From: Brian Dennis (bdennis@internetworkexpert.com)
Date: Fri Dec 05 2003 - 14:45:57 GMT-3
<Quote>
You would not want this message sent only to the all-multicast-routers
address, 224.0.0.2, because there may not be a router between you and the
source.
</Quote>
The reason the destination address of the IGMP Membership Report
message is sent to the group as opposed to 224.0.0.2 is so that other group
members see the report which will in turn cause them to suppress their own
report. This is done so that redundant messages are not sent.
Brian Dennis, CCIE #2210 (R&S/ISP-Dial/Security)
bdennis@internetworkexpert.com
Toll Free: 877-224-8987
Direct: 775-745-6404 (Outside the US and Canada)
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of Bob
Sinclair
Sent: Thursday, December 04, 2003 6:48 PM
To: Pun, Alec CL; ccielab@groupstudy.com
Subject: Re: IGMP report MAC address
Alec,
The destination IP address in an IGMP Membership Report is the multicast
group address that you want to join. So, if a host wants to join group
224.2.2.2, then it will send its IGMP message with that destination IP
address.
The IGMP Membership Report (commonly called a join message) is itself an IP
multicast. The destination MAC address of the message will be the last 23
bits of the IP mapped into the last 23 bits of the MAC address:
For example:
224.2.2.2 maps to 01-00-5E-02-02-02
224.130.2.2 also maps to 01-00-5E-02-02-02
You would not want this message sent only to the all-multicast-routers
address, 224.0.0.2, because there may not be a router between you and the
source.
IGMP version 2 routers do send their leave messages to the routers at
224.0.0.2.
Hope that helps,
-Bob Sinclair
CCIE #10427, CISSP, MCSE
bsinclair@netmasterclass.net
----- Original Message -----
From: "Pun, Alec CL" <Alec.CL.Pun@pccw.com>
To: <ccielab@groupstudy.com>
Sent: Thursday, December 04, 2003 2:59 AM
Subject: IGMP report MAC address
> Consider a IGMP join group report, what's the Destination IP address and
MAC
> address ?
> I guess the destination IP address is the well-known "all-routers" address
> (sorry that I forgot the exact IP). But how about the MAC adress ? Is it
the
> coresponding "all-routers" MAC or the multicast group MAC ?
>
> thanks
> alec
>
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