From: Derek Small (Fuse) (dwsmall@xxxxxxxxxx)
Date: Mon Mar 27 2000 - 13:28:58 GMT-3
I'm scheduled for the ATM lab on April 4th from 8:00am to 4:00pm, but can't
make it. Is there someone that has it scheduled around the same time of day
between April 12 and April 19 that would be willing to trade?
Thank You
Derek Small
dwsmall@fatkid.com
----- Original Message -----
From: Joel W. Ekis <jekis@cisco.com>
To: Muralidhar Devarasetty <dhar_murali@hotmail.com>;
<ccielab@groupstudy.com>
Sent: Thursday, February 24, 2000 9:00 AM
Subject: Re: Multicast MAC address calculation....
At 08:56 AM 2/23/2000 -0800, Muralidhar Devarasetty wrote:
>Hi all,
>I have small doubt on Multicast MAC address calculation..
>What I understand is the lower order 23 bits of multicast ip address will
be
>used with "01-005E-00-00- 00" to generate multicast MAC address.
>But since the multicast ip address range is upto 28 bits of classD I feel
>there will be a clash , I mean same Multicast MAC will be generated for
>different multicast ip addresses.
You win the prize! There is an overlap - 32:1 (that 2 ^ 5). You can thank
a cheap university for the conflict. The developer of multicast, a
university professor (or grad student, I forget) asked for the complete 28
bits, but his boss wouldn't pay to register all of it. He would only agree
to fund 23 bits worth - this was just an academic exercise after all. (Once
again the theory of '64k of RAM is all you'll ever need' bites the
networking industry!!)
Think about TokenRing now. They have ONE 'multicast' MAC address
(c0-00-00-04-00-00)! That's about 269 Million to 1 overlap. Will Win2k run
on T/R? Active Directory uses multicast to function. Interesting
problem...
>If this is the case when there will be a packet to one of the multicast ip
>address will it be send to all routers/machines who are having same
>multicast MAC??
PIM won't have this problem, it functions at L3. Flows will be delivered to
the proper switch. However a well designed NIC should recognize flows that
were delivered to it that are unnecessary and NOT send them to the CPU.
>IF not How will switch differentiate?
The switch can't. The switch only understands MAC addresses and it could be
any of 32 different flows. If CGMP is used, the router will inform the
switch that a flow should go to a specific port based on the IGMP L3
information. IGMP Snooping will allow a non-CGMP switch to read the L3
information in the IGMP packet to decide what flow belongs to a specific
port. IGMP Snooping is CPU intensive - the switch must read every multicast
packet to see the few IGMP informational packets that are mixed in the flow.
New switches like the 6509 that don't support CGMP use dedicated ASICs to
off-load the snooping task from the CPU.
The best book ever written on Multicast is by Beau Williamson - Developing
IP Multicast Networks. I used this to study for the Lab - it rocks.
(Disclaimer - I'm a Cisco SE, Beau also works for Cisco).
>Any ideas? or my understanding is wrong?
Nope, you are right. Very astute.
>Thanks in advance
>Murali
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