RE: switchort block multicast

From: Scott Morris (swm@emanon.com)
Date: Sat Sep 03 2005 - 11:03:06 GMT-3

IGMP only applies with IP multicast joins. That will be presented as MAC
addresses that begin with 01-00-5E. However, there are many more
multicast/group packets at Layer 2 than just those. This command applies to
ALL Layer 2 multicast traffic, not just the Layer 2 presentation of IP
multicast.

How do you know whether a frame received is multicast or not? (Hint, look
up the I/G bit)

-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
simon hart
Sent: Saturday, September 03, 2005 9:58 AM
To: Group Study; Scott Morris
Subject: RE: switchort block multicast

Scott,

Cannot truly understand the point you are making. I realise that a switch
will base its forwarding decisions based upon L2 addresses resident with its
mac table (CAM). I understand that if a switch recieves a frame for an
unknown address it will flood out of all ports.

My investigations seem to prove that if igmp snooping is enabled on the
switch, then unknown multicast frames are not flooded out of all switch
ports (assigned to the corresponding vlan). On the contrary it will only be
sent out of ports that are mrouter enabled.

If i turn off igmp snooping, and then send an unknown multicast frame, it
will be flooded out of all ports associated with that vlan, unless I use the
switchport block multicast command.

Or are you saying that switchport block multicast is for blocking multicast
frames outside of 0100.5exx scope when igmp snooping is enabled ?

Thanks

Simon

-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of Scott
Morris
Sent: 03 September 2005 14:41
To: 'simon hart'; 'Group Study'
Subject: RE: switchort block multicast

You're thinking of IP multicast.

The switch is a layer 2 device and is basing decisions on Layer 2 multicast
traffic.

-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
simon hart
Sent: Saturday, September 03, 2005 9:11 AM
To: Group Study
Subject: switchort block multicast

Hi All,

I am trying to understand the validity of this command. This command blocks
unknown multicast traffic.

If IGMP snooping is enabled on a switch, then surely this command is not
necessary. My understanding is that with igmp snooping unless a reciever on
a port has specifically joined a group then multicast traffic will not be
forward from that port.
If a reciever joins a group then the port will forward the mcast stream by
virtue of the fact it now has a known mac address to deliver too.

Therefore would this command be only used when igmp snooping is disabled

Thanks

Simon 

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