route from the IP routing table, use the following command: Router#clear ip route A.B.C.D To debug OSPF operations, use the debug ip ospf command with an option listed in Figure . Useful options when troubleshooting include: Router#debug ip ospf events
Router#debug ip packet
Content 3.4 OSPF Network Types 3.4.1 OSPF Network Types Understanding that an OSPF area is made up of different types of network links is important, because the adjacency behavior is different for each network type. As a result, OSPF must be properly configured to function correctly over certain network types. Default settings do not always work properly under some network topologies. There are three major types of networks that are defined by OSPF: OSPF operation and configuration on each of these network types is the focus of this lesson. Interactive Media Activity Drag and Drop: OSPF Network Types Upon completion of this activity, the student will be able to identify the different OSPF network types.
Content 3.4 OSPF Network Types 3.4.2 Adjacency Behavior for a Point-to-Point Link A point-to-point network joins a single pair of routers. A T1 serial line configured with a link-layer protocol such as PPP or High-Level Data Link Control (HDLC) is an example of a point-to-point network. On point-to-point networks, the router dynamically detects its neighboring routers by multicasting its hello packets to all OSPF routers, using the address 224.0.0.5. Neighboring routers become adjacent whenever they can communicate directly. Because there can be only two routers on a point-to-point link, there is no need for a DR or BDR. Usually, the IP source address of an OSPF packet is set to the address of the outgoing interface on the router. It is possible to use IP unnumbered interfaces with OSPF. In this case, the IP source address is set to the IP address of another interface on the router. The default OSPF hello and dead intervals on point-to-point links are 10 seconds and 40 seconds, respectively.
Content 3.4 OSPF Network Types 3.4.3 Adjacency Behavior for a Broadcast Link An OSPF router on a multiaccess broadcast network such as Ethernet forms an adjacency with its DR and BDR. Adjacent routers have synchronized LSDBs. A common media segment is the basis for adjacency, for example, two routers connected on the same Ethernet segment. When routers first come up on the Ethernet, they perform the hello process and then elect the DR and BDR. The routers then attempt to form adjacencies with the DR and BDR. The routers on a segment must elect a DR and a BDR to represent the multiaccess broadcast network. The BDR does not perform any DR functions when the DR is operating. Instead, the BDR receives all the information, but the DR performs the LSA forwarding and LSDB synchronization tasks. The BDR performs the DR tasks only if the DR fails. If the DR fails, the BDR automatically becomes the DR, and a new BDR election occurs. Since routers may have more than one Ethernet interface, one router might be a DR for one network on one interface and a BDR for a network on another interface. The DR and BDR improve network functioning in the following ways: Note
After a DR and BDR have been selected, any router added to the network establishes adjacencies with the DR and BDR only.
Content 3.4 OSPF Network Types 3.4.4 Selecting the DR and BDR One of the main functions of a DR is to ensure that all the routers on the same LAN segment have identical databases by passing its database to any new routers that come up. It is more efficient to have one router represent the other routers to a new router rather than all the routers pass the same information. Routers on the LAN also maintain a partial-neighbor relationship, a two-way adjacency state, with the other routers on the LAN that are not the DR or BDR (DROTHERs). To elect a DR and BDR, the routers view the OSPF priority value of the other routers during the hello packet exchange process and then use the following conditions to determine which router to select: The BDR uses the wait timer to determine whether the DR is out of service. If the BDR does not confirm that the DR is forwarding LSAs before the timer expires, the BDR assumes that the DR is out of service. Note
The highest IP address on an active interface is normally used as the router ID. However, you can override this selection by configuring an IP address on a loopback interface or using the router-id router configuration command. In a multiaccess broadcast environment, each network segment has its own DR and BDR. A router connected to multiple multiaccess broadcast networks can be a DR on one segment and a regular router on another segment. Note
The DR concept is at the link level. A DR is selected for every multiaccess broadcast link in the OSPF network. Use the ip ospf priority interface command to designate which router interfaces on a multiaccess link are the DR and the BDR. The default priority is 1, and the range is from 0 to 255. The interface with the highest priority becomes the DR, and the interface with the second-highest priority becomes the BDR. Interfaces set to zero priority cannot be involved in the DR or BDR election process. Here is a configuration example: interface FastEthernet 0/0
ip ospf priority 10 Note
The priority of an interface takes effect only when the existing DR goes down. A DR does not relinquish its status just because a new interface reports a higher priority in its hello packet.
Interactive Media Activity Checkbox: DR and BDR Election Upon completion of this activity, the student will be able to identify the process of DR and BDR election.
Content 3.4 OSPF Network Types 3.4.5 Adjacency Behavior for a NBMA Network When a single interface interconnects multiple sites over an NBMA