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OSPF: Dangers of having non-redundant backbone ABRs isolated areas

[attachment=0]ospf-non-standard-area-design.png[/attachment]

This is Part 2 of an OSPF story. Part 1 can be found here: [url]http://forum.ivorde.ro/ospf-multi-area-scenario-with-isolated-areas-cisco-juniper-part-1-t19221.html[/url]


In the top diagram I have a non-standard OSPF network (only OSPF AS internal routes) that works well and optimized if all links and nodes are up.

I will show throughout this part some failing scenarios and how parts of the network can remain separated from each other even though there are physical paths.

Based on references from part 1 of this article, Juniper and Cisco have different behavior when it comes to acting as an ABR - more in depth RFC 3509.

Cisco approach: It needs one interface in area 0 - backbone to advertise it self in all areas that it is an ABR (B bit set) and to convert type 1 and type 3 LSAs from one area into type 3 LSA on another area. Without an interface in area 0, it will be a router inside both areas without sending summary LSAs with destinations from other non-backbone area.

Juniper approach: Juniper device will act as an ABR if it has two interfaces in two different areas, even two non-backbone areas. Thus, it will take Router and Summary LSAs from any area (let's say 20) and send Summary LSAs with those destinations to area 30.

Following information will be based on Juniper approach (part 3 will be based on a GNS lab).

In the picture above backbone area is the one on top and chained to it are Areas 10 and 20, next to these areas is chained area 40. Area 40 is isolated from the backbone.

Quiz 1: From Host A traffic to Host B will go via.... ? (remember this setup is based on Junos devices).

Answer: HostA - R3 - R4 - HostB
[code]
HostA# traceroute 10.43.4.2
traceroute to 10.43.4.2 (10.43.4.2), 128 hops max, 52 byte packets
1 10.43.3.1 (10.43.3.1) 2.284 ms 2.993 ms 5.547 ms
2 10.42.25.2 (10.42.25.2) 6.617 ms 5.657 ms 8.197 ms
3 10.43.4.2 (10.43.4.2) 11.866 ms 7.795 ms 5.978 ms
[/code]


Why is that ? Let's look at OSPF database on R4 ( I prefer to start from destination ):
[code]
[edit]
admin@R4# run show ospf database advertising-router self lsa-id 10.255.0.4 extensive area 20

OSPF database, Area 0.0.0.20
Type ID Adv Rtr Seq Age Opt Cksum Len
Router *10.255.0.4 10.255.0.4 0x8000000c 201 0x22 0x3621 72
bits 0x1, link count 4
id 10.42.7.1, data 10.42.7.2, Type Transit (2)
Topology count: 0, Default metric: 1
id 10.43.4.0, data 255.255.255.0, Type Stub (3)
Topology count: 0, Default metric: 1
id 10.255.0.4, data 255.255.255.255, Type Stub (3)
Topology count: 0, Default metric: 0
id 172.16.4.0, data 255.255.255.0, Type Stub (3)
Topology count: 0, Default metric: 1
Topology default (ID 0)
Type: Transit, Node ID: 10.42.7.1
Metric: 1, Bidirectional
Gen timer 00:46:39
Aging timer 00:56:39
Installed 00:03:21 ago, expires in 00:56:39, sent 00:03:19 ago
Last changed 00:46:21 ago, Change count: 5, Ours
[edit]
admin@R4# run show ospf database advertising-router self lsa-id 10.43.4.0 extensive area 40

OSPF database, Area 0.0.0.40
Type ID Adv Rtr Seq Age Opt Cksum Len
Summary *10.43.4.0 10.255.0.4 0x80000003 523 0x22 0x688a 28
mask 255.255.255.0
Topology default (ID 0) -> Metric: 1
Gen timer 00:40:58
Aging timer 00:51:16
Installed 00:08:43 ago, expires in 00:51:17, sent 00:08:40 ago
Last changed 00:47:40 ago, Change count: 1, Ours
[/code]

R4 injects the destination subnet 10.43.4.0/24 in area 20 in a type 1 Router LSA (interface is added to area 20) - Notice Type 1 LSA bits 0x1 - Acting as ABR.
R4 injects the destination subnet 10.43.4.0/24 in area 40 as a type 4 Summary LSA.

Both LSAs have Advertising Router ID R4 10.255.0.4.

Next we look at R3:
[code]
[edit]
admin@R3# run show ospf database area 40 lsa-id 10.43.4.0 extensive

OSPF database, Area 0.0.0.40
Type ID Adv Rtr Seq Age Opt Cksum Len
Summary 10.43.4.0 10.255.0.4 0x80000003 695 0x22 0x688a 28
mask 255.255.255.0
Topology default (ID 0) -> Metric: 1
Aging timer 00:48:24
Installed 00:11:31 ago, expires in 00:48:25
Last changed 00:50:31 ago, Change count: 1
[/code]

Based on this type 3 LSA that R3 received from R4, it forwards traffic to R4.

Quiz 2: What if link between R3 and R4 is down ? From Host A traffic to Host B will go via.... ?


Answer: HostA - R3 - R1 - R2 - R4 - HostB
[code]
HostA# traceroute 10.43.4.2
traceroute to 10.43.4.2 (10.43.4.2), 128 hops max, 52 byte packets
1 10.43.3.1 (10.43.3.1) 5.586 ms 2.685 ms 4.955 ms
2 10.42.6.2 (10.42.6.2) 9.922 ms 6.287 ms 5.984 ms
3 10.42.4.1 (10.42.4.1) 7.919 ms 19.464 ms 12.181 ms
4 10.42.7.2 (10.42.7.2) 14.368 ms 9.715 ms 14.054 ms
5 10.43.4.2 (10.43.4.2) 10.119 ms 12.926 ms 13.783 ms
[/code]

So far so good. Redundancy works and I'm happy. But why does it work ?

Again, I start looking at R4. We know R4 injects Type 1 - Router LSA into area 20 that reaches R1 (type 1 has area scope).
[code]
[edit]
admin@R1# run show ospf database area 20 lsa-id 10.255.0.4 extensive

OSPF database, Area 0.0.0.20
Type ID Adv Rtr Seq Age Opt Cksum Len
Router 10.255.0.4 10.255.0.4 0x8000000c 828 0x22 0x3621 72
bits 0x1, link count 4
id 10.42.7.1, data 10.42.7.2, Type Transit (2)
Topology count: 0, Default metric: 1
id 10.43.4.0, data 255.255.255.0, Type Stub (3)
Topology count: 0, Default metric: 1
id 10.255.0.4, data 255.255.255.255, Type Stub (3)
Topology count: 0, Default metric: 0
id 172.16.4.0, data 255.255.255.0, Type Stub (3)
Topology count: 0, Default metric: 1
Topology default (ID 0)
Type: Transit, Node ID: 10.42.7.1
Metric: 1, Bidirectional
Aging timer 00:46:12
Installed 00:13:42 ago, expires in 00:46:12
Last changed 00:56:46 ago, Change count: 5
[/code]

Notice the bit 0x1, Advertising router R4 and network 10.43.4.0/24 Stub network.
Now we look at information that R1 injects into Area 10 (towards VR1 and R3):

[code]
[edit]
admin@R1# run show ospf database advertising-router self

OSPF database, Area 0.0.0.10
Type ID Adv Rtr Seq Age Opt Cksum Len
Router *10.255.0.1 10.255.0.1 0x8000000b 1491 0x22 0x5393 72
Network *10.42.6.2 10.255.0.1 0x80000005 1678 0x22 0x3b9e 32
Summary *10.42.3.0 10.255.0.1 0x80000002 553 0x22 0x9d5a 28
Summary *10.42.4.0 10.255.0.1 0x80000003 2416 0x22 0x8077 28
Summary *10.42.7.0 10.255.0.1 0x80000002 366 0x22 0x5f97 28
Summary *10.43.4.0 10.255.0.1 0x80000002 178 0x22 0x9064 28
Summary *10.255.0.2 10.255.0.1 0x80000001 2416 0x22 0x9c88 28
Summary *10.255.0.4 10.255.0.1 0x80000001 2416 0x22 0x928f 28
Summary *172.16.4.0 10.255.0.1 0x80000001 2416 0x22 0x95d8 28

OSPF database, Area 0.0.0.20
Type ID Adv Rtr Seq Age Opt Cksum Len
Router *10.255.0.1 10.255.0.1 0x8000000a 741 0x22 0x524f 36
Summary *10.42.1.0 10.255.0.1 0x80000006 2054 0x22 0x8f6a 28
Summary *10.42.6.0 10.255.0.1 0x80000006 1116 0x22 0x589c 28
Summary *10.43.3.0 10.255.0.1 0x80000002 1865 0x22 0x9165 28
Summary *10.255.0.1 10.255.0.1 0x80000007 928 0x22 0x9090 28
Summary *10.255.0.3 10.255.0.1 0x80000005 1303 0x22 0x8a95 28
Summary *10.255.0.255 10.255.0.1 0x80000006 2240 0x22 0x9c86 28

[edit]
admin@R1# run show ospf database advertising-router self lsa-id 10.255.0.1 extensive

OSPF database, Area 0.0.0.10
Type ID Adv Rtr Seq Age Opt Cksum Len
Router *10.255.0.1 10.255.0.1 0x8000000b 1497 0x22 0x5393 72
bits 0x1, link count 4
id 10.42.1.2, data 10.42.1.1, Type Transit (2)
Topology count: 0, Default metric: 1
id 10.42.6.2, data 10.42.6.2, Type Transit (2)
Topology count: 0, Default metric: 1
id 10.255.0.255, data 255.255.255.255, Type Stub (3)
Topology count: 0, Default metric: 0
id 10.255.0.1, data 255.255.255.255, Type Stub (3)
Topology count: 0, Default metric: 0
Topology default (ID 0)
Type: Transit, Node ID: 10.42.6.2
Metric: 1, Bidirectional
Type: Transit, Node ID: 10.42.1.2
Metric: 1, Bidirectional
Gen timer 00:25:02
Aging timer 00:35:03
Installed 00:24:57 ago, expires in 00:35:03, sent 00:24:55 ago
Last changed 03:10:43 ago, Change count: 4, Ours

OSPF database, Area 0.0.0.20
Type ID Adv Rtr Seq Age Opt Cksum Len
Router *10.255.0.1 10.255.0.1 0x8000000a 747 0x22 0x524f 36
bits 0x1, link count 1
id 10.42.4.1, data 10.42.4.0, Type Transit (2)
Topology count: 0, Default metric: 1
Topology default (ID 0)
Type: Transit, Node ID: 10.42.4.1
Metric: 1, Bidirectional
Gen timer 00:37:33
Aging timer 00:47:33
Installed 00:12:27 ago, expires in 00:47:33, sent 00:12:25 ago
Last changed 00:40:23 ago, Change count: 3, Ours
Summary *10.255.0.1 10.255.0.1 0x80000007 934 0x22 0x9090 28
mask 255.255.255.255
Topology default (ID 0) -> Metric: 0
Gen timer 00:34:25
Aging timer 00:44:25
Installed 00:15:34 ago, expires in 00:44:26, sent 00:15:32 ago
Last changed 03:16:33 ago, Change count: 1, Ours

[edit]
admin@R1# run show ospf database advertising-router self lsa-id 10.43.4.0 extensive

OSPF database, Area 0.0.0.10
Type ID Adv Rtr Seq Age Opt Cksum Len
Summary *10.43.4.0 10.255.0.1 0x80000002 189 0x22 0x9064 28
mask 255.255.255.0
Topology default (ID 0) -> Metric: 3
Gen timer 00:46:50
Aging timer 00:56:50
Installed 00:03:09 ago, expires in 00:56:51, sent 00:03:07 ago
Last changed 00:40:27 ago, Change count: 1, Ours


[/code]

R1 is present in Areas 10 and 20. It advertises it self in both Area 10 and area 20 as ABR (bit 0x1). In area 10 it sends a summary LSA with it's own ID as advertising router (send intra-area traffic for this network through me). This type 3 LSA reaches R3 R3 builds a route based on it.

Next questions will be more interesting as they start touching backbone area 0 :).

Quiz 3: What if link between R1 and R2 is down ? From Host A traffic to Host B will go via.... ?

Answer: HostA - R3 - R1 - VR1 - VR2 - R2 - R4 - HostB
[code]
HostA# traceroute 10.43.4.2
traceroute to 10.43.4.2 (10.43.4.2), 128 hops max, 52 byte packets
1 10.43.3.1 (10.43.3.1) 10.864 ms 4.013 ms 6.928 ms
2 10.42.6.2 (10.42.6.2) 6.692 ms 8.018 ms 8.881 ms
3 10.42.1.2 (10.42.1.2) 10.131 ms 8.800 ms 6.316 ms
4 10.42.2.2 (10.42.2.2) 15.703 ms 10.183 ms 10.739 ms
5 10.42.3.1 (10.42.3.1) 17.160 ms 16.460 ms 19.429 ms
6 10.42.7.2 (10.42.7.2) 17.031 ms 11.959 ms 13.024 ms
7 10.43.4.2 (10.43.4.2) 16.934 ms 11.868 ms 15.137 ms
[/code]

Interesting. Again I'm happy all works. But why does it work ?

This answer is obvious. It's traffic from area 10 via area 0 to area 20.

Quiz 4: What if all links are up except for link between VR2 and R2 ? From VR1 loopback traffic to Host B will go via.... ?

For curiosity sake, before this question is answered, I will show a traceroute from VR1 loopback to HostB:
[code]
[edit]
admin@R1# run traceroute 10.43.4.2 routing-instance VR1 source 10.255.0.11
traceroute to 10.43.4.2 (10.43.4.2) from 10.255.0.11, 30 hops max, 40 byte packets
1 10.42.2.2 (10.42.2.2) 5.640 ms 8.252 ms 6.458 ms
2 10.42.3.1 (10.42.3.1) 8.924 ms 9.235 ms 5.363 ms
3 10.42.7.2 (10.42.7.2) 12.476 ms 10.405 ms 9.979 ms
4 10.43.4.2 (10.43.4.2) 8.910 ms 6.221 ms 6.266 ms
[/code]
This path is VR1 - VR2 - R2 - R4 - HostB. Why ? Because VR1 has an idea about 10.43.4.0/24 network via a type 3 LSA - Summary which was injected into Area 0 by VR2 and in this case.

To satisfy even more curious minds: type 3 LSA reaches VR1 via Area 0 from VR2 and via area 10 from R1. VR1 also converts the LSA from area 0 and injects it into area 10. Result ? LSA loop. This LSA will be present in area 10 from R1 and from VR1:
[code]
[edit]
admin@R1# run show ospf database instance VR1 lsa-id 10.43.4.0

OSPF database, Area 0.0.0.0
Type ID Adv Rtr Seq Age Opt Cksum Len
Summary 10.43.4.0 10.255.0.12 0x80000001 159 0x22 0x509a 28

OSPF database, Area 0.0.0.10
Type ID Adv Rtr Seq Age Opt Cksum Len
Summary 10.43.4.0 10.255.0.1 0x80000001 275 0x22 0x9263 28
Summary *10.43.4.0 10.255.0.11 0x80000001 157 0x22 0x608a 28
[/code]


Now, for the answer: They don't go anywhere and packets are dropped :) Finally I am trully happy (not because of this because we reached a point where this design starts to show flaws.

Let's see why. After link between VR2 and R2 fails, area 0 is isolated from area 20 (where this 10.43.4.0/24 subnet is present).
Initially, my questions were: why ? What ? HM .. ?

Let's see, VR1 has a summary LSA for 10.43.4.0 via area 10:
[code]
[edit]
admin@R1# run show ospf database instance VR1 lsa-id 10.43.4.0

OSPF database, Area 0.0.0.10
Type ID Adv Rtr Seq Age Opt Cksum Len
Summary 10.43.4.0 10.255.0.1 0x80000001 603 0x22 0x9263 28
[/code]

Why isn't it using it to construct a route:
[code]
[edit]
admin@R1# run show route 10.43.4.2 table VR1.inet.0

VR1.inet.0: 29 destinations, 45 routes (29 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

0.0.0.0/0 *[Aggregate/130] 1w1d 18:52:03, metric2 0
> to 1.1.1.1 via ge-0/0/1.0
to 1.1.2.1 via ge-0/0/2.0
[/code]
This points to outside the AS.

Since this network is received ONLY via non-backbone area by VR1 through a summary LSA (VR2 stopped sending the type 3 LSA from area 20 because link between VR2 and R2 failed) and VR1 has a full adjacency on the backbone still, it ignores these summaries.
How do we fix this ? Virtual links (Control plane), tunnels (forwarding plane).


[code]
[edit]
admin@R1# run show ospf neighbor instance VR1
Address Interface State ID Pri Dead
10.42.2.2 ge-0/0/6.0 Full 10.255.0.12 128 30
10.42.1.1 ge-0/0/5.0 Full 10.255.0.1 128 37

[edit]
admin@R1# run show ospf neighbor instance VR1
Address Interface State ID Pri Dead
10.42.1.1 ge-0/0/5.0 Full 10.255.0.1 128 31
[edit]
admin@R1# run show route 10.43.4.2

inet.0: 24 destinations, 24 routes (24 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

10.43.4.0/24 *[OSPF/10] 00:31:50, metric 3
> to 10.42.4.1 via ge-0/0/7.0

VR1.inet.0: 36 destinations, 40 routes (36 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

10.43.4.0/24 *[OSPF/10] 00:13:55, metric 4
> to 10.42.1.1 via ge-0/0/5.0
[/code]

So now VR1 has no adjacency in area 0, Only in area 10 and doesn't ignore tye summary lsa from R1, but traffic is still not flowing:
[code]
[edit]
admin@R1# run traceroute 10.43.4.2 routing-instance VR1 source 10.255.0.11 wait 1
traceroute to 10.43.4.2 (10.43.4.2) from 10.255.0.11, 30 hops max, 40 byte packets
1 10.42.1.1 (10.42.1.1) 10.165 ms 7.935 ms 9.783 ms
2 * * *
3 *
[/code]

Hmm so it leaves VR1, R1 and nothig from R2. I look on R1 to see what does it's database contain:
[code]
[edit]
admin@R1# run show ospf database

OSPF database, Area 0.0.0.10
Type ID Adv Rtr Seq Age Opt Cksum Len
Router *10.255.0.1 10.255.0.1 0x8000000c 1316 0x22 0x5194 72
Router 10.255.0.3 10.255.0.3 0x8000000b 1378 0x22 0x909f 60
Router 10.255.0.11 10.255.0.11 0x80000008 1617 0x22 0x99f8 36
Network 10.42.1.2 10.255.0.11 0x80000007 508 0x22 0x7a50 32
Network *10.42.6.2 10.255.0.1 0x80000006 1504 0x22 0x399f 32
Summary 10.42.2.0 10.255.0.11 0x80000008 965 0x22 0x5693 28
Summary *10.42.3.0 10.255.0.1 0x80000003 1524 0x22 0x9b5b 28
Summary *10.42.4.0 10.255.0.1 0x80000003 2039 0x22 0x8077 28
Summary *10.42.7.0 10.255.0.1 0x80000002 304 0x22 0x5f97 28
Summary 10.42.25.0 10.255.0.3 0x80000002 2033 0x22 0x944c 28
Summary *10.43.4.0 10.255.0.1 0x80000002 122 0x22 0x9064 28
Summary *10.255.0.2 10.255.0.1 0x80000001 2039 0x22 0x9c88 28
Summary *10.255.0.4 10.255.0.1 0x80000001 2039 0x22 0x928f 28
Summary 10.255.0.11 10.255.0.11 0x80000007 666 0x22 0xef1d 28
Summary *172.16.4.0 10.255.0.1 0x80000001 2039 0x22 0x95d8 28

OSPF database, Area 0.0.0.20
Type ID Adv Rtr Seq Age Opt Cksum Len
Router *10.255.0.1 10.255.0.1 0x8000000c 2040 0x22 0x4e51 36
Router 10.255.0.2 10.255.0.2 0x80000017 1525 0x22 0x7e0d 84
Router 10.255.0.4 10.255.0.4 0x8000000d 1465 0x22 0x3422 72
Router 10.255.0.12 10.255.0.12 0x8000000c 1925 0x22 0xa3e5 36
Network 10.42.4.1 10.255.0.2 0x80000004 343 0x22 0x518a 32
Network 10.42.7.1 10.255.0.2 0x80000006 1053 0x22 0x4a8b 32
Summary *10.42.1.0 10.255.0.1 0x80000007 1883 0x22 0x8d6b 28
Summary 10.42.1.0 10.255.0.12 0x80000006 1953 0x22 0x5796 28
Summary *10.42.6.0 10.255.0.1 0x80000007 941 0x22 0x569d 28
Summary 10.42.6.0 10.255.0.12 0x80000006 1811 0x22 0x2abd 28
Summary 10.42.25.0 10.255.0.4 0x80000005 2037 0x22 0x8854 28
Summary *10.43.3.0 10.255.0.1 0x80000003 1693 0x22 0x8f66 28
Summary 10.43.3.0 10.255.0.12 0x80000003 1669 0x22 0x6187 28
Summary *10.255.0.1 10.255.0.1 0x80000008 754 0x22 0x8e91 28
Summary *10.255.0.3 10.255.0.1 0x80000006 1129 0x22 0x8896 28
Summary 10.255.0.12 10.255.0.12 0x80000007 2545 0x22 0xdf2b 28
Summary *10.255.0.255 10.255.0.1 0x80000008 567 0x22 0x9888 28
[/code]

So R1 has a Summary LSA ID 10.255.0.11 from VR1 on Area 10, but it does not send it to area 20.

Conclussion one: OSPF (I know Cisco and find out now also Juniper) does not use a type 3 LSA from non-backbone area to construct another type 3 lsa in another non-backbone area.

What if I move VR1 loopback from area 0 into area 10 ?
[code]
[edit]
admin@R1# run show ospf database lsa-id 10.255.0.11 router extensive

OSPF database, Area 0.0.0.10
Type ID Adv Rtr Seq Age Opt Cksum Len
Router 10.255.0.11 10.255.0.11 0x80000009 23 0x22 0xb8b3 48
bits 0x1, link count 2
id 10.42.1.2, data 10.42.1.2, Type Transit (2)
Topology count: 0, Default metric: 1
id 10.255.0.11, data 255.255.255.255, Type Stub (3)
Topology count: 0, Default metric: 0
Topology default (ID 0)
Type: Transit, Node ID: 10.42.1.2
Metric: 1, Bidirectional
Aging timer 00:59:36
Installed 00:00:22 ago, expires in 00:59:37, sent 00:00:22 ago
Last changed 00:00:22 ago, Change count: 3

[edit]
admin@R1# run show ospf database

OSPF database, Area 0.0.0.10
Type ID Adv Rtr Seq Age Opt Cksum Len
Router *10.255.0.1 10.255.0.1 0x8000000c 1502 0x22 0x5194 72
Router 10.255.0.3 10.255.0.3 0x8000000b 1564 0x22 0x909f 60
Router 10.255.0.11 10.255.0.11 0x80000009 29 0x22 0xb8b3 48
Network 10.42.1.2 10.255.0.11 0x80000007 694 0x22 0x7a50 32
Network *10.42.6.2 10.255.0.1 0x80000006 1690 0x22 0x399f 32
Summary 10.42.2.0 10.255.0.11 0x80000008 1151 0x22 0x5693 28
Summary *10.42.3.0 10.255.0.1 0x80000003 1710 0x22 0x9b5b 28
Summary *10.42.4.0 10.255.0.1 0x80000003 2225 0x22 0x8077 28
Summary *10.42.7.0 10.255.0.1 0x80000002 490 0x22 0x5f97 28
Summary 10.42.25.0 10.255.0.3 0x80000002 2219 0x22 0x944c 28
Summary *10.43.4.0 10.255.0.1 0x80000002 308 0x22 0x9064 28
Summary *10.255.0.2 10.255.0.1 0x80000002 125 0x22 0x9a89 28
Summary *10.255.0.4 10.255.0.1 0x80000001 2225 0x22 0x928f 28
Summary *172.16.4.0 10.255.0.1 0x80000001 2225 0x22 0x95d8 28

OSPF database, Area 0.0.0.20
Type ID Adv Rtr Seq Age Opt Cksum Len
Router *10.255.0.1 10.255.0.1 0x8000000c 2226 0x22 0x4e51 36
Router 10.255.0.2 10.255.0.2 0x80000017 1711 0x22 0x7e0d 84
Router 10.255.0.4 10.255.0.4 0x8000000d 1651 0x22 0x3422 72
Router 10.255.0.12 10.255.0.12 0x8000000c 2111 0x22 0xa3e5 36
Network 10.42.4.1 10.255.0.2 0x80000004 529 0x22 0x518a 32
Network 10.42.7.1 10.255.0.2 0x80000006 1239 0x22 0x4a8b 32
Summary *10.42.1.0 10.255.0.1 0x80000007 2069 0x22 0x8d6b 28
Summary 10.42.1.0 10.255.0.12 0x80000006 2139 0x22 0x5796 28
Summary *10.42.6.0 10.255.0.1 0x80000007 1127 0x22 0x569d 28
Summary 10.42.6.0 10.255.0.12 0x80000006 1997 0x22 0x2abd 28
Summary 10.42.25.0 10.255.0.4 0x80000005 2223 0x22 0x8854 28
Summary *10.43.3.0 10.255.0.1 0x80000003 1879 0x22 0x8f66 28
Summary 10.43.3.0 10.255.0.12 0x80000003 1855 0x22 0x6187 28
Summary *10.255.0.1 10.255.0.1 0x80000008 940 0x22 0x8e91 28
Summary *10.255.0.3 10.255.0.1 0x80000006 1315 0x22 0x8896 28
Summary *10.255.0.11 10.255.0.1 0x80000001 28 0x22 0x42d9 28
Summary 10.255.0.12 10.255.0.12 0x80000007 2731 0x22 0xdf2b 28
Summary *10.255.0.255 10.255.0.1 0x80000008 753 0x22 0x9888 28
[/code]

After this change, VR1 injects it's Router LSA into area 10 with loopback 10.255.0.11 (after moving loopback from area 0 to area 10).
Now R1 uses this destination and injects a Summary LSA into area 20.

[code]
admin@R1# run traceroute 10.43.4.2 routing-instance VR1 source 10.255.0.11 wait 1
traceroute to 10.43.4.2 (10.43.4.2) from 10.255.0.11, 30 hops max, 40 byte packets
1 10.42.1.1 (10.42.1.1) 3.893 ms 4.099 ms 4.096 ms
2 10.42.4.1 (10.42.4.1) 6.265 ms 6.445 ms 6.399 ms
3 10.42.7.2 (10.42.7.2) 10.237 ms 9.455 ms 8.363 ms
4 10.43.4.2 (10.43.4.2) 8.187 ms 8.460 ms 6.248 ms
[/code]

Traffic from VR1 loopback to HostB now flows via non backbone area: area 10 and area 20.


Conclussion finale: A destination network is injected in area 1 via Router LSA, injected into area 2 as summary LSA and is not sent into any other area that is non transit area. Destinations local to an area (present in Router LSA) are sent to a non-backbone router as Summary LSA as well as to backbone.