[c-nsp] Multicast flooding

Jason Lixfeld jason at lixfeld.ca
Mon Apr 8 18:50:32 EDT 2013

Quick question about multicast flooding/blocking...

I know that by default a switch will forward multicast on all ports in the same broadcast domain.  I think I've got it disabled with switchport block multicast and igmp snooping enabled on the multicast vlan (4004) on the 15.0(2)SE1 3560G-24TS I have running here.  However, whenever I bounce a port configured with vlan 4004 on it (trunk or access), I get a period of 120 seconds where all ports in that VLAN are flooded with multicast traffic from Gi0/24.  After 120 seconds has elapsed, the multicast traffic from Gi0/24 dies off and the flooding to any other connected vlan 4004 ports subsides.

There is an ME3600 connected to Gi0/24 and the multicast source is connected to an access port on that ME3600 in VLAN 4004.  The switchport block multicast is configured there as well.

It seems like I'm missing, or, have misconfigured a knob somewhere, but I'm not quite sure what or where.

Any insight?

interface range GigabitEthernet0/1 - 10
 description Facing Client
 switchport trunk encapsulation dot1q
 switchport trunk native vlan 4082
 switchport trunk allowed vlan 4004,4032,4082,4086
 switchport mode trunk
 switchport block multicast
 switchport block unicast
 spanning-tree portfast
interface GigabitEthernet0/24
 description Facing Multicast Source
 switchport access vlan 4004
 switchport block multicast
 switchport block unicast

Systems-Dev_Pit#sh ip igmp snooping vlan 4004 
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 4004:
IGMP snooping                       : Enabled
CAPWAP enabled                      : Disabled
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

Thanks in advance.

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