Categories R&S

OSPF – Implement and troubleshoot area types – Not-so-stubby area

In this article, we will dive into configuring not-so-stubby-area.

 

Theory:

 

NSSA uses type 7 LSAs in order to allow external network to be advertise into the area.

Type 7 LSAs are converting by the ABR into type 5 LSAs.

 

Requirements :

 

To demonstrate how OSPF not-so-stubby area  works, we first need to modify our configuration.

On R11, we remove interface lo0 from OSPF.

 

R11(config)#interface Loopback0
R11(config-if)#no ip ospf 10 area 100

 

Let’s create a route map and redistribute lo0 into OSPF.

 

R11(config)#route-map LO0_TO_OSPF
R11(config-route-map)#match interface loopback 0

R11(config)#router ospf 10
R11(config-router)#redistribute connected route-map LO0_TO_OSPF subnets

 

Currently OSPF area 100 is a totally stubby area, let’s see how the route to R11 lo0 and lo1 are seen from R9 and R1.

 

R9#sh ip route 111.11.11.11
% Subnet not in table
R9#sh ip route 111.111.111.111
Routing entry for 111.111.111.0/24
  Known via "ospf 10", distance 110, metric 21, type inter area
  Last update from 10.10.3.10 on Ethernet0/1, 00:05:07 ago
  Routing Descriptor Blocks:
  * 10.10.3.10, from 100.100.100.100, 00:05:07 ago, via Ethernet0/1
      Route metric is 21, traffic share count is 1

R1#sh ip route 111.11.11.11
% Subnet not in table
R1#sh ip route 111.111.111.111
Routing entry for 111.111.111.0/24
  Known via "ospf 10", distance 110, metric 31, type inter area
  Last update from 10.10.18.2 on Ethernet0/2, 00:05:33 ago

 

Checking at R11 ospf database, 111.11.11.11 is seen as a type 5 LSA so it is not redistribute into OSPF.

 

R11#sh ip ospf data ext 111.11.11.11
            OSPF Router with ID (111.111.111.111) (Process ID 10)
                Type-5 AS External Link States
  LS age: 382
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 111.11.11.11 (External Network Number )
  Advertising Router: 111.111.111.111
  LS Seq Number: 80000001
  Checksum: 0xC38E
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 20
        Forward Address: 0.0.0.0
        External Route Tag: 0

 

Diagram:

OSPF - Implement and troubleshoot area types - Not-so-stubby area

 

Configuration and verification:

 

Now let’s solve this by configuring area 100 as a NSSA.

 

R10(config)#router ospf 10
R10(config-router)#no area 100 stub no-summary
R10(config-router)#no area 100 stub
R10(config-router)#area 100 nssa

R11(config)#router ospf 10
R11(config-router)#no area 100 stub no-summary
R11(config-router)#no area 100 stub
R11(config-router)#area 100 nssa

 

We now see area 100 as a NSSA, R10 is performing type-7/type-5 LSA translation.

 

R10#sh ip ospf 10
 Routing Process "ospf 10" with ID 100.100.100.100
 Start time: 00:00:22.413, Time elapsed: 00:39:55.311
SNIP
    Area 100
        Number of interfaces in this area is 1
        It is a NSSA area
        Perform type-7/type-5 LSA translation

R11#sh ip ospf 10
 Routing Process "ospf 10" with ID 111.111.111.111
 Start time: 00:00:21.845, Time elapsed: 00:40:54.676
SNIP
    Area 100
        Number of interfaces in this area is 2
        It is a NSSA area
        Area has no authentication
        SPF algorithm last executed 00:02:23.289 ago

R11 routing table is now showing inter area route, but there is no more a default route, thus we can’t reach external network like 44.44.44.44. We will fix this after.

 

R11#sh ip route
Gateway of last resort is not set
      1.0.0.0/32 is subnetted, 1 subnets
O IA     1.1.1.1 [110/31] via 10.10.101.1, 00:04:11, Ethernet0/0
      2.0.0.0/32 is subnetted, 1 subnets
O IA     2.2.2.2 [110/41] via 10.10.101.1, 00:04:11, Ethernet0/0
      3.0.0.0/32 is subnetted, 1 subnets
O IA     3.3.3.3 [110/31] via 10.10.101.1, 00:04:11, Ethernet0/0
      4.0.0.0/32 is subnetted, 1 subnets
O IA     4.4.4.4 [110/41] via 10.10.101.1, 00:04:11, Ethernet0/0
      5.0.0.0/32 is subnetted, 1 subnets
O IA     5.5.5.5 [110/51] via 10.10.101.1, 00:04:11, Ethernet0/0
      6.0.0.0/32 is subnetted, 1 subnets
R11#sh ip route 44.44.44.44
% Network not in table

 

In R11 OSPF database, we now have a new LSA which is type 7.

 

R11#sh ip ospf data
SNIP

                Type-7 AS External Link States (Area 100)

Link ID         ADV Router      Age         Seq#       Checksum Tag
111.11.11.11    111.111.111.111 435         0x80000001 0x00F297 0

                Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag
111.11.11.11    111.111.111.111 1159        0x80000001 0x00C38E 0

 

On R10, the route to 111.11.11.11 is seen as an N2 (OSPF NSSA external type 2) route.

 

R10#sh ip route 111.11.11.11
Routing entry for 111.11.11.11/32
  Known via "ospf 10", distance 110, metric 20, type NSSA extern 2, forward metric 11
  Last update from 10.10.101.2 on Ethernet0/2, 00:13:03 ago
  Routing Descriptor Blocks:
  * 10.10.101.2, from 111.111.111.111, 00:13:03 ago, via Ethernet0/2
      Route metric is 20, traffic share count is 1

 

The routing bit is set on the Type 7 LSA.

 

R10#sh ip ospf data nssa-external 111.11.11.11
            OSPF Router with ID (100.100.100.100) (Process ID 10)
                Type-7 AS External Link States (Area 100)
  Routing Bit Set on this LSA in topology Base with MTID 0
  LS age: 851
  Options: (No TOS-capability, Type 7/5 translation, DC, Upward)
  LS Type: AS External Link
  Link State ID: 111.11.11.11 (External Network Number )
  Advertising Router: 111.111.111.111
  LS Seq Number: 80000001
  Checksum: 0xF297
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 20
        Forward Address: 111.111.111.111
        External Route Tag: 0

 

R10 is performing the type 7 to type 5 translation, so on R9 we see the route as an OSPF external type 2.

 

R9#sh ip route 111.11.11.11
Routing entry for 111.11.11.11/32
  Known via "ospf 10", distance 110, metric 20, type extern 2, forward metric 21
  Last update from 10.10.3.10 on Ethernet0/1, 00:17:28 ago
  Routing Descriptor Blocks:
  * 10.10.3.10, from 100.100.100.100, 00:17:28 ago, via Ethernet0/1
      Route metric is 20, traffic share count is 1

R9#sh ip ospf data ext 111.11.11.11
            OSPF Router with ID (99.99.99.99) (Process ID 10)
                Type-5 AS External Link States
  Routing Bit Set on this LSA in topology Base with MTID 0
  LS age: 1079
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 111.11.11.11 (External Network Number )
  Advertising Router: 100.100.100.100
  LS Seq Number: 80000001
  Checksum: 0xD2ED
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 20
        Forward Address: 111.111.111.111
        External Route Tag: 0

 

R1 is also seeing the route to 111.11.11.11 as an OSPF external type 2.

 

R1#sh ip route 111.11.11.11
Routing entry for 111.11.11.11/32
  Known via "ospf 10", distance 110, metric 20, type extern 2, forward metric 31
  Last update from 10.10.18.2 on Ethernet0/2, 00:29:35 ago
  Routing Descriptor Blocks:
  * 10.10.18.2, from 100.100.100.100, 00:29:35 ago, via Ethernet0/2
      Route metric is 20, traffic share count is 1

 

We have seen earlier that we don’t have a default route originated by the ABR on R11 anymore.

R11 have no reachability outside of the OSPF domain.

 

R11#ping 44.44.44.44
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 44.44.44.44, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
R11#sh ip route 44.44.44.44
% Network not in table

 

We can add the default-information-originate option on R10 in order to originate a default route as an OSPF NSSA external type 2.

 

R10(config)#router ospf 10
R10(config-router)#area 100 nssa default-information-originate

 

We now have a default route on R11 and we get reachability outside of the OSPF domain.

 

R11#sh ip route
Gateway of last resort is 10.10.101.1 to network 0.0.0.0
O*N2  0.0.0.0/0 [110/1] via 10.10.101.1, 00:00:19, Ethernet0/0
      1.0.0.0/32 is subnetted, 1 subnets

R11#ping 44.44.44.44
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 44.44.44.44, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/5 ms

 

On R10, we can see that it is generating the NSSA default route.

 

R10#sh ip ospf 10
 Routing Process "ospf 10" with ID 100.100.100.100
 Start time: 00:00:22.413, Time elapsed: 03:29:04.978
SNIP
    Area 100
        Number of interfaces in this area is 1
        It is a NSSA area
        Perform type-7/type-5 LSA translation
        Generates NSSA default route with cost 1

 

 

Adding the default-information-originate option is one way to get reachability outside of the OSPF domain, we will see in our next section that we can use another area type to get this working.

 

 

Thank you for reading.

 

OSPF – Implement and troubleshoot area types – Not-so-stubby area