From: CCIEin2006 (ciscocciein2006@gmail.com)
Date: Mon Oct 29 2007 - 20:18:55 ART
You're right,
That was a bad example because R2 and R3 were in same area. Here is more
what I intended:
R4-A1-R5-A2-R6
| |
A0 A0
| |
R1-A0-R2-A0-R3
For R4 to get to the segment between R5 and R6, will it take the shorter
path through area 1 and area 2 or will it take the longer path through area
0?
P.S. - I hate virtual links too - they're like the duct tape of OSPF. You
never have these problems with EIGRP :-)
In this setup will R2 use
On 10/29/07, Scott Vermillion <scott_ccie_list@it-ag.com> wrote:
>
> 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
>
> |
>
> A0
>
>
> 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 ]
> *Sent:* Monday, October 29, 2007 1:38 PM
> *To:* Scott Vermillion
> *Cc:* CCIEin2006; Cisco certification
>
>
> *Subject:* Re: Why must all areas connect to Area 0?
>
>
>
> Look at the "capability transit" command
>
>
>
>
http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cr/hirp_r/
rte_osph.htm#wp999437
>
>
>
>
> On 10/29/07, *Scott Vermillion* <scott_ccie_list@it-ag.com> wrote:
>
> It must flow to Area 0. You cannot build a virtual link directly between
> Areas 1 and 2; all virtual links either connect two pieces of Area0 or
> they
> connect a non-0 area to Area 0. In other words, all virtual links involve
>
> Area 0!
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto: nobody@groupstudy.com] On Behalf Of
> CCIEin2006
> Sent: Monday, October 29, 2007 12:42 PM
> To: Cisco certification
> Subject: Re: Why must all areas connect to Area 0?
>
> So in this scenario
>
> Area1-Area2
> \ /
> Area0
>
> Area 1 and 2 are directly connected. Would the data need to flow to area0
> or
> can the traffic flow directly?
>
> What if we configured a virtual link between them?
>
> On 10/29/07, CCIEin2006 < ciscocciein2006@gmail.com> wrote:
> >
> > Hi folks,
> >
> > I was reading over Jeff Doyle's blog and came across his favorite
> > interview question:
> > Why does OSPF require all traffic between non-backbone areas to pass
> > through a backbone area (area 0)?
> >
> > Answer:
> > Because inter-area OSPF is distance vector, it is vulnerable to routing
> > loops. It avoids loops by mandating a loop-free inter-area topology, in
> > which traffic from one area can only reach another area through area 0.
> >
> > Can someone elaborate on that answer a little bit? Exactly how does
> having
> > a connection to Area0 prevent routing loops? Is it similar to spanning
> tree
> > in the area 0 is the root of the spanning tree?
> >
> > Also this answer does not take into consideration redistribution from
> > another routing protocol right?
> >
> > Thank You
> >
> > Here is the article:
> > http://www.networkworld.com/community/node/19293
>
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>
>
>
> --
> Narbik Kocharians
> CCIE# 12410 (R&S, SP, Security)
> CCSI# 30832
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