Solved: Re: Routing problem due to OSPF issue with "O" versus "IA O"

aimarchitect · ‎05-25-2012

I would like to dscuss how an OSPF router might insert an intra area "O" OSPF route into the routing table when it seems that the route should have been an inter-area "IA O" route and therfore [due to precedence of O before IA O, regardless of cost] lost to a higher cost intra area "O" route.

The attached diagram helps illustrate the question. Take note of the two ABR routers, C2 and S2.

C2 and S2 each have an interface in Area 14. 10.180.21.0/24 is configured in Area 14. The router interface addresses in this network/Area are .17 and .22 respectively. The OSPF cost on these interfaces is 1.

C2 and S2 also each have an interface in Area 0. 10.180.16.0/24 is configured in Area 0. The router interface addresses in this network/Area are .4 and .6 respectively. The OSPF cost on these interfaces is 5.

C2 and S2 are also configured with router-ids 10.180.20.4 and 10.180.20.6 respectively. These host addresses are configured in Area 0 on each router.

Which interface should C2 use reach 10.180.20.6 (S2 router-id) and why?

Running a "show ip route | i 10.180.20.6" command on C2 does (but should not?) show this:

C2#sh ip route | i 10.180.20.6

O 10.180.20.6/32 [110/2] via 10.180.21.22, 1d08h, Vlan14

How can a route from C2 to 10.180.20.6 (S2) with the next hop 10.180.21.22 be considered an "O" if it is not going through Area 0?

How exactly can C2 know/classify the route to 10.180.20.6 (S2) via 10.180.21.22 as an "O" instead of an "IA O"?

Thanks for any responses...

Giuseppe Larosa · ‎07-12-2012

Hello Greg,

thanks for your kind update to this thread.

It has been wise to open a TAC service request.

The virtual link was not mentioned in previous posts, however it was not immediate to think of this kind of consequences of its presence.

Best Regards

Giuseppe

.

View solution in original post

Edwin Summers · ‎05-25-2012

I may not fully understand your question, but the route learned by C2 to 10.180.20.6 *would* be an intra-area route. Intra-area routes originate and are learned in the same area (it doesn't have to traverse Area 0). Intra-area routes are indicated by the "O". As the route was originated in Area 1, and learned on C2's Area 1 interface (next hop is 10.180.21.22, it is an intra-area ("O") route.

Inter-area routes are routes that originate in one area and learned in another. Those are the "IA O" routes.

Hopefully I've helped some? Best, -Ed

Oops! Just read Giuseppe's comment and realized I missed your note that the loopbacks were configured to Area 0. Sorry for the confusion! -Ed

Giuseppe Larosa · ‎05-26-2012

Hello Greg,

if I understand correctly Vlan14 is part of area 14 and from the network diagram we see that both C2 and S2 nodes are ABR between area 0 and area 14 ABR(0,14).

You say that loopback addresses have been advertised with network .... area 0 command.

I agree the expected output should be an O route in area 0 with IP next-hop 10.180.16.6.

Here we an O route in area 14.

So first I would check if IP connectivity and OSPF relationships are correctly setup in area 0 for the lan segment associated to the IP subnet 10.180.16.0/24.

You can use

show ip ospf neighbors

on C2 to see if all the expected neighbors are seen or not.

The second aspect that can be examined is the OSPF database

you can use on C2 node:

show ip ospf database advertising-router 10.180.20.6

to see all LSA generated by S2.

show ip ospf database router 10.180.20.6

You should see two Router LSA one built for area 0 and one for area 14 if IP connectivity and OSPF are fine in subnet 10.180.16.0/24.

the show ip route that you have provided looks like compatible with a scenario where C2 receives only the router LSA in area 14 from S2 and no router LSA for S2 in area 0 with loopback interface IP address matching a network ... area 14 command on S2. This may be caused by some possible cases: S2 actually configured as an internal router in area 14, S2 or C2 without any active link to area 0.

You can use

show ip ospf

to have information about OSPF on S2 and you should verify it is running for area 0 and area 14.

Hope to help

Giuseppe

aimarchitect · ‎05-27-2012

Giuseppe,

Thank you for taking the time to read my post.

I'm glad you understand the problem...the route from C2 to the loopback router-id address 10.180.20.6 on S2 should not be intra-area via 10.180.21.22 in Area 14 (VLAN 14). Instead it should be intra-area via 10.180.16.6 in Area 0 (VLAN 10).

Exactly what process and data does C2 use to make its deciscions about classifying routes as intra-area or inter-area?

Here is the output of some show commands run on routers C2 and S2:

C2#sh ip ospf nei

Neighbor ID Pri State Dead Time Address Interface

10.180.20.6 1 2WAY/DROTHER 964 msec 10.180.16.6 Vlan10

10.180.20.6 254 FULL/BDR 00:00:30 10.180.21.22 Vlan14

[output omitted]

C2#sh ip ospf database adv-router 10.180.20.6

OSPF Router with ID (10.180.20.4) (Process ID 65525)

Router Link States (Area 0)

Link ID ADV Router Age Seq# Checksum Link count

10.180.20.6 10.180.20.6 1392 0x80008555 0x001215 7

Net Link States (Area 0)

Link ID ADV Router Age Seq# Checksum

[output omitted]

Summary Net Link States (Area 0)

Link ID ADV Router Age Seq# Checksum

[output omitted]

Summary ASB Link States (Area 0)

Link ID ADV Router Age Seq# Checksum

[output omitted]

Router Link States (Area 14)

Link ID ADV Router Age Seq# Checksum Link count

10.180.20.6 10.180.20.6 1514 0x800084FB 0x005192 4

Summary Net Link States (Area 14)

Link ID ADV Router Age Seq# Checksum

10.180.20.6 10.180.20.6 1758 0x800084C8 0x00B18C

[output omitted]

Summary ASB Link States (Area 14)

Link ID ADV Router Age Seq# Checksum

[output omitted]

Type-5 AS External Link States

Link ID ADV Router Age Seq# Checksum Tag

[output omitted]

C2#

C2#sh ip ospf int brie

Interface PID Area IP Address/Mask Cost State Nbrs F/C

Vl10 65525 0 10.180.16.4/23 5 DROTH 2/9

Vl14 65525 14 10.180.21.17/28 1 DR 6/6

[output omitted]

C2#sh run | s ospf

router ospf 65525

router-id 10.180.20.4

log-adjacency-changes

area 0 authentication message-digest

area 14 authentication message-digest

redistribute static subnets

network 10.180.20.4 0.0.0.0 area 0

network 10.180.16.0 0.0.1.255 area 0

network 10.180.21.16 0.0.0.15 area 14

[output omitted]

C2#sh ip proto

*** IP Routing is NSF aware ***

Routing Protocol is "ospf 65525"

Outgoing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Router ID 10.180.20.4

It is an area border and autonomous system boundary router

Redistributing External Routes from,

static, includes subnets in redistribution

Number of areas in this router is 2. 2 normal 0 stub 0 nssa

Maximum path: 4

Routing for Networks:

[output omitted]

10.180.16.0 0.0.1.255 area 0

10.180.20.4 0.0.0.0 area 0

10.180.21.16 0.0.0.15 area 14

Passive Interface(s):

[output omitted]

Routing Information Sources:

Gateway Distance Last Update

10.180.20.6 110 02:01:25

[output omitted]

S2#sh ip ospf nei

Neighbor ID Pri State Dead Time Address Interface

10.180.20.4 1 2WAY/DROTHER 872 msec 10.180.16.4 Vlan10

10.180.20.4 255 FULL/DR 00:00:38 10.180.21.17 Vlan14

[output omitted]

S2#sh ip ospf database adv-router 10.180.20.4

OSPF Router with ID (10.180.20.6) (Process ID 65525)

Router Link States (Area 0)

Link ID ADV Router Age Seq# Checksum Link count

10.180.20.4 10.180.20.4 19 0x8000DE41 0x007EFA 13

Net Link States (Area 0)

Link ID ADV Router Age Seq# Checksum

[output omitted]

Summary Net Link States (Area 0)

Link ID ADV Router Age Seq# Checksum

[output omitted]

Summary ASB Link States (Area 0)

Link ID ADV Router Age Seq# Checksum

[output omitted]

Router Link States (Area 14)

Link ID ADV Router Age Seq# Checksum Link count

10.180.20.4 10.180.20.4 60 0x8000B133 0x00C010 2

Net Link States (Area 14)

Link ID ADV Router Age Seq# Checksum

[output omitted]

Summary Net Link States (Area 14)

Link ID ADV Router Age Seq# Checksum

10.180.20.4 10.180.20.4 60 0x8000B121 0x009923

Summary ASB Link States (Area 14)

Link ID ADV Router Age Seq# Checksum

[output omitted]

Type-5 AS External Link States

Link ID ADV Router Age Seq# Checksum Tag

[output omitted]

S2#

S2#sh ip ospf int brie

Interface PID Area IP Address/Mask Cost State Nbrs F/C

Vl10 65525 0 10.180.16.6/23 5 DROTH 2/9

Vl14 65525 14 10.180.21.22/28 1 BDR 6/6

[output omitted]

S2#sh run | s ospf

router ospf 65525

router-id 10.180.20.6

log-adjacency-changes

area 0 authentication message-digest

area 14 authentication message-digest

redistribute static subnets

network 10.180.20.6 0.0.0.0 area 0

network 10.180.16.0 0.0.1.255 area 0

network 10.180.21.16 0.0.0.15 area 14

[output omitted]

S2#sh ip proto

*** IP Routing is NSF aware ***

Routing Protocol is "ospf 65525"

Outgoing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Router ID 10.180.20.6

It is an area border and autonomous system boundary router

Redistributing External Routes from,

static, includes subnets in redistribution

Number of areas in this router is 2. 2 normal 0 stub 0 nssa

Maximum path: 4

Routing for Networks:

[output omitted]

10.180.16.0 0.0.1.255 area 0

10.180.20.6 0.0.0.0 area 0

10.180.21.16 0.0.0.15 area 14

Passive Interface(s):

[output omitted]

Routing Information Sources:

Gateway Distance Last Update

[output omitted]

10.180.20.4 110 01:59:35

Giuseppe Larosa · ‎05-28-2012

Hello Greg,

both routers C2 and S2 are ABR between area 0 and area 14.

However, at the same time both routers are ASBR because they inject routes into the OSPF domain

>>Redistributing External Routes from,

static, includes subnets in redistribution

Both area 0 and area 14 are standard areas:

>> Number of areas in this router is 2. 2 normal 0 stub 0 nssa

It is the fact that S2 is an ASBR that changes the game rules: routing to S2 Router-id can be performed by using LSA type 4 information.

LSA type 4 information is present in area 0 and in area 14 for 10180.20.6/32.

At this point the route in area 14 is preferred for the lowest cost over the path in area 0.

With this lab setup we see the effective use of LSA type 4.

You have omitted the output but the section relevant ot LSA type 4 is

Summary ASB Link States (Area 14)

Link ID ADV Router Age Seq# Checksum

[output omitted]

and there is an LSA Type4 in area 14 generated by S2 that describes how to reach S2 router-id

This makes the route via Vlan14 comparable ( of the same type O instead of O IA) to that via vlan10.

the

show ip ospf database router 10.180.20.6 detail

on C2 should show that the router LSA in area 14 does not include the loopback address 10.180.20.6

If S2 wouldn't be an ASBR the only route would have been via vlan10 in area 0 learned by Router LSA in area 0.

Being S2 an ASBR the comparison is performed on LSA type4 information,

Hope to help

Giuseppe

aimarchitect · ‎05-31-2012

Hi Giuseppe,

You are right, I see that a Type 1 LSA for S2 router-id 10.180.20.6 is not present in the Area 14 database on C2. And yes, a Type 4 LSA for S2 router-id 10.180.20.6 is presnt in the Area 0 and Area 14 databases on C2 (see show ip ospf datbase command output below).

Do I understand correctly that C2 takes the Type 4 LSA from the Area 14 database for 10.180.20.6 to make this routing table entry?

C2#sh ip route 10.180.20.6

Routing entry for 10.180.20.6/32

Known via "ospf 65525", distance 110, metric 2, type intra area

Last update from 10.180.21.22 on Vlan14, 02:09:58 ago

Routing Descriptor Blocks:

* 10.180.21.22, from 10.180.20.6, 02:09:58 ago, via Vlan14

Route metric is 2, traffic share count is 1

C2#sh ip ospf dat router 10.180.20.6

OSPF Router with ID (10.180.20.4) (Process ID 65525)

Router Link States (Area 0)

Routing Bit Set on this LSA

LS age: 318

Options: (No TOS-capability, DC)

LS Type: Router Links

Link State ID: 10.180.20.6

Advertising Router: 10.180.20.6

LS Seq Number: 800085FA

Checksum: 0xC6BA

Length: 108

Area Border Router

AS Boundary Router

Number of Links: 7

Link connected to: a Virtual Link

(Link ID) Neighboring Router ID: [output omitted]

(Link Data) Router Interface address: [output omitted]

Number of TOS metrics: 0

TOS 0 Metrics: 1

Link connected to: a Stub Network

(Link ID) Network/subnet number: [output omitted]

(Link Data) Network Mask: [output omitted]

Number of TOS metrics: 0

TOS 0 Metrics: 1

Link connected to: a Stub Network

(Link ID) Network/subnet number: [output omitted]

(Link Data) Network Mask: [output omitted]

Number of TOS metrics: 0

TOS 0 Metrics: 1

Link connected to: a Transit Network

(Link ID) Designated Router address: [output omitted]

(Link Data) Router Interface address: [output omitted]

Number of TOS metrics: 0

TOS 0 Metrics: 1

Link connected to: a Stub Network

(Link ID) Network/subnet number: 10.180.20.6

(Link Data) Network Mask: 255.255.255.255

Number of TOS metrics: 0

TOS 0 Metrics: 1

Link connected to: a Transit Network

(Link ID) Designated Router address: [output omitted]

(Link Data) Router Interface address: [output omitted]

Number of TOS metrics: 0

TOS 0 Metrics: 5

Link connected to: a Transit Network

(Link ID) Designated Router address: [output omitted]

(Link Data) Router Interface address: [output omitted]

Number of TOS metrics: 0

TOS 0 Metrics: 5

Router Link States (Area 14)

Routing Bit Set on this LSA

LS age: 317

Options: (No TOS-capability, DC)

LS Type: Router Links

Link State ID: 10.180.20.6

Advertising Router: 10.180.20.6

LS Seq Number: 800085A0

Checksum: 0x539

Length: 72

Area Border Router

AS Boundary Router

Virtual Link Endpoint

Number of Links: 4

Link connected to: another Router (point-to-point)

(Link ID) Neighboring Router ID: [output omitted]

(Link Data) Router Interface address: [output omitted]

Number of TOS metrics: 0

TOS 0 Metrics: 250

Link connected to: a Stub Network

(Link ID) Network/subnet number: [output omitted]

(Link Data) Network Mask: [output omitted]

Number of TOS metrics: 0

TOS 0 Metrics: 250

Link connected to: a Stub Network

(Link ID) Network/subnet number: [output omitted]

(Link Data) Network Mask: [output omitted]

Number of TOS metrics: 0

TOS 0 Metrics: 1

Link connected to: a Transit Network

(Link ID) Designated Router address: [output omitted]

(Link Data) Router Interface address: [output omitted]

Number of TOS metrics: 0

TOS 0 Metrics: 1

C2#sh ip ospf dat asbr-summary 10.180.20.6

OSPF Router with ID (10.180.20.4) (Process ID 65525)

Summary ASB Link States (Area 0)

LS age: 909

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links(AS Boundary Router)

Link State ID: 10.180.20.6 (AS Boundary Router address)

Advertising Router: 10.180.20.4

LS Seq Number: 8000107D

Checksum: 0xA35B

Length: 28

Network Mask: /0

TOS: 0 Metric: 1

LS age: 3 (DoNotAge)

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links(AS Boundary Router)

Link State ID: 10.180.20.6 (AS Boundary Router address)

Advertising Router: [output omitted]

LS Seq Number: 800007B7

Checksum: 0x5661

Length: 28

Network Mask: /0

TOS: 0 Metric: 10

LS age: 382

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links(AS Boundary Router)

Link State ID: 10.180.20.6 (AS Boundary Router address)

Advertising Router: [output omitted]

LS Seq Number: 80000063

Checksum: 0x2CDA

Length: 28

Network Mask: /0

TOS: 0 Metric: 16

Summary ASB Link States (Area 14)

LS age: 156

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links(AS Boundary Router)

Link State ID: 10.180.20.6 (AS Boundary Router address)

Advertising Router: 10.180.20.4

LS Seq Number: 80003055

Checksum: 0xBB47

Length: 28

Network Mask: /0

TOS: 0 Metric: 5

Adv Router is not-reachable

LS age: 572

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links(AS Boundary Router)

Link State ID: 10.180.20.6 (AS Boundary Router address)

Advertising Router: [output omitted]

LS Seq Number: 8000107B

Checksum: 0xB337

Length: 28

Network Mask: /0

TOS: 0 Metric: 10

Adv Router is not-reachable

LS age: 883

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links(AS Boundary Router)

Link State ID: 10.180.20.6 (AS Boundary Router address)

Advertising Router: [output omitted]

LS Seq Number: 8000304E

Checksum: 0x6194

Length: 28

Network Mask: /0

TOS: 0 Metric: 6

C2#

Giuseppe Larosa · ‎06-01-2012

Hello Greg,

S2 Router LSA in area 14 hasn't an entry for S2 10.180.20.6 and this is like expected.

Yes the route on C2 is derived from LSA type 4 in area 14 originated by S2 and referring to S2 router-id 10.180.20.6.

However, I don't see that specific LSA in the show command that you have reported.

Notice that the last LSA in area 14 that you have noticed (using bold) is originated by C2 itself ( advertising router 10.180.20.4).

Hope to help

Giuseppe

aimarchitect · ‎06-06-2012

Hi Giusepe,

This is tough issue and we are about to open a TAC case with the intention of getting an OSPF developer.

You are saying:

...the route on C2 is derived from LSA type 4 in area 14 originated by S2 and referring to S2 router-id 10.180.20.6.

The route destination (S2 router-id 10.180.20.6) is in Area 0, so how can an Area 14 link (VLAN 14) ever be considered Intra Area for Area 0? An Intra Area, Area 0 path should never leave Area 0, right?

I don't see a Type 4 LSA in Area 14 originated by S2 and referring to S2 router-id 10.180.20.6 Are these the right commands to run on S2 to look for it?

S2#sh ip ospf dat asbr-summary 10.180.20.6

OSPF Router with ID (10.180.20.6) (Process ID 65525)

Summary ASB Link States (Area 0)

LS age: 1989

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links(AS Boundary Router)

Link State ID: 10.180.20.6 (AS Boundary Router address)

Advertising Router: 10.180.20.4

LS Seq Number: 8000117A

Checksum: 0xA65A

Length: 28

Network Mask: /0

TOS: 0 Metric: 1

LS age: 1 (DoNotAge)

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links(AS Boundary Router)

Link State ID: 10.180.20.6 (AS Boundary Router address)

Advertising Router: [output omitted, not 10.180.20.6]

LS Seq Number: 800007B7

Checksum: 0x5661

Length: 28

Network Mask: /0

TOS: 0 Metric: 10

LS age: 186

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links(AS Boundary Router)

Link State ID: 10.180.20.6 (AS Boundary Router address)

Advertising Router: [output omitted, not 10.180.20.6]

LS Seq Number: 8000002E

Checksum: 0x96A5

Length: 28

Network Mask: /0

TOS: 0 Metric: 16

Summary ASB Link States (Area 14)

Adv Router is not-reachable

LS age: 475

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links(AS Boundary Router)

Link State ID: 10.180.20.6 (AS Boundary Router address)

Advertising Router: 10.180.20.4

LS Seq Number: 80003152

Checksum: 0xBE46

Length: 28

Network Mask: /0

TOS: 0 Metric: 5

Adv Router is not-reachable

LS age: 583

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links(AS Boundary Router)

Link State ID: 10.180.20.6 (AS Boundary Router address)

Advertising Router: [output omitted, not 10.180.20.6]

LS Seq Number: 80001178

Checksum: 0xB636

Length: 28

Network Mask: /0

TOS: 0 Metric: 10

Adv Router is not-reachable

LS age: 963

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links(AS Boundary Router)

Link State ID: 10.180.20.6 (AS Boundary Router address)

Advertising Router: [output omitted, not 10.180.20.6]

LS Seq Number: 8000314B

Checksum: 0x6493

Length: 28

Network Mask: /0

TOS: 0 Metric: 6

S2#sh ip ospf dat asbr-summary self-originate

OSPF Router with ID (10.180.20.6) (Process ID 65525)

Summary ASB Link States (Area 0)

LS age: 1469

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links(AS Boundary Router)

Link State ID: 10.180.20.4 (AS Boundary Router address)

Advertising Router: 10.180.20.6

LS Seq Number: 80001177

Checksum: 0xB44F

Length: 28

Network Mask: /0

TOS: 0 Metric: 1

LS age: 1469

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links(AS Boundary Router)

Link State ID: [output omitted, not 10.180.20.6] (AS Boundary Router address)

Advertising Router: 10.180.20.6

LS Seq Number: 80001177

Checksum: 0x50A9

Length: 28

Network Mask: /0

TOS: 0 Metric: 1

...

Summary ASB Link States (Area 14)

LS age: 1998

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links(AS Boundary Router)

Link State ID: [output omitted, not 10.180.20.6] (AS Boundary Router address)

Advertising Router: 10.180.20.6

LS Seq Number: 80003150

Checksum: 0xE821

Length: 28

Network Mask: /0

TOS: 0 Metric: 5

LS age: 1998

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links(AS Boundary Router)

Link State ID: [output omitted, not 10.180.20.6] (AS Boundary Router address)

Advertising Router: 10.180.20.6

LS Seq Number: 80003150

Checksum: 0xDE2A

Length: 28

Network Mask: /0

TOS: 0 Metric: 5

LS age: 476

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links(AS Boundary Router)

Link State ID: [output omitted, not 10.180.20.6] (AS Boundary Router address)

Advertising Router: 10.180.20.6

LS Seq Number: 8000319A

Checksum: 0x229E

Length: 28

Network Mask: /0

TOS: 0 Metric: 2

LS age: 1

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links(AS Boundary Router)

Link State ID: 10.180.20.4 (AS Boundary Router address)

Advertising Router: 10.180.20.6

LS Seq Number: 80003151

Checksum: 0xC83D

Length: 28

Network Mask: /0

TOS: 0 Metric: 5

LS age: 3

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links(AS Boundary Router)

Link State ID: [output omitted, not 10.180.20.6] (AS Boundary Router address)

Advertising Router: 10.180.20.6

LS Seq Number: 8000866B

Checksum: 0x6237

Length: 28

Network Mask: /0

TOS: 0 Metric: 1

LS age: 4

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links(AS Boundary Router)

Link State ID: [output omitted, not 10.180.20.6] (AS Boundary Router address)

Advertising Router: 10.180.20.6

LS Seq Number: 80003151

Checksum: 0x8C73

Length: 28

Network Mask: /0

TOS: 0 Metric: 5

...

aimarchitect · ‎07-12-2012

Here is an update. With the help of Cisco TAC we are on the road to understanding how the Area 0 loopback of C2 routes to the Area 0 loopback of S2 across Area 14 and calls it "intra area".

Something not pictured in the original diagram is that S2 has a Virtual Link to a router in Area 14. Still need verification, but it seems that the existence of this Virtual Link, coupled with the default "on" state for "capability transit" allows the above noted condition to occur.

Here are a couple of links (one provided by Cisco TAC) that I am looking at now with relation to this:

http://blog.ine.com/2009/09/14/understanding-ospf-transit-capability/

http://ieoc.com/forums/t/4807.aspx

Giuseppe Larosa · ‎07-12-2012

Hello Greg,

thanks for your kind update to this thread.

It has been wise to open a TAC service request.

The virtual link was not mentioned in previous posts, however it was not immediate to think of this kind of consequences of its presence.

Best Regards

Giuseppe

.

aimarchitect · ‎07-12-2012

Hi Giuseppe,

Sorry about not including the point of the Virtual Link in the original post. I was trying to simplify to the example. I was wrong to think it was not relevant.

I have now read the "Understanding OSPF Transit Capability" link above and found that it does indeed describe the behaviour we see in the network.

Especially, this part of the article describes it perfectly:

"What’s really interesting, is that an intra-area route actually takes the INTER-area path! The transit capability essentially allows the use of non-backbone inter-area routes to optimize inter-area paths and area 0 intra-area paths IF the area in question is transit!"

So this question is answered. If you see routing between Area 0 prefixes going through Non-0 Areas, and being listed in the routing table as "O" and "intra area" it may be confusing, but it is not wrong. This is the work of OSPF Transit Capability.