From: Antonio Soares (amsoares@netcabo.pt)
Date: Thu Nov 30 2006 - 20:57:54 ART
Hi Ryan,
I think this excerpt from one of the MCast Bibles (Developing IP Multicast
Networks) may help you:
"It should be obvious that this 32:1 address ambiguity can cause some
problems. For example, a host that wants to receive multicast group
224.1.1.1 will program the hardware registers in the network interface card
(NIC) to interrupt the CPU when a frame with a destination multicast MAC
address of 0x0100.5E00.0101 is received. Unfortunately, this same multicast
MAC address is also used for 31 other IP multicast groups. If any of these
31 other groups are also active on the local LAN, the host's CPU will
receive interrupts any time a frame is received for any of these other
groups. The CPU will
have to examine the IP portion of each received frame to determine whether
it is the desired group, that is, 224.1.1.1. This can have an impact on the
host's available CPU power if the amount of "spurious" group traffic is high
enough.
In addition to having a possible negative impact on hosts' CPU power, this
ambiguity can also cause problems when trying to constrain multicast
flooding in Layer 2 LAN switches based solely on these multicast MAC
addresses."
So hosts need to examine L3 information to avoid the L2 ambiguity.
Regards,
AMS
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of Ryan
Sent: quinta-feira, 30 de Novembro de 2006 20:39
To: sabrina pittarel
Cc: ccielab@groupstudy.com
Subject: Re: Grasping the Multicast 32:1 duplicate MAC address issue...
(use courier to view)
This is the same information I've read in a what seems like a dozen books
and web pages. They all say that you only use last 23bits of the IP to make
up the MAC address. Nothing new here. Yet, there is is this 32:1 overlap,
but I've not found any documentation or book that goes beyond that to
explain.
So if I try this out for myself for example:
Lets take a random Multicast IP address. 228.45.3.2 (I just typed in some
numbers, so that's as random as were going to get)
Let's make this IP into a Multicast MAC address...
228.45.3.2
01:00:5e:xx:xx:xx
xxx.45.3.2
45 . 3 . 2
0010 1101.0000 0011.0000 0010
^
2 D: 0 3: 0 2
x010 1101.0000 0011.0000 0010
So if I've done my math right, 228.45.3.2 should be: 01:00:5e:2d:03:02
If I add a '128' to it to simulate the 1 in the 24th spot that gets excluded
in the conversion,
228.173.3.2 should ALSO be: 01:00:5e:2d:03:02
So, based on this, I would have to say that for any given Multicast IP
address, there is going to be EXACTLY 32 duplicate MAC address's.
224.y.x.x
224.(y+128).x.x
225.y.x.x
225.(y+128).x.x
...
238.y.x.x
238.(y+128).x.x
239.y.x.x
239.(y+128).x.x
So, what happens with well know Multicast IP's like the 224.0.0.x address's?
This would seem to mean that you could not use any Multicast address is
y.0.0.x range or y.128.0.x range as it would conflict with the
224.0.0.xaddress's.
This correct?
-Ryan
On 11/29/06, sabrina pittarel <sabri_esame@yahoo.com> wrote:
>
>
>
> Hi Rayn,
> the way the MAC Destination Address for a multicast packet is built
> is
the following:
>
> the last 23 bits of the IP multicast address are copied into the last
> 23
bits of the MAC address, while the beginning of the MAC address is fixes and
set to 0100.5e(xx.xxxx)
> This leaves 9 bits of the IP multicast address (32-23 = 9) that are
> not
reflected anyhow in the L2 MAC. These are the first 9 bits of the IP
address.
> Of these 9 bits 4 are fixed (1110b => class D address) but 5 are
> variable
so all the combinations of these 5 bits (2^5) generates ip multicast
addresses that matches the same mac.
>
> Hope this helps,
>
> Sabrina
>
>
>
>
>
> ----- Original Message ----
> From: Ryan <ryan95842@gmail.com>
> To: ccielab@groupstudy.com
> Sent: Wednesday, November 29, 2006 6:02:17 PM
> Subject: Grasping the Multicast 32:1 duplicate MAC address issue...
>
>
>
> I'm a little confused by the Multicast duplicate MAC address issue...
>
> Is the 32:1 duplicate MAC address issue just for certain address's (32
> total)?
>
> 224.1.1.1
> 224.129.1.1
> 225.1.1.1
> 225.129.1.1
> ...
> 238.1.1.1
> 238.129.1.1
> 239.1.1.1
> 239.129.1.1
>
> Or does it apply to ANY Multicast address? So for any one Multicast
address,
> there are exactly 32 duplicates....
>
>
>
> -Ryan
>
> ______________________________________________________________________
> _ Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
>
>
> ________________________________
Access over 1 million songs - Yahoo! Music Unlimited.
This archive was generated by hypermail 2.1.4 : Fri Dec 01 2006 - 08:05:49 ART