Re: type 3 LSA from Area 0 vs. type 3 LSA from non Area 0

From: Petr Lapukhov (petrsoft@gmail.com)
Date: Thu Jun 15 2006 - 15:06:39 ART

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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