Multicasting (Ghost) across VLANs

Posted on 2011-03-17
Last Modified: 2012-08-13
Hi all,

I have read many posts regarding this and see that there are many answers to many questions.  So please bear with me if I have missed an obvious solution somewhere.

We have a training centre with multiple rooms, each room has been allocated a port one 3COM 5500-E gig (backbone) switch and that port has been allocated into a VLAN also setup on the 3COM switch.  We have a DHCP server distributing an IP scope for each room.  This is also in its own VLAN on a trunk port on the 3com switch.

Everything DHCP wise seems to be working as expected,  when  you attempt a ghost multicast session, the client connects to the ghost server and then hangs.  Unicasts work.

If you move the port allocated to the room on the 3com switch into the Ghosting VLAN, as you would expect, it works.  However, all the rooms must have their own VLAN otherwise the ghosting floods the network.

IGMP snooping is enabled on the room switches (3com 2400) as well as the 3com 5500-e switch.

Question by:sam-walker
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Expert Comment

ID: 35159916
For multicasting between VLANs you must enable your Layer 3 switch to do Multicast Routing. Is multicast routing enabled on your 5500?

Accepted Solution

RKinsp earned 500 total points
ID: 35160043
multicast routing-enable
interface vlan X
igmp enable
pim sm (or pim dm. sm is more common)

Author Comment

ID: 35160144
Thanks RKinsp, I will try this tommorrow.

From the switch it appears that there is only on VLAN interface (1) for multiple VLAN's.  Is that normal? or would you expect to have a VLAN interface for each VLAN?

Expert Comment

ID: 35160419
You need one vlan interface for every VLAN you are routing from that switch.

If you are routing between VLANs at a different point in your network, then you don't, but if i understood your design, you have no other routers/layer 3 switches, right?

LVL 45

Expert Comment

by:Craig Beck
ID: 35164338
I think you're looking at one of the switches in one of your rooms.  You need to look at the switch where all the room switches connect to.  This switch will have an IP address for each VLAN, and this is where you need to enable PIM.  Its usually easiest to configure PIM-DM per VLAN.
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Author Comment

ID: 35164580
RKinsp, yes, you are correct, there is no other layer 3 switch working on the network.

Apologies, there is a VLAN interface for every VLAN, I was looking at the correct switch but via the GUI which is not very intuitive in displaying information.

I checked the config via a telnet session and it was clear that there were VLAN interface for each VLAN as you would expect.

I have applied the changes you suggested to one VLAN and tested the ghosting, but unfortunately it did not work.  Now, please excuse my lack of knowledge here, but I did not save the config just yet, but it should take effect as it loaded into running memory, is that correct?

Note; I used pm dm as craigback suggested

Author Comment

ID: 35164593
Sorry - PIM DM

Author Comment

ID: 35165507
Hi RKinsp,

That has actually worked now!  It was my failure not to apply it to the Ghost VLAN (Doh!)

Thankyou so much for your help, your are a true Gent!

Author Closing Comment

ID: 35165512
RKinsp is a top guy.

Expert Comment

ID: 35166046
Thanks Sam. Glad to help


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