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

From: "Manral, Vishwas" <[email-masked]>
Date: Tue, 1 Oct 2002 08:35:04 -0400

Hi Dhaval,

Section 2.1.1 answers almost all ur questions of ur questions.

I guess in NBMA case we elect a DR, and as the DR is expected to be able to
talk to all routers on the subnet(if this is not possible it could lead to
problems where multiple DR's are elected for the same sub-interface etc
leading to problems) we have the "MUST" restriction.

Thanks,
Vishwas

-----Original Message-----
From: Dhaval Shah [mailto:[email-masked]]
Sent: Tuesday, October 01, 2002 5:58 AM
To: [email-masked]
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: "Pat Murphy - (650)329-4044" <[email-masked]>
Date: Tue, 1 Oct 2002 09:37:19 -0700

I have had some bad experiences using NBMA. A full mesh may
exist when the topology is stable but can easily be lost due to
outages. If an outage does not effect the links between the
DR/BDR and the remaining routers in the mesh, then the SPF
calculation of all the routers in an NBMA mesh will build
routing table as if the full mesh still existed.  This creates
blackholes in domain. If P2MP is used, a legitimate fault
tolerant routing table would still be built during these
outages, although what was once only a single hop, may now be
multiple hops. I only use NBMA when my full mesh is 99.99%
stable or, when it breaks, simply partitions. Choosing between
NBMA and P2MP is always a trade off between fault tolerance and
flooding optimization. I believe it is possible to engineer a
fault tolerant MPLS environment where NBMA could effectively be
used.

Pat

---

From: "Naidu, Venkata" <[email-masked]>
Date: Thu, 3 Oct 2002 13:39:09 -0400

-> -> A subinterface (or logical interface) is a way of subdividing a
-> -> physical interface into multiple logical interfaces.
->
->   Is this same as *multi-netting* ? I think there is a small
->   difference between *multi-netting* and *subinterfaces*.
->   Multi-netting can be applicable to BCs also (not only NBMAs)?!?

  For the benefit of the community - Operating Systems developers use
  the term "interface aliasing" for the same purpose. Router vendors
  use the term *subinterfaces*.

--
Venkata.

---

From: "Naidu, Venkata" <[email-masked]>
Date: Thu, 3 Oct 2002 16:09:41 -0400

Dave,

-> because you can't tell by looking at an incoming packet
-> the interface with which it is associated.  (But you can if you are
-> using VLANs by looking at the VLAN ID, or on ATM or FR by looking at
-> the VC information.)

  Got your point. Then, *interface aliasing* is same as *multi-netting*.
  *subinterfaces* is a special case. Thanks!

--
Venkata.