Hi Beatriz,
      The mac address that is sent(as destination) is
the exact mapping of the multicast IP address.If you
are using the Linux system then by default it supports
Mac layer multicast support.
            So if your destination IP is 224.0.0.5
then your mac destination address will be 01 00 5e 00
00 05 in hex.
       The IGMP group joining is sent as a rule for
the IP multicaste which states that whenever you join
any group of multicaste address you have to send a
group join indiaction to your router.Study multicaste
general protocol to make picture more clear.
I hope i am clear!!
Regards
Amit
--- Beatriz Silva <beatriz_hargraveMAIL.COM> wrote:
> Amit, thanks for the answer. But I still have some
> doubts ...
>
> More specifically my question is:
>
> When a packet to AllSPFRouters is sent, what is the
> MAC address written in the destination field of the
> Ethernet header ? Is it a mac multicast address or
> is there a mapping between the ip multicast address
> and the mac addresses of all of the routers that
> joined that multicast address ?
> And what must be done in the router in order for it
> to be capable to liste to packets to multicast
> address AllSPFRouters and AllDRouters (if it is the
> case) ?
> Why does the router have to send an IGMP membership
> report message to AllSPFaddress and to AllDRouters
> multicast addresses ?
>
> I hope I am not too confusing ..
>
> Thank you,
>
> Beatriz
> > > AllSPFRouters and another to AllDRouters
> ----- Original Message -----
> From: Amit Srivastava <ospfisfunYAHOO.COM>
> Date: Mon, 24 Jun 2002 02:41:34 -0700
> To: OSPFDISCUSS.MICROSOFT.COM
> Subject: Re: Multicast addresses AllSPFRouters,
> AllDRouters
>
>
> > Hi Beatriz.
> > As far as i could understand your problem the
> > answer to it is:-
> > The Ip layer keeps a buffer(table) in
> which
> > it stores all the IP addresses it is listening
> to.So
> > when a packet is send on the net the Ip layer
> would
> > check the destination ip address from the buffer
> it is
> > holding and if the address is found then it will
> give
> > the packet to the other protocol otherwise it will
> > discard the packet.
> > Regards
> > Amit
> > --- Beatriz Silva <beatriz_hargraveMAIL.COM>
> wrote:
> > > Hi !
> > >
> > > I realized that as soon as I put one of the
> routers
> > > interfaces up, one of the first things that is
> done
> > > is to send a IGMP membership report message to
> > > AllSPFRouters and another to AllDRouters. Does
> > > anybody knows why ?? Does the router interface
> keeps
> > > a table including all of the member addresses of
> > > these multicast addresses ? Is there a mapping
> from
> > > the Ip multicast address to all of the destined
> mac
> > > addresses ?
> > >
> > > Thank you very much,
> > >
> > > Beatriz
> > > --
> > >
> >
>
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