Subject: Designated routers & stub networks

From: Arthur Dimitrelis <[email-masked]>
Date: Tue, 2 Jul 2002 17:42:06 +1000

Hello all,

I'm trying to figure out the role of a designated router in a stub
network.

Consider a broadcast segment N with a single router X attached.

Firstly, can a stub network have a designated router? (a previous post
of John Moy's hints that a stub need not have a DR:
http://discuss.microsoft.com/SCRIPTS/WA-MSD.EXE?A2=ind0201&L=ospf&T=0&F=&S=&P=11786)

Here's why I think a stub network _can_ have a DR:

- The last paragraph of section 9.4 of rfc2328 implies that X will
become the DR for N:

        "Note also that if Router X is the only attached router that is
        eligible to become Designated Router, it will select itself as
        Designated Router and there will be no Backup Designated Router
        for the network."

So, as long as X's priority is not set to 0, it will elect itself the DR
of network N. I have not been able to find anything in 2328 that
mandates setting the priority of a router connected to a stub network to
zero. The only description of a stub I've been able to find is that it
has only one router (e.g. 2nd paragraph of page 15). Note that John's
January post shows that a broadcast segment with multiple routers can
still be considered a "stub" area so long as none of the routers are
elected DR).

So, if a stub network has a DR, what does that DR do? Again referring to
rfc2328, section 7.3 states:
---
    7.3.  The Designated Router

        Every broadcast and NBMA network has a Designated Router.  The
        Designated Router performs two main functions for the routing
        protocol:

        o   The Designated Router originates a network-LSA on behalf of
            the network.
---

- So the designated router of a stub network must originate a
network-LSA for that stub network. But sect. 12.4.2 says:

        12.4.2.  Network-LSAs

            A network-LSA is generated for every transit broadcast or
            NBMA network.  (A transit network is a network having two or

            more attached routers).  The network-LSA describes all the
            routers that are attached to the network.
---

- The stub network N is not a transit network. Should sec. 7.3 specify
that "every transit broadcast and NBMA network has a Designated Router."
?

Any thoughts and opinions are appreciated.

cheers,
Arthur

---

From: Anton Smirnov <[email-masked]>
Date: Tue, 2 Jul 2002 10:06:48 +0200

Arthur,
   don't mix two completely different things - network which is configured
to be transit but has only 1 OSPF speaker at present and network which is
'configured stub'. Both cases will be announced in LSAs same way. But in
second case router(s) do not usually even produce any OSPF traffic on the
network segment (Hellos in a first place), they just include reachability
information about the network in their LSAs. Apparently, in second case
there can be no DR even if multiple routers connected to the segment and
advertise it as stub.

Anton

----- Original Message -----
From: "Arthur Dimitrelis" <[email-masked]>
To: <[email-masked]>
Sent: Tuesday, July 02, 2002 09:42
Subject: [OSPF] Designated routers & stub networks

> of John Moy's hints that a stub need not have a DR:
http://discuss.microsoft.com/SCRIPTS/WA-MSD.EXE?A2=ind0201&L=ospf&T=0&F=&S=&
P=11786)

---

From: Acee Lindem <[email-masked]>
Date: Tue, 2 Jul 2002 09:48:48 -0400

Anton Smirnov wrote:

> 'configured stub'.

Arthur,

Also note that the former is part of the OSPF standard (RFC 2328) while the
latter is not. In the former case, the router with non-zero priority will elect
itself DR but it will not originate a network LSA nor will it have any
additional flooding duties (since there are no other OSPF routers on
the network).

Anton,

By configured as a stub do you mean the interface is configured
as 'passive'?

> Both cases will be announced in LSAs same way. But in

--
Acee

---

From: Anton Smirnov <[email-masked]>
Date: Tue, 2 Jul 2002 17:22:32 +0200

Acee,

> By configured as a stub do you mean the interface is configured
   yes, in Cisco wording that would be 'passive interface'. I used term
'interface, configured as stub' considering that it is clear enough and
implementation-independent enough to suit generic answer on generic
question. Term is not something widely accepted, so I put it in quotation.

> latter is not.
   This is discussible point (that is, I don't agree :-).
   The first, standard clearly discusses ways to handle during SPF multiple
paths to stub networks and their appearance in multiple Router LSAs. That is
the only thing required from standard to make above mentioned behavior
standardized and correct. The only other case when multiple paths to stub
network can happen is L2 partitioning of formerly transit network.
Apparently, nothing can help in latter case and I hope you agree that this
wasn't primary goal of that part of standard.
   The second, much more generic point. Industry standard cares (and states
this on more than one occasion) only about things which are transmitted on
the wire (being put in a very broad sense). If two behaviors of the box -
first described in standard and another is what actually happening inside
the box - are indistinguishable from other network devices, then both
behaviors are standard compliant.

Anton

---

From: Acee Lindem <[email-masked]>
Date: Tue, 2 Jul 2002 11:37:53 -0400

> question. Term is not something widely accepted, so I put it in quotation.

We use the same terminology. I think most vendors do.

>
>
>
>>Also note that the former is part of the OSPF standard (RFC 2328) while
>>
> the
>
>>latter is not.
>>

I did *not* say implementing the 'passive' option was not standards compliant.
What I said was that it is not part of RFC 2328 - you won't find a description
on the appropriate behavior for a passive interface and the 'passive' attribute
is not explicitly listed in the configurable constants included in Appendix C.
I included this since Arthur's original question related DR operation as defined
in RFC 2328.

>>>   don't mix two completely different things - network which is
>>>
> configured
>
>>>to be transit but has only 1 OSPF speaker at present and network which
>>>
> is
>
>>>'configured stub'.
>>>
>>
>>Arthur,
>>
>>Also note that the former is part of the OSPF standard (RFC 2328) while
>>
> the
>
>>latter is not. In the former case, the router with non-zero priority will
>>
> elect
>
>>>second case router(s) do not usually even produce any OSPF traffic on
>>>
> the
>
>>>network segment (Hellos in a first place), they just include
>>>
> reachability
>

--
Acee