Cisco Catalyst 3560E

Unanswered Question
Apr 14th, 2010


I have a Cisco Catalyst 3560E switch and I want to set it up for multicasting.

Since I have little experience with switches & routers, could you give me some guidelines to help me set it up?

Thank you,


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Giuseppe Larosa Wed, 04/14/2010 - 09:32

Hello Yper,

first of all it is important to determine if the switch will work at L3 (= does inter vlan routing) or it acts only at L2.

in first case you do the following:

enable multicast ip routing

conf t

ip multicast-routing

For each interface Vlan (they are named SVI) defined on it where you want to receive and/or send multicast traffic you can use:

int Vlan X

ip pim dense-mode

in the hypothesis that your multicast traffic is confined in a single campus.

If you want to route multicast traffic over the WAN the best way is to use IP PIM Sparse-Mode that requires one device to act as Rendez-vous Point or RP.

if the device is acting as L2 only (= no intervlan routing or inter vlan routing done by another device) you need to look at IGMP snooping possible issues.

IGMP snooping attempts to optimize forwarding of multicast frames of each multicast group by learning what user devices are interested on receiving a specific group. IGMP snooping acts by intercepting IGMP reports from end user devices.

And here comes the possible issue: IGMP reports are sent in response to an IGMP query sent by a multicast enabled router.

If no router is present on the broadcast domain the risk is that IGMP snooping will block traffic.

To avoid this IGMP snooping can be disabled on the vlan (not recommended) or the switch can:

a) become the multicast router on LAN


b) enable IGMP snooping querier on device (if supported it is for example on C6500)


Hope to help



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