From: Vikram Dadlaney (vdadlaney@gmail.com)
Date: Fri Jun 16 2006 - 12:28:33 ART
Ok please ignore the LSage comment in my last email. I don't know what I was
thinking. I apologize but I don't quite see the reason for the virtual-link
over here. Is it being specifically to prefer the intra area LSA as opposed
to the one coming from the backbone? Could you please advise if I might be
missing something over here? Thx
I think this discussion way back in 2001 might be relevant over here.
http://www.groupstudy.com/archives/cisco/200211/msg01301.html
HTH.
On 6/15/06, Pierre-Alex <paguanel@hotmail.com> wrote:
>
> Petr,
>
> The LSA comparison I enclosed in my last e-mail was that very scenario you
> are
> talking about.
>
> But no matter how high I put the cost on R4 interface towards R2 ,the path
> via
> Area 0 is always prefered.
>
>
> Area 12 (network 10.1.2.0)
> |
> R2---virtual-link area 23---------R3---
> | |
> | |
> ----Area 0-------------R4--Area34--|
>
>
>
> This is leaving me believe that the rule cannot be broken unless R4
> interface
> in area 0 goes down, and
>
> then everything is routed via the virtual-link.
>
> Have you worked on scenarios where you were able to take a shortcut via
> the
> virtual-link?
>
>
> Kind Regards,
>
> Pierre-Alex
>
> ----- Original Message -----
> From: Petr Lapukhov
> To: Pierre-Alex
> Cc: ccielab@groupstudy.com
> Sent: Thursday, June 15, 2006 8:06 PM
> Subject: Re: type 3 LSA from Area 0 vs. type 3 LSA from non Area 0
>
>
> This is a loop-prevention technique - a "star"-topology enforcement :)
> (OSPF treats summary LSA in DV-manner, and thus a mechanics
> to avoid loops is required)
>
> To "break" that rule, a virtual link is usually being put over
> non-backbone
> area.
>
> Thereby a "better" LSA is piped though area 0 (virtual-link), over a
> "shorter" path, and is preferred over "not so good" backbone LSA.
>
> HTH
> Petr
>
>
> 2006/6/15, Pierre-Alex <paguanel@hotmail.com>:
> When having to choose between a summary LSA from Area0
> and a summary LSA from non-area 0 , OSPF will always choose
> the link-state from area zero (regardless of the cost of the route).
>
> Is there any way to change this behavior and prefer the non-zero area ?
>
> Thanks
>
> Pierre-Alex
>
>
> r4#sh ip route 10.1.2.0
> Routing entry for 10.1.2.0/24
> Known via "ospf 100", distance 110, metric 65001, type inter area
> Last update from 10.2.4.2 on Serial0/0.402, 00:22:18 ago
> Routing Descriptor Blocks:
> * 10.2.4.2, from 150.1.2.2, 00:22:18 ago, via Serial0/0.402
> Route metric is 65001, traffic share count is 1
>
>
>
> Summary Net Link States (Area 0)
>
> Routing Bit Set on this LSA ----> LSA CHOSEN
> LS age: 858
> Options: (No TOS-capability, DC, Upward)
> LS Type: Summary Links(Network)
> Link State ID: 10.1.2.0 (summary Network Number)
> Advertising Router: 150.1.2.2
> LS Seq Number: 8000000C
> Checksum: 0x6B1D
> Length: 28
> Network Mask: /24
> TOS: 0 Metric: 1
>
>
>
> Summary Net Link States (Area 34)
>
> LS age: 1533
> Options: (No TOS-capability, DC, Upward)
> LS Type: Summary Links(Network)
> Link State ID: 10.1.2.0 (summary Network Number)
> Advertising Router: 150.1.3.3
> LS Seq Number: 80000001
> Checksum: 0x126F
> Length: 28
> Network Mask: /24
> TOS: 0 Metric: 782
>
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