2) why should routers connected by a point to point link having
interface addresses assigned,
should advertise a stub link a stub connection to the other
router's interface address.
This dates back to the fact that P2P links were represented as a host
route to the destination network on BSD based systems. Those of you
who are familiar with GateD will remember this. Note that there is
also an option to represent/advertise P2P links as a subnets. Most if
not all routers vendors advertise the subnet.
Do you have a choice here? I read the RFC that if the P2P link has
a host route (mask = 255.255.255.255) you have to advertise a stub
link to the other router's interface address.
From RFC 2328:
12.4.1.1. Describing point-to-point interfaces
For point-to-point interfaces, one or more link
descriptions are added to the router-LSA as follows:
o If the neighboring router is fully adjacent,
add a
Type 1 link (point-to-point). The Link ID
should be
set to the Router ID of the neighboring
router. For
numbered point-to-point networks, the Link Data
should specify the IP interface address. For
unnumbered point-to-point networks, the Link Data
field should specify the interface's MIB-II
[Ref8]
ifIndex value. The cost should be set to the
output
cost of the point-to-point interface.
o In addition, as long as the state of the
interface
is "Point-to-Point" (and regardless of the
neighboring router state), a Type 3 link (stub
network) should be added. There are two forms
that
this stub link can take:
Option 1
Assuming that the neighboring router's IP
address is known, set the Link ID of the
Type 3
link to the neighbor's IP address, the
Link Data
to the mask 0xffffffff (indicating a host
route), and the cost to the interface's
configured output cost.[15]
Option 2
If a subnet has been assigned to the point-
to-
point link, set the Link ID of the Type 3
link
to the subnet's IP address, the Link Data
to the
subnet's mask, and the cost to the
interface's
configured output cost.[16]