Multicasting across VLAN/subnets?

[shinedog74]

We're using this product from Acronis called Snap Deploy, used for imaging workstations. Products like Snap Deploy and Ghost can use multicasting to send out an image to multiple stations at once.

I ran into a problem with this at a rather large client where they have like 20+ VLANs set up, think classrooms where each one is set up on its own VLAN. The problem is that the routers don't seem to pass multicasting through the VLAN interfaces, unicast works fine though but it would be a lot faster if we could multicast an entire room at once.

I can hold my own on a lot of basic Cisco config stuff but I'm a little dark on this one so I guess my questions are:

1.) Is it possible to pass multicast traffic from VLAN1 into other VLANS?

2.) If so on #1, can anyone point me in the right direction, perhaps a link or some commands that I can research further? I tried Googling but not sure if I am passing the right keywords as I haven't found much.

We basically only have one server with a single IP address that needs to pass multicast traffic into multiple VLANs. Thanks for any info provided!

[Amit Singh · Accepted solution]

In a LAN environment to use multicast effectively you can use either of the following methods:

1. GMRP, GARP multicast registering protocol

2. CGMP, Cisco group mulicast protocol

3. IGMP-Snooping and MVR (multicast Vlan registration)

If you have a router behind the Server then you have to use IGMP on the router as well. My Idea for this would be configruing IGMP-Snooping/MVR on the the switches and have the clients join the multicast session.

Which switch you are using? Please have look at configuring IGMP-Snooping on the switches:

http://www.cisco.com/en/US/products/hw/switches/ps5023/products_configuration_guide_chapter09186a00801cdea3.html

HTH,Please rate if it does

-amit singh

[ebanks2006 · Accepted solution]

You need to enable ip multicast routing for a router to forward multicast traffic between VLAN's.

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fipr_c/ipcpt3/1cfmulti.htm

Be aware that unless you're running something like IGMP or CGMP on the access switches supporting the end-nodes on each VLAN, the switches will flood the multicast frames to every port on the VLAN. For something as bandwidth intensive as system imaging, that could bring a VLAN to its knees. IGMP and CGMP are protocols that intelligently detect what hosts should be enrolled in a given multicast, and then flood the multicast only to those ports.

For your application (one classroom on a VLAN where probably every system in that classroom needs to get imaged at a whack), this may be less of a concern, but just remember that in essence, you'll be unleashing a multicast storm on the VLAN. That's fine for everyone on the VLAN that's supposed to be getting imaged by Acronis, but bad for anyone else trying to talk on that VLAN - they won't be able to get a word in edgewise.