This article is the second (but not the last) part of the OSPF lab configuration presented in the previous article(Configuring OSPF in a single area). This time we will configure the "secondary" areas (area 10 and 23) leaving area 30 next time.
The program of this part:
  • Multi-area configuration 
  •  EIGRP Basic Redistribution in OSPF 
  •  Area Totally NSSA.
Topology of How to configure Multi-area OSPF

Multi-area OSPF
Multi-area OSPF

First stage
Let's start with the simple part, the configuration of the area 23. Here the only thing we have to do for the moment is to integrate the interface Fa0 / 1 of ABR2 into the area 23 as well as the interface F0 / 1 of ABR3.
For now, we will not do anything else here. We will return to this area when configuring a virtual-link for the area 30 in the next article.

On ABR2

ABR2(config)#router ospf 1
ABR2(config-router)#network 192.168.23.0 0.0.0.255 area 23
ABR2(config-router)#exit
ABR2(config)#

On ABR3

ABR3(config)#router ospf 1
ABR3(config-router)#router-id 4.4.4.4
ABR3(config-router)#network 192.168.23.0 0.0.0.255 area 23
ABR3(config-router)#exit
ABR3(config)#

Now ABR2 and ABR3 have established an adjacency relation in area 23. Let us check this.

Show ip ospf neighbors

ABR2#show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
4.4.4.4           1   FULL/BDR        00:00:39    192.168.23.254  FastEthernet0/1
ABR2#

ABR2 now has an interface configured in area 23 and 3 interfaces in area0, which makes it an Area Border Router (ABR), ie a router located at the border of two areas.

The delimitation of the network into areas makes it possible to reduce the size of the OSPF topology on each router, which of course makes it possible to considerably reduce the time required to recalculate a route in the event of a change in the network.

Another important point, the ABRs (and the ASBRs which we will talk about later) are the only points of the network where it can define a summary for OSPF.

Second step
Let's now move to area 10, which closely resembles area 23, except that ASBR is at the boundary between the OSPF (Autonomous System) AS ... ie the OSPF domain and the AS EIGRP. We're going to set up two things here.
Firstly, area 10 is sort of a cul-de-sac for OSPF, which will allow us to define it as a "stub" area, and even "Totally stub", which will have the effect of avoiding Propagation of some OSPF information within the area. In this case (Totally Stub), the Type 3 (Summary) and Type 5 (External Routes) LSAs will be blocked.

We will just have to bring a nuance to that. ASBR will have to redistribute the EIGRP routes in OSPF, which therefore creates external routes (ie type 5 LSAs), except this is not possible in a Totally Stub area, so we will make it a " Totally NSSA ", which will have the effect of generating LSAs of type 7 which will be propagated in the area 10 and which will be retransformed into LSA type 5 when passing through the area 0.

On ABR1

ABR1(config)#interface serial 0/2
ABR1(config-if)#bandwidth 128
ABR1(config-if)#exit
ABR1(config)#router ospf 1
ABR1(config-router)#network 192.168.10.0 0.0.0.3 area 10
ABR1(config-router)#area 10 nssa no-summary
ABR1(config-router)#exit
ABR1(config)#

About R1

R1(config)#interface serial 0/0
R1(config-if)#bandwidth 128
R1(config-if)#exit
R1(config)#interface serial 0/1
R1(config-if)#bandwidth 128
R1(config-if)#exit
R1(config)#router ospf 1
R1(config-router)#network 192.168.10.0 0.0.0.3 area 10
R1(config-router)#network 192.168.10.4 0.0.0.3 area 10
R1(config-router)#area 10 nssa no-summary
R1(config-router)#exit
R1(config)#

About ASBR

ASBR(config)#interface serial 0/0
ASBR(config-if)#bandwidth 128
ASBR(config-if)# exit
ASBR(config)#router ospf 1
ASBR(config-router)#router-id 7.7.7.7
ASBR(config-router)#network 192.168.10.4 0.0.0.3 area 10
ASBR(config-router)#area 10 nssa no-summary
ASBR(config-router)#redistribute eigrp 100 subnets metric 1 metric-type 1
ASBR(config-router)#exit
ASBR(config)#

When configuring a stub area (or totally stub etc), this must be done on all routers in the area, otherwise the adjacency will end and the routes will no longer be exchanged.
The redistribution of EIGRP in OSPF uses three options:
  • Subnets : OSPF force to redistribute subnets, otherwise OSPF would announce class-full networks from redistribution. 
  • Metric 1 : defined the base metric of redistributed routes to 1 (I took a route for a FastEthernet network as a base) 
  • Metric-type 1 : define the routes distributed as an E1 type, which will cause each router to add its own cost to the route metric by putting it in its routing table, unlike an E2 route whose Metric will be constant at any point in the network.

The configuration of area 10 is now complete. Let's look at the result ...
Show ip route (on BBR1)

BBR1>sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     192.168.10.0/30 is subnetted, 2 subnets
O IA    192.168.10.0 [110/782] via 10.0.0.3, 00:29:02, FastEthernet0/0
O IA    192.168.10.4 [110/1563] via 10.0.0.3, 00:12:18, FastEthernet0/0
     10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks
O       10.0.1.8/30 [110/782] via 10.0.0.3, 00:31:06, FastEthernet0/0
                    [110/782] via 10.0.0.2, 00:31:06, FastEthernet0/0
O       10.0.1.12/30 [110/782] via 10.0.0.4, 00:30:55, FastEthernet0/0
                     [110/782] via 10.0.0.2, 00:31:06, FastEthernet0/0
C       10.0.0.0/24 is directly connected, FastEthernet0/0
C       10.0.1.0/30 is directly connected, Serial0/0
C       10.0.1.4/30 is directly connected, Serial0/1
O IA 192.168.23.0/24 [110/11] via 10.0.0.4, 00:31:06, FastEthernet0/0
O E1 192.168.100.0/24 [110/1564] via 10.0.0.3, 00:09:19, FastEthernet0/0
BBR1>

BBR1 has therefore learned well the roads of the area 10 , as well as the road redistributed since EIGRP .
Now let's analyze the effect of the Totally NSSA area ...

Show ip route on R1

R1#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is 192.168.10.1 to network 0.0.0.0

     192.168.10.0/30 is subnetted, 2 subnets
C       192.168.10.0 is directly connected, Serial0/0
C       192.168.10.4 is directly connected, Serial0/1
O N1 192.168.100.0/24 [110/782] via 192.168.10.6, 00:13:08, Serial0/1
O*IA 0.0.0.0/0 [110/782] via 192.168.10.1, 00:16:08, Serial0/0
R1#

A default route! ... hey yes this is the main effect of a Totally Stub area. Since Type 3 LSAs are no longer propagated in the Totally Stub area, they must be replaced ... and what is more effective than a default route? ABR1 is therefore the default point of exit of area 10, which corresponds perfectly to the topology. The result is also visible on R1 in the OSPF database.

Show ip ospf database

R1#sh ip ospf database

            OSPF Router with ID (6.6.6.6) (Process ID 1)

                Router Link States (Area 10)

Link ID         ADV Router      Age         Seq#       Checksum Link count
2.2.2.2         2.2.2.2         1566        0x80000004 0x00D417 2
6.6.6.6         6.6.6.6         1418        0x80000007 0x00A5ED 4
7.7.7.7         7.7.7.7         1402        0x80000003 0x00F4C7 2

                Summary Net Link States (Area 10)

Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         2.2.2.2         1566        0x80000001 0x00FC31

                Type-7 AS External Link States (Area 10)

Link ID         ADV Router      Age         Seq#       Checksum Tag
192.168.100.0   7.7.7.7         1381        0x80000003 0x003F85 0
R1#

We see here that 2.2.2.2 (ABR1) announces a default network (0.0.0.0) . The only summary present in the database while in a "normal" area there would be at least one per remote area.

Moreover, since we are in an area stub, the LSAs of type 5 are blocked, (the LSAs which indicate the roads coming from a redistribution). Here, as we have defined the area in NSSA, LSAs type 5 are replaced by Type 7 (simple artifice to bypass the rule). These LSAs are pre- pared up to ABR1 which will convert them back into LSA type 5 in area 0.

Show ip ospf database on BBR1

BBR1#sh ip ospf database

            OSPF Router with ID (1.1.1.1) (Process ID 1)

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         1052        0x80000004 0x005CF1 5
2.2.2.2         2.2.2.2         87          0x80000006 0x001819 5
3.3.3.3         3.3.3.3         1248        0x80000009 0x00DF37 5
5.5.5.5         5.5.5.5         1042        0x80000005 0x002BE0 5

                Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
10.0.0.4        3.3.3.3         1248        0x80000004 0x005D8B

                Summary Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.10.0    2.2.2.2         1079        0x80000002 0x00B6FC
192.168.10.4    2.2.2.2         87          0x80000002 0x002C73
192.168.23.0    3.3.3.3         220         0x80000003 0x00DFC4

                Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag
192.168.100.0   2.2.2.2         1751        0x80000003 0x006A78 0
BBR1#

The area 0 being a standard area (it can not be otherwise), we find here an LSA type 5 for the redistributed road. Note also that it can be seen here that there is as much summary as there are networks in the other areas, these being announced in the form of Type 3 LSA by the ABRs.

So much for this second part. We still have a lot of work, so configuring OSPF on frame-relay in a Hub & Spoke topology, creating a virtual-link and then optimizing OSPF through summarizing the configuration Timers etc ...

          Also read Configuring OSPF in a Single Area

 
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