The SPF algorithm builds a directed graph—paths made up of a series of points connected by direct links. One of the consequences of this directed-graph approach is that the algorithm has no way to handle a multiaccess network, such as an Ethernet VLAN. The solution used by OSPF is to elect one router, called the Designated Router (DR), to represent the entire segment. Point-to-point links fit the SPF model perfectly and don't need any special modeling method. On a point-to-point link, no DR is elected and all traffic is multicast to 22.214.171.124.
OSPF supports five network types:
■ NBMA—Default for multipoint serial interfaces. RFC-compliant mode that uses DRs and requires manual neighbor configuration.
■ Point-to-multipoint (P2MP)—Doesn't use DRs so adjacencies increase logarithmically with routers. Resilient RFC compliant mode that automatically discovers neighbors.
■ Point-to-multipoint nonbroadcast (P2MNB)—Proprietary mode that is used on Layer 2 facilities where dynamic neighbor discovery is not supported. Requires manual neighbor configuration.
■ Broadcast—Default mode for LANs. Uses DRs and automatic neigh- O bor discovery. Proprietary when used on WAN interface. p
■ Point-to-point (P2P)—Proprietary mode that discovers neighbors and S doesn't require a DR. I
If the default interface type is unsatisfactory, you can statically configure it with the command ip ospf network under interface configuration mode:
Router(config-if)#ip ospf network point-to-multipoint
When using the NBMA or P2MP nonbroadcast mode, neighbors must be manually defined under the routing process: Router(config-router)#neighbor 172.16.0.1
On a multiaccess link, one of the routers is elected as a DR and another as a backup DR (BDR). All other routers on that link become adjacent only to the DR and BDR, not to each other (they stop at the two-way state). The DR is responsible for creating and flooding a network LSA (type 2) advertising the multiaccess link. NonDR (DROTHER) routers communicate with DRs using the IP address 126.96.36.199. The DRs use IP address 188.8.131.52 to pass information to other routers.
The DR and BDR are elected as follows:
Step 1. A router starting the OSPF process listens for OSPF hellos. If none are heard within the dead time, it declares itself the DR.
Step 2. If hellos from any other routers are heard, the router with the highest OSPF priority is elected DR, and the election process starts again for BDR. A priority of zero removes a router from the election.
Step 3. If two or more routers have the same OSPF priority, the router with the highest RID is elected DR, and the election process starts again for BDR.
After a DR is elected, elections do not take place again unless the DR or BDR are lost. Because of this, the DR is sometimes the first device that comes online with a nonzero priority.
The best way to control DR election is to set OSPF priority for the DR and BDR for other routers. The default priority is one. A priority of zero means that a router cannot act as DR or BDR; it can be a DROTHER only. Priority q can be set with the ip ospf priority command in interface configuration J2 mode.
^ Router(config)#int fa 0/1
O Router(config-if)#ip ospf priority 2
Nonbroadcast Multiaccess (NBMA) Networks
Routing protocols assume that multiaccess links support broadcast and have full-mesh connectivity from any device to any device. In terms of OSPF, this means the following:
■ All Frame Relay or ATM maps should include the broadcast attribute.
■ The DR and BDR should have full virtual circuit connectivity to all other devices.
■ Hub-and-spoke environments should either configure the DR as the hub or use point-to-point subinterfaces, which require no DR.
■ Partial-mesh environments should be configured using point-to-point subinterfaces, especially when no single device has full connectivity to all other devices. If there is a subset of the topology with full connectivity, then that subset can use a multipoint subinterface.
■ Full-mesh environments can be configured using the physical interface, but often logical interfaces are used to take advantage of the other benefits of subinterfaces.
■ It may be necessary to statically identify neighbor IP addresses.
Was this article helpful?