shen jing wrote:

> Sina:
>
> Thank you very much. But, I still can not understand why choose local
> IP address.

so you didn't read the previous e-mail ;-)

>
> As you mentioned before, each P2P connection can be advertised by three
> means:
>

I never said that :-) in p2p numbered or unnumbered we always announce a
link to the neighbor, this is needed in order to built the SPT so you will
always have a type 1 link ( p2p) with Link ID = neighbor Router ID and Link
Data : Router's interface IP address

now for p2p unnumbered we are done since we don't have any IP address to
announce
for p2p numbered in addition to the above type 1 link we will add a type 3
link ( Stub) and there are 2 option for that

a) both routers advertise a link to the common IP address
b) every router announce a link to the neighbor IP address

I hope this is clear so far

>
> (Link ID = IP address of p2p link , Link Data = Network Mask ) for
> common network between two ends
> (Link ID = neighbor IP address , Link Data = / 32 mask ) for other
> numbered network connection
> (link ID = neighbor Router ID , Link Data = Router's interface IP
> address) for unnumbered connection
>
> These shows P2MP can implemented the last two method, what I don't
> understand is :
>

again there are not 3 methods, in case of P2mp you will announce a link type
1 to every remote adjacent neighbor ( as in case of P2P ) this is just to
say who are your neighbors now in addition to that you would need to
announce the IP address and I explained in my previous e-mail why we choose
our own IP address

>
> 1.) why P2MP can not be unnumbered?

a router, in order to send a packet to the next hop router it will need the
outgoing interface and the next hop IP address, the outgoing interface will
indicate the path toward your next hop router and the next hop IP address is
used to send the packet to the corresponding router should more than 1
router be reachable out of the outgoing interface.
in P2P you don't really need the next hop IP address ( the remote IP address
of the link ) since your outgoing interface uniquely identify a given router
so you could have unnumbered ( no IP address on the outgoing interface
toward the next hop router ) however in P2mp you will reach all your remote
neighbors through the same interface and you would need the next hop IP
address ( so the link should be numbered ) in order to send the packet to
the correct router

> if neighor RouterID and local
> interface IP is available, we can use
> the third method.

as mentioned before there is no 3d method, what you refer as 3d method is
the connectivity to the remote neighbor and should be included in P2P
numbered, P2P unnumbered and P2mP

>
> 2.) If R1 in the following paragraph only advertise a link state which
> only covers its connection to local 'stub'
> , which is to avoid advertising 3 different connections, how
> other router find out this LSA represents a P2MP
> connection?
>

R1 advertise three link type 1 ( since there are 3 neighbors ) + one link
type 3 to its own interface IP address
what do you mean by "how the others router find out his LSA represents a
P2MP connection "
the other router will just see that R1 is advertising a link ( type 1 ) to
them as they do announce in their LSA as well and the IP address advertised
by R1 is processed as a Stub information.

Sina

> Thank you .
>
> Jing Shen
>
> > 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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