Jing,

Jing Shen wrote:

> I'm sorry if my question is stupid or off the topic.
>
> I'm a newcomer of OSPF, I'm reading rfc2328 to learn how OSPF works.
> I met some problem to understand the content , would you please do me
> a favor to do some help ?
>
> 1. What's the difference between NBMA mode and P2MP mode?
> ATM is mentioned as NBMA network, but as I know, if an switch
> in ATM network want to communicate with multiple switches/Host
> it must setup multiple SVCs . While P2MP also need such action.
>

SPF should see the network as a collection of node and P2P link now in case
we have multi-access network ( media with more than 2 attached router ) we
have to consider it either as a collection of p2p link ( connection from a
given routers to all the attached router ) in this case the netwrok type is
p2mp or emulate the network as a node itself represented by DR and all the
routers will announce a link to the transit network represented by
Pseudonode ( DR ) and the Pseudonode will announce a link back to all the
attached routers in this case the netwrok type is broadcast or NBMA
therefore we have the following option for multi-access network

I- the multi-access network has broadcast capability ( for ex Ethernet )
in this case the network is considered as Pseudonode, DR election takes
place and the DR is representing the transit network, so the adjacency over
this media is 2 unidirectional connection one from every attached router (
DR included ) to the Pseudonode
( DR ) and one connection from Pseudonode to all the attached routers. in
this type of network because of broadcast capability neighbor discovery is
dynamic
note that although a broadcast network is by default a " broadcast network
type" but it could be changed to P2mP and you won't have any DR / BDR
election as mentioned above

II- multi-access without broadcast capability in this case we have 2
options

a ) either we will consider the media as a Pseudonode and DR / BDR election
will take place but in this case it requires all the routers be fully
meshed or at least DR / BDR have link to all the other routers also because
of the lack of broadcast capability a configuration might be required in
order to tell a routers who are its potential neighbors

b) or we will consider the adjacency over the media as just a collection of
P2P link in this case there is no DR / BDR election again because of lack
of broadcast capability a configuration might be needed in order to tell a
router who are its neighbor

 NBMA mode is actually a) and in this mode a DR/BDR election will take
place but for this to work you would need at least a connection from DR /
BDR to all the other routers

P2mP mode is actually b) and in this mode we consider the different
connection between routers as a collection of P2P link and there would be
no DR/ BDR election

also OSPF is layer 3 protocol so we don't care how the layer 2 set up a
link, all we care is an end to end connection to the given neighbor(s)

>
> 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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