Hi ,
  as per my understanding, on virtual links packets are sent as unicasts.

  I am also attaching part of RFC 2328 section 8.1 Sending protocol
packets.

"The IP destination address for the packet is selected as
        follows. On physical point-to-point networks, the IP
        destination is always set to the address AllSPFRouters. On all
        other network types (including virtual links), the majority of
        OSPF packets are sent as unicasts, i.e., sent directly to the
        other end of the adjacency. "

  Thanks.
                          -Samvid

----- Original Message -----
From: "Amit Srivastava" <ospfisfunYAHOO.COM>
To: <OSPFdiscuss.microsoft.com>
Sent: Friday, June 28, 2002 1:24 AM
Subject: Re: Multicast addresses AllSPFRouters, AllDRouters

> Hi samvid!!
> When you say that all the ospf packet do not
> travel more than one hop away then i think you are not
> considering the virtual links!!!
> Plz think on that aspect and the picture
> would be more clear!!
> Regards
> Amit
>
> --- Samvid Shah <samvidFUTSOFT.COM> wrote:
> > Hi Group,
> > 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
> > multicast address."
> >
> > now coming to membership reports generated by
> > OSPF router,
> > as per RFC 2328, section 4.4 (page 44)
> >
> > "IP multicast
> > 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.)
> >
> > Thanks.
> > -Samvid
> >
> > ----- Original Message -----
> > From: "Amit Srivastava" <ospfisfunYAHOO.COM>
> > To: <OSPFdiscuss.microsoft.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 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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> === message truncated ===
>
>
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