IP multicast - best practice

Answered Question
Jul 15th, 2009
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Hi. I'm setting up IP Multicast for the first time (I'm not experienced with Multicast at all), and I'd like a few pointers in how to set it up in the best way. Attached is a sketch of the network as it is now.


In my lab, I've managed to get it to work: the IOS switches seems to have multicast turned on by default. To get the routers to work, I just entered "ip multicast-routing" globally and "ip pim dense-mode" on the participating interfaces.


What I like to know, is how to set up multicast in my network, so only the clients on 10.1.20.0/24 get the multicast traffic from 10.1.10.37. In other words: as little noise as possible :-)

Correct Answer by Jon Marshall about 7 years 8 months ago

Just to add to Giuseppe's/Joseph's posts.


Because you only have a small network you can use static RP. So assuming you choose the 6500 as the RP


1) on the 6500 and 7200 add this command in global config mode -


ip pim rp-address 10.74.2.5


2) Under each of the interfaces on the 6500 and 4500 add


"ip pim sparse-mode"


3) As already noted enable IGMP snooping on all your L2 switches.


4) Make sure that the TTL of the multicast packets are at least set to 3


Jon

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Giuseppe Larosa Wed, 07/15/2009 - 01:55
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Hello Ivar,

test in your lab PIM sparse mode is the mode that provides better control.


>> is how to set up multicast in my network, so only the clients on 10.1.20.0/24 get the multicast traffic from 10.1.10.37. In other words: as little noise as possible :-)


Sparse Mode


Hope to help

Giuseppe


ivarstrandberg Wed, 07/15/2009 - 02:30
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When I set all four interfaces to Sparse Mode, nothing happens. Do I need to use RP or something when using Sparse?

Giuseppe Larosa Wed, 07/15/2009 - 04:01
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Hello Ivar,


>> Do I need to use RP or something when using Sparse?


yes indeed sorry I didn't write it on first post and also IGMP snooping on layer2 switches is important (may be more important in a campus environment)

you can check RP with:

conf t

ip pim rp

on L3 switch


to verify:

sh ip pim rp mapping


to check IGMP snooping that should be enabled by default

sh ip igmp snooping


see if it says enabled on clients' vlan


edit:

I've seen the network diagram

you need ip pim rp on all L3 devices on path


Hope to help

Giuseppe





Correct Answer
Jon Marshall Wed, 07/15/2009 - 04:16
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Just to add to Giuseppe's/Joseph's posts.


Because you only have a small network you can use static RP. So assuming you choose the 6500 as the RP


1) on the 6500 and 7200 add this command in global config mode -


ip pim rp-address 10.74.2.5


2) Under each of the interfaces on the 6500 and 4500 add


"ip pim sparse-mode"


3) As already noted enable IGMP snooping on all your L2 switches.


4) Make sure that the TTL of the multicast packets are at least set to 3


Jon

ivarstrandberg Wed, 07/15/2009 - 04:44
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Just the info I was looking for, thanks :-)


One more question though: How can I make sure that the TTL of the multicast packets are set to at least 3? :)

Jon Marshall Wed, 07/15/2009 - 04:49
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"How can I make sure that the TTL of the multicast packets are set to at least 3? :)"


That's done within the application itself ie. nothing to do with the network setup as such. Chances are you will be fine but some multicast apps set the TTL to 1 which means it won't go across routed interfaces. There should be some way within the app to tell it to increase the TTL if that is the case.


Jon



Joseph W. Doherty Wed, 07/15/2009 - 05:04
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As Jon notes, it should be set by the application, although I recall most hosts default to something like 32, 64 or 128. Whatever; if dense-mode has been working, i.e. TTL has been sufficient, then so it's likely so should sparse-mode. (Reason likely - sparse-mode might introduce [initially/permanently] more hops between source and receiver. [But we're now getting much deeper into multicast then you want, or need at the moment, to know. ;) ])

Joseph W. Doherty Wed, 07/15/2009 - 02:34
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Besides Giuseppe's recommendation for using sparse-mode rather than dense-mode to reduce the needless distribution of multicast-traffic, on your switches, if they support IGMP snooping, you would want to insure it's enabled too, to stop the needless distribution of multicast to ports on a network segment.


BTW, sparse-mode is little more complex to configure than dense-mode since a RP needs to be defined.


Also, if your network is such where multiple routers share a network segment, PIM snooping might be used to supress multicast traffic to router ports that don't need the traffic.


PS:

"Attached is a sketch of the network as it is now."


Appears to be missing.

cbeswick Wed, 07/15/2009 - 06:45
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Hi,


There is maybe a another way to do this and that is to use auto-rp. By using this feature you do not need to manually configure a rendevous point for every multicast group, instead the network takes care of it automatically.


To enable this you configure a minimum of 2 routers to announce themselves as candidate rendevous points, and a minimum of 2 routers to act as mapping agents. The mapping agents then choose between the 2 candidate routers and automatically assign a router to a multicast group.


It sounds complicated, but once setup you can just leave it alone and let it do its thing. We have been using this configuration for over 3 years now without issue. We also use IGMP snooping on all access layer switches and PIM snooping on core / distribution devices.


In order to use this you need to configure ip pim sparse-dense-mode on your backbone. This is needed for auto-rp to function.


The BCMSN book used to discuss this configuration, but I do not know if it is still included on the course.


HTH.

Giuseppe Larosa Wed, 07/15/2009 - 07:33
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Hello Chris,

good note

there are only three routers in the picture so using manual RP setting is acceptable here.


Hope to help

Giuseppe


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