RE: Why must all areas connect to Area 0?

From: Scott Vermillion (scott_ccie_list@it-ag.com)
Date: Mon Oct 29 2007 - 18:11:44 ART

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Hi CCIEin2006,

The ASCII art worked out great.

Narbik has me questioning myself at this point (which is a good thing),
since I don't really know what that transit thing is all about. But
generically speaking, the "rules" of OSPF state that your R2 and R3 ABRs
would need at least one connection to the backbone, so from that
perspective, I think that yes, you are correct that you would need a virtual
links to maintain a "proper" OSPF design. Having said that, assuming that
the basic hello parameters matched up between R2 and R3 for Area 3, it seems
that they would attempt to form an adjacency. But in attempting to
synchronize their databases, I would expect some trouble (sans the
aforementioned virtual links). They are ABRs, yet they have no Area 0
connectivity. Now that you've got me thinking about this, it's something
I'm interested to see the debug of in the lab. However, I have several more
hours of work to do in the lab I'm presently building, so I can't try it
until later this evening.

It just dawned on me that I may still be missing your question. Are you
asking how traffic would flow if you *did* build the virtual links? Would a
packet entering into R2 bound for a network attached to R3 transit A1 and A2
or would it directly transit A3? If that's the question, I think it simply
transits A3, as your OSPF topology at that point does not mirror the
topology as drawn below. At that point you basically have A0 in the center,
to which R1 is attached and is acting as ABR for A1 and A2, and you also
have R2 and R3 "attached" to the backbone, both serving as ABR for A3 only.
In that case the traffic in question would not be inter-area traffic at all;
it would be intra-area A3-only traffic. Seemingly?

"Have I ever mentioned that virtual links are my least favorite aspect of
OSPF?!"

Narbik's link was to the command reference; perhaps I can find more
information about this on the Config Guide side of the house once I've
wrapped up what I'm working on at the moment. Some context and picture
would likely be illuminating.

Regards,

Scott

From: CCIEin2006 [mailto:ciscocciein2006@gmail.com]
Sent: Monday, October 29, 2007 2:14 PM
To: Scott Vermillion
Cc: Narbik Kocharians; Cisco certification
Subject: Re: Why must all areas connect to Area 0?

Here's a scenario for you to try Scott (hope the ASCII art comes out
clearly):

R2--A3--R3
\ /
A1 A2

\ /

R1 is connected to Area 0 and connects to R2 and R3 via area 1 and 2
respectively.

R2 and R3 have a direct connection to each other via area 3.

1. I am assuming for this to work I would need a virtual link between R1 and
R2 and another virtual link between R1 and R3 - is that correct?

2. Considering transit capability is enabled by default, would R2 and R3
sent traffic directly to each other via area 3?

Thanks in advance.

On 10/29/07, Scott Vermillion <scott_ccie_list@it-ag.com> wrote:

Hey Narbik,

Don't educate me too much before your upcoming bootcamp! But I couldn't
really decipher the context of this:

"OSPF area capability transit is enabled by default, allowing the OSPF Area
Border Router to install better-cost routes to the backbone area through the
transit area instead of the virtual links. If you want to retain a traffic
pattern through the virtual-link path, you can disable capability transit by
entering the no capability transit command. If paths through the transit
area are discovered, they are most likely to be more optimal paths, or at
least equal to, the virtual-link path. To reenable capability transit, enter
the capability transit command."

Have I ever mentioned that virtual links are my least favorite aspect of
OSPF?!

I just happen to have an active OSPF component in the lab I'm currently
working if anyone has suggestions as to how to see this in action (and come
to understand how it applies to the discussion at hand - I'm near 100% sure
Narbik is suggesting I'm incorrect in my below statement, but it's not
exactly jumping out at me).

Regards,

Scott

From: Narbik Kocharians [mailto:narbikk@gmail.com
<mailto:narbikk@gmail.com> ]
Sent: Monday, October 29, 2007 1:38 PM
To: Scott Vermillion
Cc: CCIEin2006; Cisco certification