Comments on: Identifying Ethernet Multicast

By: amar

amar — Wed, 28 Sep 2011 08:51:17 +0000

Just wondering…..
Even the broadcast address will have 1 in most significant bytes most significant bit.
So how does switch differentiates between broadcast and multicast address?
Looks like its not only based on most significant bit alone.

By: Rubén

Rubén — Wed, 08 Sep 2010 14:01:25 +0000

Hi Brad,

I have a Cisco 3560 with IGMP snooping configured to control the multicast traffic. However, the switch is filtering frames addressed to a 0180.XXXX.XXXX multicast MAC address within a VLAN.

I mean, the frames to this multicast address arrive to the switch from a port, and they are not forwarded to other port in the same VLAN.

As far as I know, the switch must forward these multicast frames (no IP multicast packet, but multicast frame). Is it true? Should the switch forward them by default? Is there any way to force it forward these multicast frames within the same VLAN?

Thanks. Rubén.

By: frankm

frankm — Fri, 20 Feb 2009 17:38:08 +0000

Hey terry

yes, if the second hex character is odd or B, D or F, then its a multicast MAC address.

By: Terryn

Terryn — Wed, 04 Feb 2009 16:06:14 +0000

Hi Brad,

Would it be safe to say, reading a MAC address in HEX, that if the 2nd char in from the left is even (or 0) its a Unicast address and if its odd it is a multicast address? Or have I got it horribly wrong?

Thanks learning, much needed.