Multicasting

[alan-d]

I'm studying for MCSA/MCSE and currently going over DHCP.

 

I seem to be having problems getting my head around Multicasting I know how to set up a multicast group etc but can't seem to get to grips on why it should be used.

 

Any pointers to sites etc that may explain it in laymans terms? I've tried googling but I just seem to find the MS textbook explainations.

[ChrisC]

I've always found it easier to unicast, seeing as multicasting 2 machines just halves the speed of transfer.....

 

And another problem would be if the network is old or slow, and you're multicasting to lots of machines, it would take forever AND a day, or possibly you'd just loose packets, and that would take eeeeven longer.

 

Chris

[Ric_]

In theory, multicasting reduces the amount of bandwidth consumed when sending data to multiple machines simultaneously. If you unicast data, it is sent out across the network as many times as there are clients where as multicasting will only send the data once.

 

You do need to properly configure your switches and they must be intelligent enough to understand multicasting properly for it to work.

[alan-d]

Thanks Ric - it has all just dropped into place

 

Makes sense now I must be getting too old for this study lark!

 

Just one more thing - Am I right in saying that on an internal network the multicast scope is only valid for the period you set it for and after that you have to reactivate it?

 

It seems odd that it would need to be set everytime a video conference took place, assuming it was set for 30 days and each conference was monthly.

 

Does that make sense?

[Geoff]

Scope expiry is an entirely Windowism. Do not set an expiry if you want a multicast things out into the wider world or to non-ms hardware/software.

 

If your just using things internally on your network, you should use an administrative scope (as defined in RFC 2365).

 

If for some reason you require to multicast onto the internet you will require a global scope allocated by RIPE. You will also have to arrange correct routing with your ISP on to the MBone.

 

In your paticular example, I would not bother setting an expiry. Video conferencing is likely to be a unpredictable and regular occurance. Additionally, some clients may not understand the Multicast Address Dynamic Client Allocation Protocol (I refer you to the first point I made above).