Re: IGMP Snooping on 2950 and still flooding multicast

comdirect · ‎05-06-2003

Hi,

I'm configuring IGMP Snooping on Cisco Catalyst 2950 Version 12.1(12c)EA1 Enhanced Image.

IGMP snooping seems correctly configured :

sw#sho ip igmp snooping vlan 1

vlan 1

----------

IGMP snooping is globally enabled

IGMP snooping is enabled on this Vlan

IGMP snooping immediate-leave is disabled on this Vlan

IGMP snooping mrouter learn mode is pim-dvmrp on this Vlan

IGMP snooping is running in IGMP_ONLY mode on this Vlan

But I still receive a lot of multicast without having joined any IGMP groups (on my fa0/8 interface) :

sw#sho int fa0/8 counters

Port InOctets InUcastPkts InMcastPkts InBcastPkts

Fa0/8 3387781 41569 48 50

Port OutOctets OutUcastPkts OutMcastPkts OutBcastPkts

Fa0/8 75207017 42402 60322 1962

sw#sho mac-address-table multicast

Vlan Mac Address Type Ports

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

1 0100.5e00.0001 IGMP Gi0/2

1 0100.5e00.0002 IGMP Gi0/2

1 0100.5e00.000a IGMP Gi0/2

1 0100.5e00.00fa IGMP Gi0/2

1 0100.5e7f.fffa IGMP Fa0/8, Gi0/2

5 0100.5e00.0001 IGMP Gi0/2

5 0100.5e00.000a IGMP Gi0/2

20 0100.5e00.0002 IGMP Gi0/2

20 0100.5e00.000a IGMP Gi0/2

The verbose multicast group (proprietary application) is 230.0.0.1 aka 0100.5e00.0001 on VLAN 1. As I used a sniffer, I know that I'm still receiving multicast packets for 230.0.0.1.

Any help would be appreciated.

Please reply to my email address as well (reytans_at_wanadoo.fr).

Thanks,

Stephane

ikraehenmann · ‎05-10-2003

What you probably see are multicasts in the range 0100.5e00.00xx. The Cisco switches do not apply IGMP snooping on these addresses. Why?

224.0.0.x is dedicated to link-local multicasting (by IANA). These addresses map to the MAC addresses mentioned above. Quite a few applications use these addresses (just think of the routing protocols).

If you restriced forwarding to those who registered through IGMP (and therefore create a multicast CAM entry), these applications wouldn't work no longer.

Keep that in mind, when you choose multicast groups for your multicast applications.

Igi