networks like Ethernet, an LSDB is synchronized between the router that is not a DR or a BDR (that is, a DROTHER) and its DR and BDR. Interactive Media Activity Drag and Drop: OSPF Router Types Upon completion of this activity, the student will be able to identify the four different types of OSPF routers.

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Content 3.1 Review of OSPF Fundamentals and Features 3.1.7 OSPF Terminology Figure provides a high-level view of OSPF terminology. Figures through provide an overview of various OSPF terms. Web Links OSPF Design Guide
http://www.cisco.com/warp/public/104/1.html

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Content 3.1 Review of OSPF Fundamentals and Features 3.1.8 Calculating the OSPF Metric Edsger Dijkstra designed a mathematical algorithm for calculating the shortest path from one point to another in a graph. Link-state routing protocols use Dijkstra’s algorithm to calculate the best paths through a network. By assigning a cost to each link in the network, and by placing the specific node at the root of a tree and adding the costs toward each given destination, the branches of the tree can be calculated to determine the best path. The best paths are put in the forwarding database (routing table). For OSPF, the default behavior is that the interface cost is calculated based on its configured bandwidth. An OSPF cost can also be manually defined for each interface by using the ip ospf cost command, which overrides the default cost value. Figure illustrates an example of a Dijkstra calculation. The calculation occurs as follows:

• Router H advertises its presence to router E. Router E then passes on the advertisements of router H and its own advertisements to its neighbors (routers C and G). Router G passes these and its own advertisements to router D, and so on.
• These LSAs follow the split horizon rule, which dictates that a router should never advertise an LSA to the router from which it came. In the example, router E does not advertise the LSAs of router H back to router H.
• Router X has four neighboring routers: A, B, C, and D. From these routers, it receives the LSAs from all other routers in the network. From these LSAs, it can also deduce the links between all routers and draw the web of routers depicted in the figure.
• Each Ethernet link in the figure is assigned an OSPF cost of 10. By summing the costs to each destination, the router can deduce the best path to each destination.
• The right side of the figure shows the resulting best paths (the SPF tree). From these best paths, shown with solid lines, routes to destination networks attached to each router are offered to the routing table. For each route, the next-hop address is the appropriate neighboring router (A, B, C, or D).

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Content 3.1 Review of OSPF Fundamentals and Features 3.1.9 Link-State Data Structures Each LSA entry has its own aging timer, which the link-state age field carries. For OSPF, the default is 30 minutes (expressed in seconds). After an LSA entry ages, the router that originated the entry sends an LSA with a higher sequence number in a Link-State Update (LSU) packet. The LSU can contain one or more LSAs. This LSA validation method saves on bandwidth compared to distance vector routers, which send their entire routing table at short intervals. When each router receives the LSU, it does the following:

• If the LSA does not already exist, the router adds the entry to its LSDB, sends a link-state acknowledgment (LSAck) back, floods the information to other routers, runs SPF, and updates its routing table.
• If the entry already exists and the received LSA has the same sequence number, the router ignores the LSA entry.
• If the entry already exists but the LSA includes newer information (it has a higher sequence number), the router adds the entry to its LSDB, sends an LSAck back, floods the information to other routers, runs SPF, and updates its routing table.
• If the entry already exists but the LSA includes older information, it sends an LSU to the sender with its newer information.

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Content 3.2 Review of OSPF Operation 3.2.1 OSPF Packet Types The OSPF protocol exchanges five packet types:

• Hello
• Database description (DBD)
• Link-state request (LSR)
• Link-state update (LSU)
• Link-state acknowledgement (LSAck)

These packet types interact to build OSPF neighbor adjacencies and maintain the OSPF topology database. Figure displays a brief description of each packet. Interactive Media Activity Drag and Drop: OSPF Packet Types Upon completion of this activity, the student will be able to identify the different OSPF packet types.

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Content 3.2 Review of OSPF Operation 3.2.2 OSPF Packet Header Format All five OSPF packets are encapsulated directly into an IP payload, as shown in Figure . The OSPF packet does not use TCP or User Datagram Protocol (UDP). OSPF requires a reliable packet transport scheme. Since TCP is not used, it has defined its own acknowledgment routine that uses an acknowledgment packet (OSPF packet type 5). In the IP header, a protocol identifier of 89 defines all OSPF packets. Each of the OSPF packets begins with the same header format. This header has the following fields:

• Version number: For OSPF version 2 or 3
• Type: Differentiates the five OSPF packet types
• Packet length: Length of packet in bytes
• Router ID: Defines which router is the source of the packet
• Area ID: Defines the area where the packet originated
• Checksum: Used for packet-header error detection to ensure that the OSPF packet was not corrupted during transmission
• Authentication type: An option that specifies either no authentication, clear-text passwords, or encrypted Message Digest 5 (MD5) formats for router authentication
• Authentication: Used in the authentication scheme
• Data (for hello packet): Includes a list of known neighborsData (for DBD packet): Contains a summary of LSDB, which includes all known router IDs and their last sequence number, among a number of other fields
• Data (for LSR packet): Contains the type of LSU needed and the router ID that has the needed LSU
• Data (for LSU packet): Contains the full LSA entries; multiple LSA entries can fit in one OSPF update packet
• Data (for LSAck packet): Is empty
  

Interactive Media Activity Drag and Drop: OSPF Packet Header Upon completion of this activity, the student will be able to identify the different fields in an OSPF packet header.

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Content 3.2 Review of OSPF Operation 3.2.3 Establishing OSPF Neighbor Adjacencies Neighbor OSPF routers must recognize and establish a relationship with each other before they can share routing updates. This process is done using the hello protocol. The hello protocol establishes and maintains neighbor relationships by ensuring bidirectional (two-way) communication between neighbors. Bidirectional communication occurs when a router recognizes itself listed in the hello packet received from a neighbor. Each interface participating in OSPF uses IP multicast address 224.0.0.5 to send hello packets periodically.