Subject: Re: when to run in NBMA or P2MP mode?

From: Manav Bhatia <[email-masked]>
Date: Tue, 1 Oct 2002 12:41:13 +0530

Dhaval,
Let me give it a shot!

As i understand, you want to clarify the following
- Why the restriction of being able to reach all neighbors directly is
placed in order to run in NBMA mode when we always send unicasts to our
neighbors?
- When would we like to run as p2mp
- How can one determine which mode to run in?

The reason why we want all the neighbors to be reached directly is because
we want to use mechanisms similar to those used in the broadcast subnets.
This helps making the NBMA segments more efficient in terms of :
- neighbor maintenance
- database synchronization
- database representation
- reducing the O(n^2) relationships to O(n)

Emulating NBMA as a normal broadcast network helps the representation of an
NBMA subnet within OSPF LSDB as indistinguishable from the representation
of a broadcast subnet.
However, NBMA segments can fail when the two attached neighbors are not
able to communicate directly with each other.If, for example, a PVC fails
or is misconfigured or if an SVC cannot be established, due to capacity or
policy reasons, routing over NBMA will fail!

When to use point-to-multipoint mode?

Used if the nonbroadcast subnets cannot support a large number of routers
in complete mesh.

To connect 200 routers in a full mesh over a Frame Relay/ATM subnet, one
would require (200)(200-1)/2 = 19,900 PVCs! Even when SVCs are used,
capacity limitations may prohibit pairwise connectivity. One cant thus use
the NBMA model as each router cant now communicate directly with the every
other router! The only alternative thus left is p2mp.
The problem comes when operating a full mesh FR/ATM network in p2mp mode
because now the work involved in neighbor maintenance, flooding and
database representation increases as O(n^2), where n is the nos. of routers
attached to the subnet, instead of O(n) that can be achieved with the NBMA
model.

For the final question i guess the answer is that if you cant provide full
mesh support then its better to model the network as p2mp.

Regards,
Manav

----- Original Message -----
From: "Dhaval Shah" <[email-masked]>
To: <[email-masked]>
Sent: Tuesday, October 01, 2002 5:58 AM
Subject: when to run in NBMA or P2MP mode?

| Hi
|         According to RFC 2328, Ospf can run in one of the two modes on a
| non-broadcast networks, NBMA or P2MP. RFC says that you can run in NBMA
mode
| if all routers attached to the network can communicate directly and you
should
| run in P2MP mode if thats not possible.
|         However, while sending hellos out on non-broadcast networks,
irrespective of
| which mode you run on, you always send them unicast to your neighbors.
|         The essential difference between NBMA and P2MP is only that there
is DR/BDR
| election and Network LSA generation in NBMA mode and not in P2MP mode.
|         So why is the need for the restriction that one MUST be able to
reach all
| neighbors directly in order to run in NBMA mode? What are some of the
example
| physical networks where one would like to run in P2MP mode and not in
NBMA
| mode? How can one determine which mode to run when?
|
| thanks
| Dhaval.
|
|

---

From: Dave Katz <[email-masked]>
Date: Tue, 1 Oct 2002 09:17:10 -0700

I'd go further than that.  NBMA mode should be avoided like the
plague, because the network will break if you lose connectivity
between the DR and any other router (a single failed VC is all it
takes.)

It is a much more natural fit to model an NBMA network as a collection
of point-to-point links, since then link failures are treated like they
would be in any point-to-point network, namely, as long as the network
is not partitioned, routing will be correct.

P2MP mode is, for the most part, a hack to get around router o/s's that
didn't allow the configuration of point-to-point subinterfaces (since
P2MP is effectively a simulation of P2P subinterfaces within OSPF.)  There
may be reasons that P2MP is more desirable than P2P (if subinterfaces are
very expensive in your o/s, or if you didn't feel link renumbering your
single-subnet NBMA network.)  On the other hand, if you use P2P mode you
could run your NBMA network unnumbered if you so desired.

The theoretical advantages of the pseudo-LAN model (complexity reduction)
are unlikely to be meaningful in practice.  The largest NBMA networks
I've seen (hundreds of routers) have typically been fully meshed but used
pure point-to-point mode in the IGP.  These days the links and CPUs are
fast enough so that networks of practical size are not limited by the
performance of the protocol.

---

From: Misha Dovek <[email-masked]>
Date: Thu, 3 Oct 2002 14:32:24 +0530

> could run your NBMA network unnumbered if you so desired.

what exactly is the difference between p2p interfaces and p2p subinterfaces.
I tried searching for that in google but couldnt find any information. I
come across this term a lot of times and would be glad if somebody could
explain me about it.

Misha

---

From: Dave Katz <[email-masked]>
Date: Thu, 3 Oct 2002 09:52:18 -0700

A subinterface (or logical interface) is a way of subdividing a
physical interface into multiple logical interfaces.  NBMA interface
are often treated this way--one or more PVCs is treated as a logical
interface.  If only one PVC is bound to a logical interface, that
logical interface can appear to the rest of the system as a standard
point-to-point interface.  You can also bind multiple PVCs and treat
it as a multipoint interface, but this is rarely done in practice.
Subinterfaces are often used with other media, such as VLANs--each
VLAN is treated as a separate multipoint subinterface.

From the point of view of router applications (such as OSPF), there is
no difference between a true P2P interface and a P2P subinterface,
which of course is the whole point fo the exercise.

--Dave

---

From: Misha Dovek <[email-masked]>
Date: Fri, 4 Oct 2002 11:41:36 +0530

Hi,
Can anyone please give me real life usages of p2p subinterfaces?

Misha

---

From: Manav Bhatia <[email-masked]>
Date: Fri, 4 Oct 2002 13:41:25 +0530

Misha,
Certain protocols cannot be supported on partially meshed networks because
they require "split horizon," in which a packet received on an interface
cannot be transmitted out the same interface even if the packet is received
and transmitted on different VCs.

Configuring subinterfaces ensures that a single physical interface is
treated as multiple virtual interfaces (as pointed out by Dave). This
capability allows us to overcome split horizon rules. Packets received on
one virtual interface can now be forwarded out another virtual interface,
even if they are configured on the same physical interface.

Subinterfaces address the limitations of Frame Relay networks by providing
a way to subdivide a partially meshed Frame Relay network into a number of
smaller, fully meshed (or p2p) subnetworks. Each subnetwork is assigned its
own network number and appears to the protocols as if it is reachable
through a separate interface.

Hope this helps.

Regards,
Manav

----- Original Message -----
From: "Misha Dovek" <[email-masked]>
To: <[email-masked]>
Sent: Friday, October 04, 2002 11:41 AM
Subject: p2p subinterfaces - why ?

| Hi,
| Can anyone please give me real life usages of p2p subinterfaces?
|
| Misha

---

From: Dave Katz <[email-masked]>
Date: Fri, 4 Oct 2002 09:28:05 -0700

Every large ISP that I know of uses them.

   Hi,

   Misha

---

From: Erblichs <[email-masked]>
Date: Tue, 8 Oct 2002 08:41:00 -0700

Group,

        First, I would like to say that this is a very good
        explanation.

        I would like to add just one item to this.

        With a DR / BDR in a BMA or a NBMA environment,
        the number of neighbors that can simultaneously
        perform database exchange (exchange start to full)
        should also be considered when chosing between
        your two network configuration modes. A high value
        should decrease the amount of time to achieve
        initial database synchronization vs P2MP mode.

        Thus, if the capabilities of your routing nodes
        differ and you implement one of the MA modes,
        then the nodes with the higher capacities should
        probably be elected as the DR and the BDR.

        Mitchell Erblich
        ====================