Multicast on a small LAN environment

Unanswered Question
Jul 20th, 2009

Hi,

I'd like to enable multicast on a small environment consisting of two 3750 and couple of 2960. Will just enabling "ip multicast-routing" and "ip pim sparse-dense-mode" on all the VLAN that need multicast do the trick? The source address is local to the subnet so are its receivers.

regards,

I have this problem too.
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Tshi M Mon, 07/20/2009 - 11:34

Thanks Rob for the quick reply. I will give this a try on Thursday.

Jon Marshall Mon, 07/20/2009 - 12:02

Etienne

Also configure igmp snooping on your switches otherwise multicast packets are just broadcast.

Jon

Joseph W. Doherty Mon, 07/20/2009 - 12:13

If I rememember correctly, since you multicast source and receivers are on the same segment, you might not need to enable multicast-routing, although I believe the interface PIM is needed to activated IGMP query. Even for that, if switches not doing IGMP snooping, multicast should be seen by all hosts on multicast source segment.

Also, if you enable some form of dense mode, and since you haven't defined a RP, you might end up flooding other subnets with multicast until it's pruned.

I.e.

If there no's IGMP snooping, multicast should "naturally" work for source and receivers on same subnet.

If IGMP snooping is active, you'll need a IGMP querier, but shouldn't need multicast routing (again for multicast source and receivers on same subnet).

Lastly, once you activate multicast routing, dense mode will flood and prune other subnets which can be precluded by using sparse mode.

Tshi M Tue, 07/21/2009 - 04:48

Hi Joseph,

It appears that IGMP snooping is enabled by default (see below). If I get your point, if IGMP snooping is enabled, I will have to enable ip pim-dense-mode to enable igmp querier. Is that right?

Global IGMP Snooping configuration:

-------------------------------------------

IGMP snooping : Enabled

IGMPv3 snooping (minimal) : Enabled

Report suppression : Enabled

TCN solicit query : Disabled

TCN flood query count : 2

Robustness variable : 2

Last member query count : 2

Last member query interval : 1000

Vlan 1:

--------

IGMP snooping : Enabled

IGMPv2 immediate leave : Disabled

Multicast router learning mode : pim-dvmrp

CGMP interoperability mode : IGMP_ONLY

Robustness variable : 2

Last member query count : 2

Last member query interval : 1000

Vlan 10:

--------

IGMP snooping : Enabled

IGMPv2 immediate leave : Disabled

Multicast router learning mode : pim-dvmrp

CGMP interoperability mode : IGMP_ONLY

Robustness variable : 2

Last member query count : 2

Last member query interval : 1000

Joseph W. Doherty Tue, 07/21/2009 - 04:55

Specifically PIM-DM, no, but I recall a configuration of PIM on the interface is what activates the IGMP querier. (BTW, some switches can do this too [i.e. be the querier].)

Jon Marshall Tue, 07/21/2009 - 04:59

Etienne

"If I get your point, if IGMP snooping is enabled, I will have to enable ip pim-dense-mode to enable igmp querier. Is that right? "

Not necessarily. You do need something to query so that IGMP snooping can listen to the answer. However some switches are capable of being the IGMP querier themselves without having to enable pim on the L3 interfaces. If you do not need to pass multicast traffic across L3 boundaries have a look at the IGMP querier function -

2960 -

http://www.cisco.com/en/US/docs/switches/lan/catalyst2960/software/release/12.2_46_se/command/reference/cli1.html#wp6248718

3750 -

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_46_se/command/reference/cli1.html#wp6248718

Jon

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