Nonbroadcast and Multi-point / multi-access

nuglobe

I just can't grasp what this is. Some help please :

steve-o87

Nonbroadcast Multiple Access Network(NBMA), is a network to which multiple devices are attached, but the data is transmitted from one device to another over a virtual circuit. NBMA do not support broadcast or multicast for that matter.
Some examples:
ATM
Frame relay
X.25

Broadcast Multi-access - every device will receive the broadcast packet, due to multi-access nature of the medium, probably ethernet, being used.

Broadcast networks - I think the name says it all

I hope this helps!

nuglobe

So how does DR play in to one of these and not the other?

steve-o87

A DR/BDR is elected to keep the number of router adjacencies down on the ospf network because the non-designated routers will only form an adjacency with them.

Another reason for DR/BDR is say a link went down on a router it sends a message to 224.0.0.6 (AllDRRouters) and the DR triggers an update on 224.0.0.5 to inform the rest of the network of the change. So to keep the network up-to-date and ensure that everybody got the message.

On a broadcast multi-access every router can reach every other so a DR/BDR are elected. On point-point there you dont need this dr/bdr election because there will only ever be two devices on the line. Instead they get a master/slave role. NBMA also elects a DR/BDR but it needs a little more tweaking than Broadcast multi-access to get it 100%. Point-multipoint works the same way as point-point as far as DR/BDR are concerned.