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

From: Petr Lapukhov (petr@internetworkexpert.com)
Date: Fri Jun 16 2006 - 15:09:43 ART

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Pierre,

Here is a simple scenario to demostrate the idea:

Diagram

---
Lo5
|
R5
|   \___R3
|   /
R4
Configuration:
1) R4 and R5 are connected via link in Area 1 (with Serial 0/1 interfaces)
    This path has Cost = 1.
2) R3, R4 and R5 form Area 0 (with Serial 0/0 interfaces at R4, R5).
    Cost has default value.
3) R5 has Loopback5 (164.1.155.5/32) in Area 5.
The task is to evaluate how R4 routes to Lo5, with and w/o virtual-link
between R4 and R5. I want put more output, since it's easy to
simulate both cases.
--
First case: NO virtual link over Area 1.
Rack1R4#show ip route 164.1.155.5
Routing entry for 164.1.155.5/32
  Known via "ospf 1", distance 110, metric 846, type inter area
  Last update from 164.1.34.3 on Serial0/0, 00:07:24 ago
  Routing Descriptor Blocks:
  * 164.1.34.3, from 150.1.5.5, 00:07:24 ago, via Serial0/0
      Route metric is 846, traffic share count is 1
Path via Area 0 is preferred regardless or cost.
---
Put a virtual link from R4 to R5 via Area 1 and check routes
again:
Rack1R4#show ip ospf virtual-links
Virtual Link OSPF_VL2 to router 150.1.5.5 is up
  Run as demand circuit
  DoNotAge LSA allowed.
  Transit area 1, via interface Serial0/1, Cost of using 1
  Transmit Delay is 1 sec, State POINT_TO_POINT,
.....
Rack1R4#show ip route 164.1.155.5
Routing entry for 164.1.155.5/32
  Known via "ospf 1", distance 110, metric 2, type inter area
  Last update from 164.1.45.5 on Serial0/1, 00:00:29 ago
  Routing Descriptor Blocks:
  * 164.1.45.5, from 150.1.5.5, 00:00:29 ago, via Serial0/1
      Route metric is 2, traffic share count is 1
The "best" path is now chosen, because LSAs flow through "Area 0 put
over Area 1".
-- 
Petr Lapukhov, CCIE #16379
petr@internetworkexpert.com
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