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OSPF Configuration

Can anybody explain to me when to configure the different modes for OSPF operation? For example, I have read different stories about configuring in NBMA mode (that this is for ATM, Frame Relay and X.25 as they do not support broadcasts), then you have point-to-multipoint non-broadcast (cannot see the difference). Can anybody explain to me how these modes differ and when to use each one?

best regards,


Cisco Employee

Re: OSPF Configuration

here is what the documentation says :



A point-to-point subinterface has the properties of any physical point-to-

point interface. As far as OSPF is concerned, an adjacency is always

formed over a point-to-point subinterface with no DR or BDR election.



An OSPF point-to-multipoint interface is defined as a numbered point-to-point

interface having one or more neighbors. This concept takes the pre-viously

discussed point-to-point concept one step further. Administra-tors

do not have to worry about having multiple subnets for each point-to-

point link. The cloud is configured as one subnet. This should work

well for people who are migrating into the point-to-point concept with no

change in IP addressing on the cloud. Also, they would not have to worry

about DRs and neighbor statements. OSPF point-to-multipoint works by

exchanging additional link-state updates that contain a number of infor-mation

elements that describe connectivity to the neighboring routers.



The selection of the DR becomes

an issue because the DR and BDR need to have full physical connectivity

with all routers that exist on the cloud. Also, because of the lack of

broadcast capabilities, the DR and BDR need to have a static list of all

other routers attached to the cloud. This is achieved using the neighbor


neighbor ip-address [priority number] [poll-interval seconds]

where the “ip-address” and “priority” are the IP address and the OSPF

priority given to the neighbor. A neighbor with priority 0 is considered

ineligible for DR election.



This approach is a workaround for using the “neighbor” command which

statically lists all existing neighbors. The interface will be logically

set to broadcast and will behave as if the router were connected to a

LAN. DR and BDR election will still be performed so special care should

be taken to assure either a full mesh topology or a static selection of

the DR based on the interface priority.

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