Shen,

shen jing wrote:

> Thank you for your kindness and great help.
> But, as you mentioned, under p2mp mode , connection
> between routers is considered as a collection of p2p connections,
> there is no DR/BDR elected. But, why p2mp use local interface
> IP address in additoion to directed edge to remote router?
>

Good question :-) in case of P2mP link since we consider this as a collection of
P2P link for every fully adjacent neighbor we will announce a link type 1 with
Link Id = neighbor Router ID and Link Data = Router's interface IP address

now as in case of P2P numbered ( note that p2mp cannot be unnumbered ) we should
announce the IP address information. let's take un example

           R3
        /
     /
R1-------------R2
    \
       \
          \
            R4

consider that this is WAN PVC and only R1 has a link to all the routers and we
have a common subnet say / 29
now if you remember we said for numbered P2P we have 2 options
a) announce a Stub link to the common subnet IP address
b) announce a Stub link to the neighbor IP address

we cannot do a) since that would mean that R1, R2, R3, R4 will announce a Stub to
the common IP address ( /29 ) which imply they can reach this subnet however R2,
R3, R4 has only link to R1 and they cannot reach directly some part of the
subnet so this won't work

we could do b) but in this case we would have to add a type 3 link ( Stub ) for
every neighbor and imagine you have 50 neighbor so you would need to add 50 Stub
link, this is not the most optimal way to do it and R1 in any case will use the
same interface to reach them so it will just advertise a Stub link to its own IP
address.

Sina

>
> James Shen
>
> >
> > >
> > > 2. Why OSPF use peer router interface address as stub network for point
> > > to point
> > > connection? In this mode, can the interfaces of the two router
> > > belong to two
> > > subnet ?
> > >
> >
> > P2P link could be either numbered or unnumbered, in both case a router will
> > announce in its Router LSA a link to the remote neighbor
> > link ID = neighbor Router ID and Link Data = Router's interface IP address
> > now if a link is unnumbered we don't have any thing else to add however if
> > a link is numbered we should announce the IP address of the link so in
> > addition to the above we will create a Stub link in order to announce this
> > additional information, there are 2 possible way
> >
> > - either both routers announce a link to the common IP address so
> > Link ID = IP address of p2p link and Link Data = Network Mask
> > - or every router announce a link to the remote neighbor IP address ( for
> > example imagine the 2 end of the link are not on the same subnet since it's
> > a p2p link ) so
> > Link ID = neighbor IP address and Link Data = / 32 mask
> >
> > Sina
> >
> > >
> > > Thank you very much.
> > >
> > > James Shen

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