From: Joe Smith (j333smith@hotmail.com)
Date: Wed Dec 22 2004 - 12:13:28 GMT-3
That's how OSPF works. R7 will use the intra area route, doesn't even
consider the Inter area routes. R6 being in a different area has a choice
between the two summary (Inter area) routes and of course it selects the
route with the lower metric. In this case R6 passes the traffic to R7 and
then R7 will use the tunnel.
J3
>From: gladston@br.ibm.com
>Reply-To: gladston@br.ibm.com
>To: ccielab@groupstudy.com
>Subject: OSPF Cost Date: Wed, 22 Dec 2004 08:52:41 -0500
>
>R7 is an ABR router connected to R3 via tunnel interface on area 0 and
>connected to the same R3 via connected interface on a nonbackbone area. R7
>cost to a R3's route 172.16.123.0 is much bigger than R6 cost to the same
>route. R6 uses R7 to reach R3's route.
>R7 uses the tunnel interface cost on the calculation while R6, even though
>uses R7 path, does not use this cost; R6 uses the physically cost between
>R7 and R3.
>
>Any doc recommendation to understand this behavior?
>
>r7#sir 172.16.123.0
>Routing entry for 172.16.123.0/24
> Known via "ospf 1", distance 110, metric 11175, type intra area
> Last update from 172.16.137.3 on Tunnel36, 00:09:17 ago
> Routing Descriptor Blocks:
> * 172.16.137.3, from 3.3.3.3, 00:09:17 ago, via Tunnel36
> Route metric is 11175, traffic share count is 1
>
>r6#sir 172.16.123.0
>Routing entry for 172.16.123.0/24
> Known via "ospf 1", distance 110, metric 139, type inter area
> Last update from 172.16.67.7 on Serial1, 00:07:33 ago
> Routing Descriptor Blocks:
> * 172.16.67.7, from 3.3.3.3, 00:07:33 ago, via Serial1
> Route metric is 139, traffic share count is 1
>
>
>r6-----r7-----(tunnel and direct connection)----r3
>
>The direct connection is nonbackbone area
>The tunnel is backbone area.
>
>The summary LSA for the route is almost the same on both r6 and r7; the
>difference is the message "Routing Bit Set on this LSA" on r6.
>
>r7#sh ip os database summary
>
> LS age: 991
> Options: (No TOS-capability, DC, Upward)
> LS Type: Summary Links(Network)
> Link State ID: 172.16.123.0 (summary Network Number)
> Advertising Router: 3.3.3.3
> LS Seq Number: 8000002E
> Checksum: 0x8308
> Length: 28
> Network Mask: /24
> TOS: 0 Metric: 64
>
> LS age: 1589
> Options: (No TOS-capability, DC, Upward)
> LS Type: Summary Links(Network)
> Link State ID: 172.16.123.0 (summary Network Number)
> Advertising Router: 7.7.7.7
> LS Seq Number: 80000001
> Checksum: 0xF322
> Length: 28
> Network Mask: /24
> TOS: 0 Metric: 11175
>
>r6#sh ip os database summary
>
> Routing Bit Set on this LSA
> LS age: 1042
> Options: (No TOS-capability, DC, Upward)
> LS Type: Summary Links(Network)
> Link State ID: 172.16.123.0 (summary Network Number)
> Advertising Router: 3.3.3.3
> LS Seq Number: 8000002E
> Checksum: 0x8308
> Length: 28
> Network Mask: /24
> TOS: 0 Metric: 64
>
> LS age: 1639
> Options: (No TOS-capability, DC, Upward)
> LS Type: Summary Links(Network)
> Link State ID: 172.16.123.0 (summary Network Number)
> Advertising Router: 7.7.7.7
> LS Seq Number: 80000001
> Checksum: 0xF322
> Length: 28
> Network Mask: /24
> TOS: 0 Metric: 11175
>
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