0.255 area 1
R1(config-router)# area 1 stub

34. OSPF verification
• R1# show ip ospf neighbors
• R1# show ip ospf interfaces
• R1# show ip ospf database
• R1# show ip route
• R1# show ip interfaces brief

35. Issues not covered
• OSPF Area Types
• NSSA
• Totally stubby
• Totally NSSA
• OSPF Network Types
• Broadcast subnets
• NBMA Subnets
• OSPF in the WAN
• OSPF Management
• Virtual Links
• OSPF and IPv6
• Many others

36. Thank You!