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

From: Pierre-Alex (paguanel@hotmail.com)
Date: Thu Jun 15 2006 - 17:57:30 ART

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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