therefore, the DR must be router A. Figure displays the show ip ospf neighbor command parameters.
Content 3.5 Implementing OSPF in an NBMA Network 3.5.5 Point-to-Multipoint Configuration Networks in point-to-multipoint mode are designed to work with partial-mesh or star topologies. In RFC 2328-compliant point-to-multipoint mode, OSPF treats all router-to-router connections over a nonbroadcast network as if they were point-to-point links. DRs are not used in point-to-multipoint mode, and a type 2 network LSA (as described in the next lesson) is not flooded to adjacent routers. Instead, OSPF point-to-multipoint works by exchanging additional LSUs that are designed to automatically discover neighboring routers and add them to the neighbor table. In large networks, using point-to-multipoint mode reduces the number of PVCs required for complete connectivity, because you are not required to have a full-mesh topology. In addition, not having a full-mesh topology reduces the number of neighbor entries in the neighbor table. Point-to-multipoint mode has the following characteristics: Figure shows partial configurations of routers A and C in point-to-multipoint mode. This configuration does not require subinterfaces and uses only a single subnet. In point-to-multipoint mode, a DR or BDR is not required; therefore, DR and BDR election and priorities are not a concern. The show ip ospf interface command in Figure displays key OSPF details for each interface. The OSPF network type, area number, cost, and state of the interface are all displayed. The hello interval for a point-to-multipoint interface is 30 seconds, with a dead interval of 120 seconds. The point-to-multipoint and nonbroadcast modes default to a 30-second hello timer, while the point-to-point and broadcast modes default to a 10-second hello timer. The hello and dead timers on the neighboring interfaces must match for the neighbors to form successful adjacencies. The listed adjacent neighboring routers are all dynamically learned. Manual configuration of neighboring routers is not necessary.
Content 3.5 Implementing OSPF in an NBMA Network 3.5.6 Point-to-Multipoint Nonbroadcast Configuration Cisco defines additional modes for the OSPF neighbor relationship. Point-to-multipoint nonbroadcast mode is a Cisco extension of the RFC-compliant point-to-multipoint mode. You must statically define neighbors, and you can modify the cost of the link to the neighboring router to reflect the different bandwidths of each link. The RFC point-to-multipoint mode was developed to support underlying point-to-multipoint virtual circuits that support multicast and broadcast; therefore, this mode allows dynamic neighboring router discovery. If multicast and broadcast are not enabled on the virtual circuits, the RFC-compliant point-to-multipoint mode cannot be used, because the router cannot dynamically discover its neighboring routers using the hello multicast packets. In this case, point-to-multipoint nonbroadcast mode should be used instead.
Content 3.5 Implementing OSPF in an NBMA Network 3.5.7 Using Subinterfaces in OSPF over a Frame Relay Configuration A physical interface can be split into multiple logical interfaces, called subinterfaces. Each subinterface is defined as a point-to-point or a point-to-multipoint interface. Subinterfaces were originally created to better handle issues caused by split horizon over an NBMA network for distance vector-based routing protocols. Each subinterface requires an IP subnet. Use the interface serial command to define subinterfaces. Figure displays the command parameters. When configuring subinterfaces, you must choose the type of interface using either the point-to-point or multipoint keyword. The default OSPF mode on a point-to-point Frame Relay subinterface is point-to-point. The default OSPF mode on a Frame Relay point-to-multipoint subinterface is nonbroadcast. The default OSPF mode on a main Frame Relay interface is also nonbroadcast.
Content 3.5 Implementing OSPF in an NBMA Network 3.5.8 Configuring a Point-to-Point Subinterface When point-to-point subinterfaces are configured, each virtual circuit (PVC and SVC) gets its own subinterface. A point-to-point subinterface has the properties of any physical point-to-point interface. There is no DR or BDR. Neighbor discovery is automatic, so neighbors do not need to be configured. Point-to-point mode is used when only two nodes exist. This mode is typically used only with point-to-point subinterfaces. Each point-to-point connection is one IP subnet. An adjacency forms over the point-to-point network with no DR or BDR election. In Figure , the router A serial 0/0/0 interface is configured with point-to-point subinterfaces. Although all three routers have only one physical serial port, router A appears to have two logical ports. Each logical port (subinterface) has its own IP address and operates as a point-to-point OSPF network type. This type of configuration does not need a DR or BDR, and does not require that neighbors be statically defined.
Content 3.5 Implementing OSPF in an NBMA Network 3.5.9 Configuring a Multipoint Subinterface When multipoint subinterfaces are configured, there are multiple virtual circuits (PVCs or SVCs) on a single subinterface. Multipoint Frame Relay subinterfaces default to OSPF nonbroadcast mode, which requires that neighbors be statically configured and a DR and BDR be elected. In Figure , router A has one point-to-point subinterface and one point-to-multipoint subinterface. The multipoint subinterface supports two other routers in a single subnet. OSPF defaults to point-to-point mode on the point-to-point subinterface, and defaults to nonbroadcast mode on the point-to-multipoint subinterface.
Content 3.5 Implementing OSPF in an NBMA Network 3.5.10 OSPF over NBMA Topology Summary Figure compares the various modes of operation for OSPF over NBMA topologies. Interactive Media Activity Drag and Drop: OSPF Network Type Characteristics Upon completion of this activity, the student will be able identify the different characteristics and DR and BDR election requirements for the different network types.
Content 3.5 Implementing OSPF in an NBMA Network 3.5.11 Tracking OSPF Adjacencies It is important to understand OSPF adjacency protocol handshakes when troubleshooting OSPF. Use the debug ip ospf adj command to track OSPF adjacencies as they come up or go down. Debugging allows you to see exactly which OSPF packets are being sent between routers, which is an invaluable troubleshooting tool. Figure displays output from the debug