1. OSPF (Open Shortest Path First)
Link-state based routing protocol: an Interior Gateway Protocol (IGP) – for inside ASs

2. What is OSPF? Open SPF Runs over IP directly, protocol number 89
Developed by IETF IGP working group, RFC2328
SPF
Each router floods link-state information through its neighbors to other routers
Based on the flooded link-state information, each router maintains a complete link-state database
Based on the link-state database, a routing table is constructed using SPF (e.g., Dijkstra’s) algorithm
Runs over IP directly, protocol number 89
Haobo Wang
(modified by M. Veeraraghavan)

3. Features of OSPF Use flexible metrics instead of only hop count
Supports variable-length subnetting
Allows load balancing among equal-cost paths
Supports multiple routes; one for each IP type of service (ToS)
Authenticates route exchanges
Quick convergence
Uses multicast rather than broadcast of its messages to reduce network load
Haobo Wang
(modified by M. Veeraraghavan)

4. Hierarchical OSPF AS is organized as two-level hierarchy
AS is partitioned into self-contained areas
Areas are interconnected by a backbone area
Areas are identified by a 32-bit area ID
is reserved for the backbone area
Four types of routers
Internal router, area border router, backbone router, autonomous system boundary router (ASBR)
Haobo Wang
(modified by M. Veeraraghavan)

5. OSPF AS organized into a 2-level hierarchy
(ASBR)
Within each area, border router
responsible for routing outside
the area
Exactly one area
is backbone area
Backbone area contains all area border routers and possibly others
Haobo Wang
(modified by M. Veeraraghavan)

6. OSPF packets Five types of OSPF packets OSPF common header Hello(1)
Database description(2)
Link-State Request(3)/Update(4)/Acknowledgement(5)
OSPF common header
Version
Type (1-5)
Packet Length
Router ID
Area ID
Checksum
Authentication Type
Authentication
Haobo Wang
(modified by M. Veeraraghavan)

7. OSPF common header fields
Version number: 2
Type: Type of OSPF packet
Packet length: in bytes, includes OSPF header
Router ID: 32-bit number assigned to each OSPF running router – uniquely identifies router within AS
Area ID: any four-byte number ( reserved for backbone area)
Checksum: error detection
Three Authentication related fields: to authenticate OSPF packets
Haobo Wang
(modified by M. Veeraraghavan)

8. Backup designated router
Hello packet
Network mask
Hello interval
Options
Priority
Dead interval
Designated router
Backup designated router
Neighbor 1 …
Neighbor n
Haobo Wang
(modified by M. Veeraraghavan)

9. Hello packet fields
Network mask: Subnet mask of the interface the packet is sent on
Hello interval: Number of seconds between Hello packets
Options: optional capabilities supported by the router
Priority: of the router – used in election of designated router
Dead interval: Number of seconds before declaring a nonresponding neighbor down
Designated router/Backup: Every broadcast network with at least two routers has a designated router. This sends network LSAs for the broadcast network. This field is if there is no designated router
Neighbors: Router ID of each neighbor from whom Hello packets have recently been received
Haobo Wang
(modified by M. Veeraraghavan)

10. Database description packet
Interface MTU
Options
Zero I M S
Database description sequence number
LSA header(s)
LSA header
LSA: Link State Advertisement
Link-state age
Options
Link-state type
Link-state ID
Advertising router
Link-state sequence number
Link-state checksum
Length
Haobo Wang
(modified by M. Veeraraghavan)

11. Database description packet fields
Interface MTU: MTU of the associated interface
Options: optional capabilities supported by the router
I bit: Init bit set to 1 if this packet is the first in the sequence of database description packets
M bit: More bit is set to 1 if more database description packets are to follow
MS bit: Master/Slave bit
Database description seq. no.: identifies the packet number sequentially so that a receiver can detect a missing packet
Haobo Wang
(modified by M. Veeraraghavan)

12. LSA header fields Link-state age: time since LSA generation
Options: optional capabilities supported by the router
Link-state type: router LSA, network LSA, summary LSA for IP networks, summary LSA for ASB routers, AS-external LSAs
Link-state ID: describes routing domain for the LSA; depends on LSA type
Advertising router: router ID of the router that generated the LSA
Link-state sequence number: Numbers LSAs sequentially to identify old and duplicate LSAs
Link-state checksum: entire contents of LSA except link-state age
Length: in bytes of LSA including LSA header
Haobo Wang
(modified by M. Veeraraghavan)

13. Link-state Request/Update/ Acknowledgement
Link-state type
Link-state ID
Advertising router
… (above 3 fields repeated for each link)
Link-state Update
Number of LSAs
LSA 1 …
LSA n
Link-state Acknowledgement
Haobo Wang
(modified by M. Veeraraghavan)

14. Router LSA 32 bits LSA header V E B #links Type #TOS Metric per-linkV E B
#links
Type
#TOS
Metric
per-link
fields
Link ID
Link Data
Repeat per-link fields for each link
Haobo Wang
(modified by M. Veeraraghavan)

15. LSA fields bit V (Virtual): 1 when router is a virtual link endpoint
bit E (External): 1 when router is an ASBR
bit B (Border): 1 when router is an area border router
# links: # of links described in this LSA
Per link:
Type:
Point-to-point connection to another router: 1
Connection to a transit network: 2
Connection to a stub network: 3
Virtual link: 4
Haobo Wang
(modified by M. Veeraraghavan)

16. LSA fields contd. Per-link:
#TOS: number of TOS metrics other than the required link metric given to this link; if none, 0
Link ID: Identifies the type of node connected to this link
If type 1, Neighboring router's Router ID
If type 2, IP address of Designated Router
If type 3, IP network/subnet number
If type 4, Neighboring router's Router ID
Link Data: depends upon type of link
For links to stub networks: Network's IP address mask
For point-to-point links: the interface's MIB-II ifIndex value.
For the other link types: the router interface's IP address.
Metric: cost of using this link
Haobo Wang
(modified by M. Veeraraghavan)

17. Link state updates
Each Link State Update packet carries a set of new link state advertisements (LSAs) one hop further away from their point of origination. A single Link State Update packet may contain the LSAs of several routers. Each LSA is tagged with the ID of the originating router and a checksum of its link state contents.
Haobo Wang
(modified by M. Veeraraghavan)

18. OSPF operations Hello protocol Database synchronization
Propagation of link-state information
Building of routing table
Haobo Wang
(modified by M. Veeraraghavan)

19. Hello Protocol
Hello packets are transmitted to all interfaces periodically
Discover neighbors, establish and maintain neighbor adjacency relationships
Elect Designated Router (DR) if there are multiple routers in a broadcast network
Haobo Wang
(modified by M. Veeraraghavan)

20. Database synchronization
Two neighboring routers exchange database description packets to synchronize their link-state databases.
Database description includes only a list of LSA headers. New or more up-to-date LSAs will be requested later
Packets sent by master are acknowledged by slave
Haobo Wang
(modified by M. Veeraraghavan)

21. Propagation of link-state information
Link-state request sent
When a router wants to update parts of its link-state database
Link-state update sent
When a link state is requested, or
When a link-state changes, or
Periodically
Link-state acknowledgement sent in response to a link-state update
Link-state updates retransmitted periodically until acknowledged
Haobo Wang
(modified by M. Veeraraghavan)

22. Flooding LSAs
A node receiving a link-state update selectively first installs each LSA in the update into its LSA database
Then it decides on which of its other links to flood the LSA
it may decide not to flood an LSA out a particular interface if there is a high probability that the attached neighbors have already received the LSA.
Haobo Wang
(modified by M. Veeraraghavan)

23. Building of routing table
Router S has knowledge of the entire area topology (complete link-state database)
Some algorithm such as Dijkstra’s is used to generate shortest path tree, rooted at router S
Only the next hop will be used in the routing table
Haobo Wang
(modified by M. Veeraraghavan)

24. Example Assume that all RT3 interfaces have 3 as their host ID AREA 1
Backbone
area 3
RT1 N1 1 N3
RT4 3
RT2 N2 1 8 6
RT3
RT6
This is the link metric
- note NOT hostID 2 N
Construct Router LSA for RT3
Haobo Wang
(modified by M. Veeraraghavan)

25. RT3’s router-LSA for Area 1
Link to N4: [COMMENT]
Link ID = ; IP Network number
Link Data = 0xffffff00; Network mask
Type = 3; connects to stub network
# TOS metrics = 0
metric = 2
LS age = 0; true on origination
Options =;
LS type = 1; indicates router-LSA
Link State ID = ; RT3's Router ID
Advertising Router = ; RT3's Router ID
bit E = 0; not an AS boundary router
bit B = 1; area border router
#links = 2
Link to N3: [COMMENT]
Link ID = ; IP address of Desig. Rtr.
Link Data = ; RT3's IP interface to net
Type = 2; connects to transit network
# TOS metrics = 0
metric = 1
Audio asks you to consult
slides 10, 12, 14, 24: if new slides
are added, these numbers will
change, but here are the slide
titles
slide 10: DDP+ LSA header
slide 12: LSA header fields
slide 14: Router LSA
slide 24: Example
Haobo Wang
(modified by M. Veeraraghavan)

26. RT3's router-LSA for the backbone
LS age = 0; always true on origination
Options = ;
LS type = 1; indicates router-LSA
Link State ID = ; RT3's router ID
Advertising Router = ; RT3's router ID
bit E = 0; not an AS boundary router
bit B = 1; area border router
#links = 1
Link to RT6
Link ID = ; Neighbor's Router ID
Link Data = ; MIB-II ifIndex of P-P link
Type = 1; connects to router
# TOS metrics = 0
metric = 8
Haobo Wang
(modified by M. Veeraraghavan)

27. References
Section of Communication Networks by A. Leon Garcia and I. Widjaja
RFC 2328 (can be obtained from
Haobo Wang
(modified by M. Veeraraghavan)