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From: Samvid Shah (samvidFUTSOFT.COM)
Date: Thu Jun 27 2002 - 15:22:07 CDT
Sorry for the late reply.
my 2 cents to this discussion.
first regarding mapping IP address to MAC address,
as per RFC 1112
"An IP host group address is mapped to an Ethernet multicast address
by placing the low-order 23-bits of the IP address into the low-order
23 bits of the Ethernet multicast address 01-00-5E-00-00-00 (hex).
Because there are 28 significant bits in an IP host group address,
more than one host group address may map to the same Ethernet
now coming to membership reports generated by OSPF router,
as per RFC 2328, section 4.4 (page 44)
Certain OSPF packets take the form of IP multicast
datagrams. Support for receiving and sending IP multicast
datagrams, along with the appropriate lower-level protocol
support, is required. The IP multicast datagrams used by
OSPF never travel more than one hop. For this reason, the
ability to forward IP multicast datagrams is not required.
For information on IP multicast, see [Ref7]."
since, IGMP (along with multicast routing protocols) is typically
used for multicast data transfer across multiple hops, I think It is not
required to send group memberships for allSPFRouters and allDRouters .
Also, since IGMP is designed for interactions between host and router, I
think, sending membership report is not required.
but if your question is,
if by sending this membership report, are we breaking any
thing here ?
I have to say , I could not find any reference that says
IGMP membership report *must not* be sent by OSPF router. (May be more
knowledgeable soul can throw a light here.)
----- Original Message -----
From: "Amit Srivastava" <ospfisfunYAHOO.COM>
Sent: Monday, June 24, 2002 5:10 AM
Subject: Re: Multicast addresses AllSPFRouters, AllDRouters
> 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 184.108.40.206
> 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!!
> --- 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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