OSPF Summary Address not advertised

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Jun 30th, 2009
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Hi, I am having a strange problem which I am sure is something simple but is driving me nuts.

I am attaching a diagram to ease the description.

Area 41 is a NSSA, all network devices in the diagram are in area 0. Core 3/4 are summarising the area 41 range.

Edge 1 is learning about this summarised range via the Metro.

Edge 2 however is learning about the summarised range via Core1 and core 2 instead of via metro 2. Both metro's are 100mb but via different providers.

Edge 2 is adjacent with Core 4 and is learning the loopback address and other Layer 3 links in area 0.

Is there some limitation that only one summarised route gets advertised out?

Both core 3 and core 4 are configured identically. Any help would be greatfully appreciated.

Cheers

Brian



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Correct Answer by andrewswanson about 8 years 3 weeks ago

hi brian

use the command "show ip ospf database summary" on edge2 - this will list all the lsas received and give you the advertising router along with the associated ospf cost. the output will hopefully show why the route to the summarized area 41 is via core 3 rather than core 4.

when you summarize area 41 on the ABRs are you specifying a cost?

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andrewswanson Wed, 07/01/2009 - 04:53
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does the outputs of the command "show ip ospf border-routers" on edge1 and 2 show the ABRs and their expected costs correctly

brian.oflynn Wed, 07/01/2009 - 05:02
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It does show both ABR's however Edge 2 is learning about the ABR's via Core 1 and 2 instead of via Metro 2.

andrewswanson Wed, 07/01/2009 - 05:09
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what is the output of "show ip ospf neighbors" on edge 2 - can you see core 4?

brian.oflynn Wed, 07/01/2009 - 05:14
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Yup, can see neighbor relationship with core 4. I am learning area 0 routes from core 4 also. It is just routes within area 41 that don't appear. Area 41 being a NSSA. Both Core 3 and Core 4 are injecting a default route into area 41 and both core 3 and 4 are summarizing area 41 to a class B range.

andrewswanson Wed, 07/01/2009 - 06:26
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does the summary lsa for area 41 from core 4 appear in the ospf database on edge2?

brian.oflynn Wed, 07/01/2009 - 06:36
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Hi Andrew,yes the summary subnet is listed in the ospf database, I can't tell if it was learnt from Core 4 however.

Correct Answer
andrewswanson Thu, 07/02/2009 - 00:32
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hi brian

use the command "show ip ospf database summary" on edge2 - this will list all the lsas received and give you the advertising router along with the associated ospf cost. the output will hopefully show why the route to the summarized area 41 is via core 3 rather than core 4.

when you summarize area 41 on the ABRs are you specifying a cost?

brian.oflynn Thu, 07/02/2009 - 00:56
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Hi Andrew, thanks a mill, can see now why it was choosing a particular path. The Metric for the summarized roue via edge 1 is one and the metric via edge 2 is one hundred! Really appreciate your assistance on this, will rate appropriately.

Cheers

Brian

andrewswanson Thu, 07/02/2009 - 01:08
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no problem brian - the ospf cost of the summary created on core 4 will be the cost of the "worst" of the summarized routes so maybe its an issue with core 4's links to the routers within area 41.

cheers

andrew

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