How to configure Multicasting

Answered Question
Apr 17th, 2007
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I have a server on a server farm L3 switch, that's going to send images to workstations that are connected to various access switches on different VLAN's. I need to set up multicasting, so either a host sends a request to this server to start a session, or the server will send a request to a number of hosts. I've read the multicasting docs on a Cisco website, but I feel really lost on how to set this up between the Server Farm switch that houses the server, the Core L3 switch that routes all the VLANs and the access L2 switches where the hosts are connected.

Correct Answer by walleyewiz about 10 years 1 month ago

You will need to remove the following command from your configuration:

ip pim spt-threshold infinity

This command, with the keyword infinitiy, prevents a router from ever switching to SPT.



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Overall Rating: 4.7 (3 ratings)
walleyewiz Tue, 04/17/2007 - 10:39
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If you just need basic functionality, you should be able to get by with PIM-DM mode. PIM-DM mode works for small networks with few routers and light multicast application requirements.

You will need to enable multicast routing on all interfaces that connect to other multicast routers by putting the following command on each interface:

int x/x

ip pim dense-mode

You will also need to do the following on each muticast router globally:

ip multicast-routing

You shouldn't need to do anything on your switches. By default, IGMP snooping is enabled globally.

In short, when in doubt put the ip pim dense-mode command on every layer 3 interface and you should be fine.

Multicast is one of those things that can be very complicated, hopefully you don't need all the complexity.


gkushnir21 Wed, 04/18/2007 - 07:18
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Thank you for your information. It was very helpful.

I have a follow up question, looking at the current configs, the core L3 switch has the following configured:

interface Vlan xx

ip pim version 1

ip pim sparse-mode

ip pim rp-address

ip pim spt-threshold infinity

This is for IPTV, so if I add multicasting to my server switch, would it conflict in any way with the existing multicast setup? And should I stick with PIM-SM since it's already setup rather than DM?

Also, that is a WAN router at another site, i don't want my new setup to use that RP, since this new multicast setup will only be on a local LAN

walleyewiz Wed, 04/18/2007 - 07:31
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If you are currently running sparse-mode, you will need to define the RP on every router so all sources know about all groups.

Yes, you should just stick with SM since it is already setup.

What kind of server switch do you have?

gkushnir21 Wed, 04/18/2007 - 07:41
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The server switch is a Cat6509 and so is the Core Switch.

So, if i need to define an RP, it will have to be defined on my Core Switch MSFC and on the Server Switch MSFC. But since I already have an RP defined on both for IPTV and it's a router at the remote site, can I define a second RP, and how would it distinguish between the two?

walleyewiz Wed, 04/18/2007 - 07:50
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I am pretty sure you can only define one RP. I guess I am a bit confused on what you are trying to do.

If you already have a mcast environment setup for IPTV, why not just enable multicast on the server segments you need?

If you do that, I would think everything should work. If you need to control access, you can always use multicast boundaries to control the groups.

Here is a link from Cisco about configuring mcast:

gkushnir21 Thu, 04/19/2007 - 13:13
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You're right, I will enable multicast on the server farm switch.

What I'm trying to understand is, if the multicast data has to travel through the RP to get from the source to all the hosts?

Or is the RP only used for keeping track of multicast groups and for join messages.

I wouldn't want to send huge amounts of data through the RP, since it's in the remote location.

royalblues Fri, 04/20/2007 - 01:56
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If the data rate warrants, leaf routers on the shared tree may initiate a switch to the data distribution tree rooted at the source. This type of distribution tree is called a shortest path tree or source tree. By default, the Cisco IOS software switches to a source tree upon receiving the first data packet from a source.

Check out this link

HTH, rate if it does


Correct Answer
walleyewiz Fri, 04/20/2007 - 05:19
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You will need to remove the following command from your configuration:

ip pim spt-threshold infinity

This command, with the keyword infinitiy, prevents a router from ever switching to SPT.




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