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mandrei99
Post  Post subject: OSPF: Dangers of non-standard area design - Juniper way  |  Posted: Thu Nov 13, 2014 12:29 pm

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OSPF: Dangers of non-standard area design - Juniper way

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

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


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



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


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


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


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


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




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


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

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



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


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

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


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  *


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


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


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


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.





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