How does Multicasting save bandwidth?

[JEDI]

Multicast is sending data from one source to group of multiple recipients (ie: software updates to workstations) and saves bandwidth vs unicast to each of those recipients.

If you're doing a 100meg patch to 10 workstations, then you need a 1 gig pipe if you want to do the upgrade simultaneously via unicast.

But don;t you still need ten 100 meg pipes w/multicast?

How does Multicast save bandwidth?

(Or more simply, how is multicast different than simultaneous unicast connections to a group of recipients?)

THX

 

[Smilin]

 

[Smilin]

Because it only sends one "stream" of data instead of multiple. All receiving computers treat the one stream as if it's meant for them. It's kinda like sending data to the 255.255.255.255 address...everyone listens.

 

[spidey07]

Exactly.

And instead of sending multiple streams you just send one. The routers and switches then send the data to the multicast group members. It is extremely efficient.

For instance I've setup video and pc ghosting here on campus. At any given moment there are PCs being imaged and dozens of 1.5 megabit video streams going on. The network doesn't even notice, everything just runs smooth as silk because all of that data is being replicated by the network devices themselves and not tying up interswitch or router links.

The key to it is having switches and routers that support multicast. Normally switches will flood every switch port with multicast frames causing unwanted traffic and load, with a mutlicast switch the switch intelligently only sends the multicast frames to ports who have group members on them.

-edit- hosts only listen if they have joined the group address, otherwise they discard.

 

[Hoober]

Multicasting is particularly useful for ghosting PC's.

Our current gear doesn't support multicasting, so imaging our labs for various classes destroys the network segment. We're working on getting multicasting enabled, but it's a big investment in capital if you have to replace all of your older gear.

 

[subflava]

spidey07 - Can you elaborate a little more about using multicast and "pc ghosting"? Do you mean "ghosting" hard drive images (which I don't see how it would benefit)? I guess I've just never heard of "pc ghosting"...

 

[Hoober]

Originally posted by: subflava
spidey07 - Can you elaborate a little more about using multicast and "pc ghosting"? Do you mean "ghosting" hard drive images (which I don't see how it would benefit)? I guess I've just never heard of "pc ghosting"...

ghosting is the same as imaging a hard drive.

 

[spidey07]

The PC support group can take a few dozen blank PCs, plug them into the network and hit enter. Within 20 minutes all PCs have an image and all software...ready to go.

I think the images are many gigabytes, so with multicasting instead of each pc taking 20 minutes linearly (say 15 pcs would take 15*20 minutes or 300 minutes) it only takes 20 minutes no matter HOW MANY PCS ARE BEING IMAGED.

Does that help? That's just one instance of just how great multicasting is. And in a large campus network with a 100 networks and a 20 routers the ability to multicast anywhere and not replicate data for each stream is crucial.

 

[JEDI]

ahh...got it

you send it once, the data hits the switch and the switch replicates the data. so the pipe between you and the switch isnt clogged.

ie: 24 port 10/100 switch. data only goes out of the ports that have machines connected to them that are in the multicast group. so if that data is 50meg in size, and there are 20 machines on that switch that are in the multicast group, then that switch is handling 1gig of data simultaneously?

EDIT: And since a 10/100 switch can handle ~10 megabytes/sec (per port) then those 20 ports would be flooded for ~5 sec, but the other 4 will behave as if there wasn't any load?

 

[spidey07]

Yedi,

You're starting to get it. The way switches are architected it is very easy to replicate or switch frames to a bunch of ports so a 1-to-many is no problem at all.

But the down side is one really should use multicast enable switches otherwise the switch will "flood" the multicast frames to every port. Imaging some different ghosting sessions going on with some video.

Well now every single port of every single switch in the entire network is now sending out 50-60 Megabits/sec of traffic. That's what you call a "net-NOT-work"

 

[JEDI]

arrg..i was editing my msg while Spidey replied.

anyway, here's my edited msg:

ie: 24 port 10/100 switch. data only goes out of the ports that have machines connected to them that are in the multicast group. so if that data is 50meg in size, and there are 20 machines on that switch that are in the multicast group, then that switch is handling 1gig of data simultaneously?

EDIT: And since a 10/100 switch can handle ~10 megabytes/sec (per port) then those 20 ports would be flooded for ~5 sec, but the other 4 will behave as if there wasn't any load?

 

[spidey07]

correction.

A 10/100 switch can push 9-11 Megabytes/second.

your 50 meg file would take 5 seconds on each port and the ports who did not have joined group members. For example here is a multicast group table from a cisco 6509...you'll see some of our multicast addresses in layer2 form (multicast IP addresses map directly to layer2 multicast addresses).

FC2-6509> (enable) sh multi
Usage: show multicast router [igmp] [mod/port] [vlan_id]
show multicast group [mac_addr] [vlan_id]
show multicast group count [vlan_id]
FC2-6509> (enable) sh multi group
IGMP enabled
GMRP disabled

VLAN Dest MAC/Route Des [CoS] Destination Ports or VCs / [Protocol Type]
----- ------------------ ----- -------------------------------------------
3 01-00-5e-00-01-16 1/1
3 01-00-5e-02-00-07 1/1
3 01-00-5e-02-00-08 1/1
3 01-00-5e-03-00-0f 1/1
3 01-00-5e-03-00-14 1/1
3 01-00-5e-03-00-15 1/1
3 01-00-5e-03-00-16 1/1
3 01-00-5e-03-00-17 1/1
3 01-00-5e-03-00-64 1/1
3 01-00-5e-03-00-65 1/1
65 01-00-5e-00-01-16 1/2
65 01-00-5e-01-02-03 1/2
91 01-00-5e-00-01-28 1/2
99 01-00-5e-00-01-16 1/1,7/1-4
99 01-00-5e-00-01-18 1/1,7/1-4
99 01-00-5e-00-01-27 1/1,7/1-4
99 01-00-5e-00-01-28 1/1,7/1-4
99 01-00-5e-03-00-0f 1/1,7/1-4
99 01-00-5e-03-00-14 1/1,7/1-4
99 01-00-5e-03-00-15 1/1,7/1-4
99 01-00-5e-03-00-16 1/1,7/1-4
99 01-00-5e-03-00-17 1/1,7/1-4
99 01-00-5e-03-00-64 1/1,7/1-4
99 01-00-5e-03-00-65 1/1,7/1-4
101 01-00-5e-00-01-3c 2/1,3/1,7/1-4
101 01-00-5e-7f-ff-fa 2/1,3/1,7/1-4
102 01-00-5e-00-01-16 4/33,4/45,5/13,7/1-4
102 01-00-5e-00-01-3c 4/16,4/26,4/31,4/42,5/2,5/8,5/31-32,5/40,6/
3,7/1-4,9/10,9/27,9/30,9/39,9/44
102 01-00-5e-7f-ff-fa 4/8-10,4/12,4/14-15,4/17,4/19-20,4/23-24,4/
29,4/33,4/35,4/37-38,4/43-45,5/1,5/3-6,5/10-11,5/13,5/16,5/19-21,5/24-25,5/27,5/
31,5/35,5/41,5/46-48,6/3,6/13,7/1-4,8/5,8/9,8/14,9/5,9/8,9/12,9/25-26,9/28,9/30,
9/34-35,9/38
103 01-00-5e-00-01-28 1/1
103 01-00-5e-00-01-3c 1/1,4/28,5/42,6/4,6/8,6/21,6/29,6/41,8/6,8/
32,8/39,9/9,9/20,9/23,9/43
103 01-00-5e-03-00-64 1/1,8/25
103 01-00-5e-7f-ff-fa 1/1,4/34,6/1,6/5-6,6/9,6/11-12,6/14-16,6/19
,6/23,6/25,6/27-28,6/34,8/3,8/7,8/10,8/13,8/18,8/21,8/25,8/28,8/35,8/38,8/43,8/4
6,8/48,9/3,9/15
104 01-00-5e-00-01-3c 2/2,3/2,7/1-4
104 01-00-5e-7f-ff-fa 2/2,3/2,7/1-4
105 01-00-5e-00-01-3c 2/3,3/3,7/1-4
105 01-00-5e-7f-ff-fa 2/3,3/3,7/1-4
106 01-00-5e-00-01-3c 2/4,3/4,7/1-4
106 01-00-5e-7f-ff-fa 2/4,3/4,7/1-4
120 01-00-5e-00-01-27 1/2
400 01-00-5e-00-01-27 1/1

Total Number of Entries = 41
FC2-6509> (enable)

 

[subflava]

Okay, so you did mean "ghosting" in the way I thought you were (copying disk images). At first, I just didn't see how you were implenting it. I suppose this is a bit OT, but I was just more interested in the specifics of how you setup the ghosting. I suppose you set these machines to boot from the network and then just use some sort of ghosting software that has network support?

Scanning Symantec's website I see a Ghost Corporate Edition 7.5. Pretty feature packed from just skimming the description. Can something like this be done with the regular desktop version of Ghost? Any other software out there that people use?

 

[JEDI]

Originally posted by: subflava
Okay, so you did mean "ghosting" in the way I thought you were (copying disk images). At first, I just didn't see how you were implenting it. I suppose this is a bit OT, but I was just more interested in the specifics of how you setup the ghosting. I suppose you set these machines to boot from the network and then just use some sort of ghosting software that has network support?

Scanning Symantec's website I see a Ghost Corporate Edition 7.5. Pretty feature packed from just skimming the description. Can something like this be done with the regular desktop version of Ghost? Any other software out there that people use?

ghost is a dos based program that fits on a floppy. everything else is just bell and whistles. or at least my copy (v6.51 enterprise edition)

i actually boot from floppy w/network drivers, connect to network, then run ghost. i suppose you can do straight from ghost, but i;ve been using ghost sonce v4 (?) and thats the way i;ve always done it.

but i've never multicast ghost. i dont do rollouts. i only ghost machines that users have screwed up so i've never had the need to multicast, nor do i have any idea how to multicast using ghost. how do u do it, btw?