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regarding BROADCAST`

Hi all,

As far i know,broadcasting in a network takes place when the destination address is not known and when the PC is switched ON i.e, when it enters in to the network for the first time and inorder to make its presence to know all the other systems the NIC will broadcast its MAC address.Is this right?

But in our company network ,we r having a central managed switch by which i can observe all the incoming and ougoing traffic.In that the broadcast packets that r being received and transmitted by all the ports are increasing all over the day.Why this is happening?Will the NIC by default broadcasts any packets?If so,why?

As our 200 users are in a single subnet,i am observing that so much of the bandwidth is being consumed for this broadcasting itself .If really broadcasting is the issue,then we will implement VLANs in our network.
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haribabupanchireddy
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haribabupanchireddy
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1 Solution
 
rsivanandanCommented:
Yes, computers when they come online do a broadcast to find the DHCP server and get an ip address. But other than that broadcasts are used for a lot of other purposes also. In your case it is not advisible to have like 200 computers in a single VLAN, even the manageability wise it is not suggested.

I would suggest you to split them into Vlans and by that your network performance will definitely increase a lot. You can do vlan separation based on a lot of criteria, like based on departments, geographical units or even separate server farms and clients (as simple as that).

So Yes, definitely creating broadcasts will make your life a lot easier.

Cheers,
Rajesh
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haribabupanchireddyAuthor Commented:
Hello Sir,

I agree with u.But i would like to know the ways in which broadcast occurs theoritically and practically.
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The--CaptainCommented:
>As far i know,broadcasting in a network takes place when the destination address is not known and when the PC is
>switched ON i.e, when it enters in to the network for the first time and inorder to make its presence to know all the other
>systems the NIC will broadcast its MAC address.Is this right?

Not if you're talking about IP (I think IPX networks are more similar to your description)...

In IP, your netmask determines which hosts you can reach on the local interface - when you try to send a packet to a host  on your network, your machine sends (if its using ethernet) an ethernet broadcast which basically says "Who has IP a.b.c.d?", and the appropriate machine responds with the MAC address of its own ethernet interface.  BTW, this type of broadcast is known as an ARP request.

If you try to send packets beyond your local network, your machine does the same thing, but asks who has the IP of the appropriate router.

Also, poorly designed protocols (ie Netbios, which is what windows typically uses for network browsing/sharing) can generate a lot of broadcast packets.

If any of this is news to you, you should really read some basic networking texts.

Cheers,
-Jon

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rsivanandanCommented:
1. DHCP Server
2. Routing Updates for some routing protocols
3. Multicasts
4. Most importantly 'Arp' broadcasts.

Say when a Machine A wants to talk to Machine B, it does a broadcast to find the mac address of machine B. This broadcast is received by all the 200 machines in your network. Now think if you had only 50 machines in a vlan, the bandwidth will be less. Same goes for DHCP as well.

Cheers,
Rajesh
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jburgaardCommented:
You also have  cache's with MAC-IP relations in it.
This cache will mitigate ARP request's. The cache will not live forever, surely not a day long. Some minutes.
In your managed sw.'s config there must be a value.
You can also show the actual cached entr.'s in the sw.

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haribabupanchireddyAuthor Commented:
Thank u all.

But still i am not clear.

For example,consider 10 systems connected to a switch and all the systems are in the ON state.Switchs MAC address table is able to show in which port which systems MAC address is and the aging time for the MAC is 300seconds.I think,if a system is idle for more than 300seconds then the MAC address will be removed from the switches MAC table and if that system again enters in to the network,then through ARP request broadcasting its address will be known.

But when all the systems are continuosly working also i am observing broadcast packets....thats what i am not able to understand...?
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rsivanandanCommented:
I believe I answered that in the last post ? It is not just the switch that is involved in the loop.

Cheers,
Rajesh
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The--CaptainCommented:
>But when all the systems are continuosly working also i am observing broadcast packets....thats what i am not able to
>understand...?

Are you running windows on many machines?  As I already said, netbios generates broadcasts as part of it's "normal" operation.

Cheers,
-Jon
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conrad_rayCommented:
You need to determine what is the bulk of these broadcasts, and if they are sourced evenly by all or by certain machines/servers - we can setup a capture session on a port that is experiencing high broadcasts to determine these with a sniffer. Then determine if the volume of broadcasts is normal for that protocol? If things seem normal and these broadcasts cannot be avoided, we can create more vlans.

For Win2k, if an entry is not used by any outgoing datagram for two minutes, the entry is removed from the ARP cache. Entries that are being referenced ARE removed from the ARP cache after ten minutes. I think the values are lower for XP.

Broadcast suppresion on switches is also a way to minimize broadcasts.

Conrad.
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haribabupanchireddyAuthor Commented:
Thanku one and all...for ur valuable explanations,now i understood the concept.
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