NBMA What is it?

[b-mackey]

Hi ,

I have a very basic question,

Can anyone please explain me definition of these terms in a practical environment:

What is meant by Broadcast Multi-access networks, an example is Etehrnet...But what exactly happens for which it is called "Broadcast" and "Multi access"

What is meant by NON Broadcast Multi-access networks, an example is Frame Relay...But what exactly happens for which it is called "Non Broadcast" and "Multi access"

Thanks a lot for your help.

Bon.

[vcjones]

Multi-access = the same physical link can be used to communicate with more than one destination. Contrast with "point-to-point" where each physical link is dedicated to a single destination.

Non-broadcast = the physical link does not support sending the same packet to more than one destination. Contrast with "broadcast" (or multicast) where the same packet can be received by all other devices on the physical media. With a NBMA network, to send a frame to 10 destinations, you have to send 10 copies of the frame, one to each destination. With a broadcast network (which, by definition, must be multiple access) you merely send a single frame with a multicast or broadcast address which is recognized by all 10 destinations.

Welcome to the world of networking! And don't let anyone kid you, networking concepts are simple common sense. The challenge is the number of concepts which you need to keep in mind at any one time :-) There are only two reasons for a networking fundamental to be confusing: the person explaining it doesn't really understand it, or the underlying reason is historical and no longer applicable. (My favorite example of the latter is the ASCII delete character, whose weird assignment actually made sense back in the days of paper tape on teletype machines.)

For fundamental questions like this or on routing loops, the best reference is still Tannebaum's book. Excellent explanations (that are actually correct, unlike many other books on the market) of a wide range of theoretcal networking issues that do not assume you already have a strong networking background but do assume that you want to understand how things work rather than just how to out guess some vendor's certification test.

Good luck and have fun!

Vincent C Jones

http://www.networkingunlimited.com

[thisisshanky]

This also adds in..

http://www.linktionary.com/n/nbma.html

[Richard Burts]

The replys from Vincent and Shanky are good. I would like to add two practical examples where the difference between Broadcast MultiAccess and Non-Broadcast MultiAccess affects the behavior of the network.

First, with Broadcast MultiAccess when a router sends a packet we know that all other stations on that segment will receive it. So, for example, when a router running OSPF sends a Hello packet to join the network and participate in election of a DR/BDR we know that all of the other routers will receive that packet, will recognize the router which is joining the network, and election of DR/BDR proceeds in the normal way. However with Non-Broadcast MultiAccess it is not necessarily true that all other stations will hear the packet. Think of a Frame Relay network with a Hub router H1 and three spoke routers S1, S2, and S3. If S1 brings up its interface and sends a Hello, the hello will be received by the Hub router but not by either of the other spoke routers. So election of a DR/BDR must have a different mechanism for NBMA.

The second example involves the concept of split horizon. Split horizon is the concept that you do not advertise back out an interface information that you learned on that interface. Some protocols such as EIGRP enforce this concept while others such as OSPF do not. This concept is valid and makes good sense for Broadcast MultiAccess such as Ethernet. If a router hears a route advertisement on its Ethernet interface, there is no reason to advertise that network back on the Ethernet interface because we know that all routers on that Ethernet have heard the routing advertisement. However this is not necessarily true for NBMA. If our example Hub and Spoke network there is LAN L1 at spoke 1 and LAN L2 at spoke 2. Spoke 1 will advertise L1 and the advertisement goes to the Hub. The Hub learns the route on its serial interface. If it does not advertise the route back out the serial interface then spoke 2 will never learn of L1. (This example is written assuming that the Hub router has the 3 PVCs on its serial interface or has all the PVCs on a multipoint subinterface. If the Hub uses Point-to-point subinterfaces then there is no issue.)

HTH

Rick

[b-mackey]

Thanks much, your reply was just awesome.