Introduction

There was a discussion on a forum about the point to multipoint network type in
OSPF. What is the purpose of it and why are /32 endpoints advertised? To understand
the solution we must first understand the problem. This topology has a NBMA network
where R1 is the hub. R1 has been elected the hub due to having the highest priority.

Topology

NBMA Networks

When using the network type non broadcast, which is the default for main interfaces
with frame relay enabled, a DR and BDR is elected. When using a hub and spoke
topology it is important that the hub is elected the DR. Why is this? If a
spoke is elected the DR the flooding process will fail. On broadcast and non broadcast
segments the DROTHERs flood the LSAs to the DR/BDR and then the DR floods them to
the other DROTHERs on the segment.

The routing has already been setup, R2 and R3 are advertising a loopback each, they
are 2.2.2.2/32 and 3.3.3.3/32 respectively. First we will have a look at the router
LSAs that are generated.

Interfaces that are connected and don’t have an OSPF adjacency are considered
stub networks. These are advertised with the network and the mask. There is
also a transit network that contains the IP of the DR and the local routers IP
for that network. Note that there is no network mask within this LSA.

If we have a look at the routing table of R2, the next-hop to reach 3.3.3.3/32
is 10.0.0.3.

On broadcast and non broadcast segments all routers are assumed to be fully
meshed but here we have a hub and spoke topology. When R2 needs to send traffic
to R3 it will try to encapsulate the frame but it does not know which DLCI to
use for 10.0.0.3 because there is no frame mapping for that.

If we debug the sending of frame relay frames we will see that the encapsulation
is failing.

The layer 3 topology is not consistent with the layer 2 topology. The layer 3 is
assumed to be fully meshed but our layer 2 is in fact hub and spoke.

The next-hop is not changed on broadcast and non broadcast segments because
all routers are assumed to be fully meshed.

Solving the Inconsistency

One way of solving the inconsistency is by adding static mappings for
the IP of R2 and R3 respectively to the correct DLCI.

This solved our problem but requires manual intervention and if new routers are
added then new mappings would have to be added as well. Before we leave the
network type non-broadcast there is one more thing I would like to point out.

Network LSA

What is the role of the network LSA? It is twofold, it conveys both topology
information and reachability information. If we look at the network LSA that R1
generates.

We see all of the attached routers and also the network mask for the segment.
This network LSA will be converted by R4 into a type3 summary LSA. If we look
from R5’s perspective we can see this prefix.

R5 can reach this network which is demonstrated by the ping.

What would happen if R2 or R3 became the DR instead of R1? Let’s try it out.

Then clear the process to see the effect.

The segment has now been split into two. R2 does not know about R3 so we have
a segment with R1 and R2 and then a segment with R1 and R3. This can be seen
from the neighbor table and from the network LSA that is generated.

This also means that SPF can’t run properly because R3 is now not connected to
the topology because it’s not part of the network LSA for 10.0.0.0/24. This
means that R5 can’t ping R3.

It can still reach R1 and R2 though.

So the network LSA is used both for building the SPF topology and for reachibility
information.

Point to Multipoint Network

There is a point to multipoint network type. This overcomes the limitations of
partially meshed networks by accomplishing two things. The first thing that happens
is that when a LSA is received on an interface, the next-hop is changed to IP of
the router LSA that the local router is connecting to. In our case this is 10.0.0.1.

This is described in RFC 2328.

From the router LSA above you can see that each router in the point to multipoint
network advertises a stub network with a /32 mask. On point to multipoint segments
the network is described as a collection of point to point links. With a regular
point to point network the stub network is described with the actual mask of
the interface that is running OSPF. What is the use of these /32 routes?
First let’s test the reachability.

Full reachability. What happens if we filter the /32 route to R3 on R2?

The host route to R3 is now gone. The route to R3 is known via the connected
network.

What about reachability?

The ping went through when we used the loopback as a source. That is because
R3 still has a host route to R2 in its routing table. We couldn’t ping 10.0.0.3
though? Why? Because without the host route pointing to a next-hop of 10.0.0.1,
R2 will try to see what DLCI to use when encapsulating the frame to 10.0.0.3
and there is no mapping for this. If we ping R2’s loopback from R3 sourcing
with the 10.0.0.3 IP, it will fail for the same reason.

Conclusion

OSPF has various network types that accomplish different things.
The point to multi point network type is used to overcome limitations at
layer 2. The network is described as a collection of point to point links
where each router advertises a stub network with a /32 mask. By doing this
the spokes can maintain connectivity between each other when sourcing traffic
from the interface connected to their common network.

The next-hop is also changed on incoming LSAs to the IP contained in the router
LSA from the router that originated the LSA. This solves the next hop issue
on non broadcast networks where the next hop is maintained because full mesh
connectivity is assumed.

OSPF – Non Broadcast and Point to Multipoint
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4 thoughts on “OSPF – Non Broadcast and Point to Multipoint

  • Pingback: OSPF host routes with network type point-to-multipoint

  • February 17, 2015 at 4:24 am
    Permalink

    Well written article.

    I have a tip about the /32 OSPF route overwriting the frame-relay interface connected route. When you do the “show ip route 10.0.0.3” twice in the R2 router, at first glance, it appears that the OSPF route (administrative distance 110) overwrote the connected route (administrative distance 0) when the /32 routes weren’t filtered. That would have violated the administrative ranking rule. But, look closer, there is no overwriting of routes because the /32 is more specific route than the connected route 10.0.0.0/24 . That is also why the extra OSPF route for each frame-relay interface has to be /32 . /24 does not work for this.

    Reply
  • July 4, 2016 at 1:48 am
    Permalink

    Great information .
    but i couldn’t understand how every ip can reach any other ip at point-to-multipoint .

    Reply
  • July 4, 2016 at 1:49 am
    Permalink

    will this work the same on subinterfaces ?

    Reply

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