connected. Some routers may try to become adjacent to all their neighbor routers. Other routers may try to become adjacent to only one or two neighbor routers. OSPF routers determine which routers to become adjacent to based on the type of network they are connected to. Once an adjacency is formed between neighbors, link-state information is exchanged. OSPF interfaces recognize three types of networks: A fourth type, point-to-multipoint, can be configured on an interface by an administrator. In a multiaccess network, the number of routers that will be connected in advance is unknown. In point-to-point networks, only two routers can be connected. In a broadcast multi-access network segment, many routers may be connected. If every router had to establish full adjacency with every other router and exchange link-state information with every neighbor, there would be too much overhead. If there are 5 routers, 10 adjacency relationships would be needed and 10 link states sent. If there are 10 routers then 45 adjacencies would be needed. In general, for n routers, n*(n-1)/2 adjacencies would need to be formed. The solution to this overhead is to hold an election for a designated router (DR). This router becomes adjacent to all other routers in the broadcast segment. All other routers on the segment send their link-state information to the DR. The DR in turn acts as the spokesperson for the segment. Using the example numbers above, only 5 and 10 sets of link states need be sent respectively. The DR sends link-state information to all other routers on the segment using the multicast address of 224.0.0.5 for all OSPF routers. Despite the gain in efficiency that electing a DR provides, there is a disadvantage. The DR represents a single point of failure. A second router is elected as a backup designated router (BDR) to take over the duties of the DR if it should fail. To ensure that both the DR and the BDR see the link states all routers send on the segment, the multicast address for all designated routers, 224.0.0.6, is used. On point-to-point networks only two nodes exist and no DR or BDR is elected. Both routers become fully adjacent with each other. Interactive Media Activity Drag and Drop: OSPF Network Types When the student has completed this activity, the student will be able to identify the different OSPF network types. Web Links Areas and Border Routers http://www.cisco.com/warp/public/ 104/2.html#3.0
Content 2.2 Single Area OSPF Concepts 2.2.6 OSPF Hello protocol When a router starts an OSPF routing process on an interface, it sends a hello packet and continues to send hellos at regular intervals. The rules that govern the exchange of OSPF hello packets are called the Hello protocol. At Layer 3 of the OSI model, the hello packets are addressed to the multicast address 224.0.0.5. This address is “all OSPF routers”. OSPF routers use hello packets to initiate new adjacencies and to ensure that neighbor routers are still functioning. Hellos are sent every 10 seconds by default on broadcast multi-access and point-to-point networks. On interfaces that connect to NBMA networks, such as Frame Relay, the default time is 30 seconds. On multi-access networks the Hello protocol elects a designated router (DR) and a backup designated router (BDR). Although the hello packet is small, it consists of the OSPF packet header. For the hello packet the type field is set to 1. The hello packet carries information that all neighbors must agree upon before an adjacency is formed, and link-state information is exchanged. Interactive Media Activity Drag and Drop: OSPF Packet Header When the student has completed this activity, the student will be able to identify the different fields in an OSPF packet header. Web Links Neighbors http://www.cisco.com/warp/public/ 104/2.html#9.0
Content 2.2 Single Area OSPF Concepts 2.2.7 Steps in the operation of OSPF OSPF routers send Hello packets on OSPF enabled interfaces. If all parameters in the OSPF Hello packets are agreed upon, the routers become neighbors. On multi-access networks, the routers elect a DR and BDR. On these networks other routers become adjacent to the DR. Adjacent routers go through a sequence of states. Adjacent routers must be in the full state before routing tables are created and traffic routed. Each router sends link-state advertisements (LSA) in link-state update (LSU) packets. These LSAs describe all of the routers links. Each router that receives an LSA from its neighbor records the LSA in the link-state database. This process is repeated for all routers in the OSPF network. When the databases are complete, each router uses the SPF algorithm to calculate a loop free logical topology to every known network. The shortest path with the lowest cost is used in building this topology, therefore the best route is selected. Routing information is now maintained. When there is a change in a link state, routers use a flooding process to notify other routers on the network about the change. The Hello protocol dead interval provides a simple mechanism for determining that an adjacent neighbor is down. - Interactive Media Activity Drag and Drop: OSPF State Flowchart When the student has completed this activity, the student will be able to identify the different OSPF neighbor states. Web Links OSPF Packets http://www.juniper.net/techpubs/ software/junos50/ swconfig50-routing/ html/ospf-overview6.html
Content 2.3 Single Area OSPF Configuration 2.3.1 Configuring OSPF routing process OSPF routing uses the concept of areas. Each router contains a complete database of link-states in a specific area. An area in the OSPF network, it may be assigned any number from 0 to 65,535. However a single area is assigned the number 0 and is known as area 0. In multi-area OSPF networks, all areas are required to connect to area 0. Area 0 is also called the backbone area.OSPF configuration requires that the configuration be enabled on the router with network addresses and area information. Network addresses are configured with a wildcard mask and not a subnet mask. The wildcard mask represents the links or host addresses that can be present in this segment. Area IDs can be written as a whole number or dotted decimal notation. To enable OSPF routing, use the global configuration command syntax: Router(config)#router ospf process-id The process ID is a number that is used to identify an OSPF routing process on the router. Multiple OSPF processes can be started on the same router. The number can be any value between 1 and 65,535. Most network administrators keep the same process ID throughout an autonomous system, but this is not a requirement. It is rarely necessary to run more than one OSPF process on a router. IP networks are advertised as follows in OSPF: Router(config-router)#network address wildcard-mask area area-id Each network must be identified with the area to which it belongs. The network address can be a whole network, a subnet, or the address of the interface. The wildcard mask represents the set of host addresses that the segment supports. This is different than a subnet mask, which is used when configuring IP addresses on interfaces. Lab Activity Lab Exercise: Configuring the OSPF Routing Process This lab is to setup an IP addressing scheme for OSPF area 0 and configure and verify OSPF routing. Lab Activity e-Lab Activity: Configuring OSPF In this lab, the students will configure and verify OSPF routing. Web Links Enabling OSPF on the Router http://www.cisco.com/warp/public/ 104/2.html#5.0
Content 2.3 Single Area OSPF Configuration 2.3.2 Configuring OSPF loopback address and router priority When the OSPF process starts, the Cisco IOS uses the highest local active IP address as its OSPF router ID. If there is no active interface, the OSPF process will not start. If the active