multicast setup

carl_townshend · ‎04-28-2009

Hi all

a few questions on muliticast

can anyone tell me what happens on the switch, does it create a seperate cam table for each group? and does it put all devices sending to the same IP ie 225.0.0.4 into the same group ?

Giuseppe Larosa · ‎04-28-2009

Hello Carl,

the switch is more interested in dynamically mapping the receivers of each group.

This subset of ports in a vlan X that are connected to users that have joined for group G can be mantained by using IGMP snooping (that is by analyzing IGMP messages).

This is a forwarding optimization.

The switch builds for each Vlan X and multicast group G a list of ports that need to receive a copy of the frames that are part of the group.

If in a Vlan Y IGMP snooping is disabled the switch can only replicate the frames belonging to multicast stream G2 on all ports of Vlan Y expect the one on which it has been received.

Conversely if for a Vlan Z with IGMP snooping enabled there is no L3 multicast device acting as IGMP querier multicast flows are blocked on the vlan because the switch cannot hear to IGMP reports from hosts.

Hope to help

Giuseppe

carl_townshend · ‎04-28-2009

so, how does it work then, pc sends multicast on a certain ip, switch then asks all other ports by a query, pcs then respond by a join request ?

does the switch say put each mutlicast ip in its own group ?

please explain ?

Giuseppe Larosa · ‎04-28-2009

Hello Carl,

please read here it can take some time but it should help

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_25_see/configuration/guide/swigmp.html

IGMP snooping = sniffing IGMP report packets on all ports to build a dynamic replication port list for each group, vlan.

Hope to help

Giuseppe