Subject: Questions on Interfaces

From: Murali Krishna <[email-masked]>
Date: Wed, 17 Jul 2002 08:00:08 -0400

I have some very basic questions.

1. What exactly are interfaces on routers?

In section 2.1 in RFC2328, for p-to-p networks, the link-state-database
graph has an edge between
a. two routers and
b. between a router and the interface on the other router.
--What does this mean?
--What is the difference between an edge from RT1 to RT2 and one from RT1
to Ib?
--If no IP address is assigned to the p-to-p interfaces on the two routers,
will there be no entry in the link-state-database graph for these
interfaces?
--If no IP address is assigned to the interfaces, what sort of
communication takes place between the two routers?

(diagram pasted below)
                                            **FROM**

                                           *      |RT1|RT2|
                +---+Ia    +---+           *   ------------
                |RT1|------|RT2|           T   RT1|   | X |
                +---+    Ib+---+           O   RT2| X |   |
                                           *    Ia|   | X |
                                           *    Ib| X |   |

                     Physical point-to-point networks

In case of pt-to-multipoint networks, each router has a stub connection to
its own IP interface address.
--Why is this so?
(if this is because the interface address cannot be broadcase since p-to-mp
is a non broadcase network, how is the establishment of such a stub
connection achieved?)

2. Page 20. Figure 3.
Why does the directed graph for the autonomous system (Fig 2 page 19) not
have entries for the two interfaces Ia and Ib on RT6 and RT 10, but are
accounted for in the SPF Tree for RT 6 in page 22 Figure 5.?

3. Why do arcs from Networks to Routers have cost 0?

If there are other RFCs that i need to go thru before referring to the OSPF
RFC, which may answer these basic questions please mention them.

Thanks
Murali.

---

From: "Manral, Vishwas" <[email-masked]>
Date: Thu, 18 Jul 2002 10:00:59 -0400

Hi Murali,

I will try to answer your queries. Replies inline prefixed by a VM>.

Thanks,
Vishwas

-----Original Message-----
From: Murali Krishna [mailto:[email-masked]]
Sent: Wednesday, July 17, 2002 5:30 PM
To: [email-masked]
Subject: Questions on Interfaces

1. What exactly are interfaces on routers?
VM> Interfaces are the points of connection on the router to the network.

In section 2.1 in RFC2328, for p-to-p networks, the link-state-database
graph has an edge between
a. two routers and
b. between a router and the interface on the other router.
--What does this mean?
VM> a. means you can reach from router 1 to router 2.
b. means you can reach the address Ib.
The idea of a. is that we can reach other networks from RT1 thru RT2. b.
however only means that you can reach the address Ib.

--What is the difference between an edge from RT1 to RT2 and one from RT1
to Ib?
VM> Explained above. Also see section 12.4.1 in the RFC. It will clarify a
lot of things.

--If no IP address is assigned to the p-to-p interfaces on the two routers,
will there be no entry in the link-state-database graph for these
interfaces?
VM> Yes there will be no stub entries in case of unnumbered links. Section
12.4.1.1.

--If no IP address is assigned to the interfaces, what sort of
communication takes place between the two routers?
VM> We still send Hellos to ALLOSPFRouters multicast address. Is this what
you are looking for?

(diagram pasted below)
                                            **FROM**

                                           *      |RT1|RT2|

2. Page 20. Figure 3.

VM> The cost between of a network is to be accounted for only once. So from
going from a router to a router the cost is accounted when going from router
to network and the network to router cost is set to 0.

If there are other RFCs that i need to go thru before referring to the OSPF
RFC, which may answer these basic questions please mention them.
VM> You can look at the archives at
http://discuss.microsoft.com/archives/ospf.html a lot of topics have been
discussed before and that will help you.