Categories R&S

OSPF – Implement and troubleshoot area types – Stub area

I will start looking at the first type of OSPF areas, the stub area.

 

Theory:

 

Stub area will remove type 5 external LSA from the OSPF database. This type 5 LSA will be replace by a default route.

OSPF stub area are mainly use to reduce the topological database size inside an area and reduce the memory requirements of routers inside that area.

 

Requirements:

Disable R11 link to R7.

Configure OSPF area 100 as stub area.

 

Diagram:

OSPF - Implement and troubleshoot area types - Stub area

 

Configuration and verification:

 

Let’s first check R11 routing table.

R11#sh ip route ospf

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, 00:03:54, Ethernet0/0
      2.0.0.0/32 is subnetted, 1 subnets
O IA     2.2.2.2 [110/41] via 10.10.101.1, 00:04:35, Ethernet0/0
      3.0.0.0/32 is subnetted, 1 subnets
O IA     3.3.3.3 [110/31] via 10.10.101.1, 00:04:35, Ethernet0/0
      4.0.0.0/32 is subnetted, 1 subnets
O IA     4.4.4.4 [110/41] via 10.10.101.1, 00:04:35, Ethernet0/0
      5.0.0.0/32 is subnetted, 1 subnets
O IA     5.5.5.5 [110/51] via 10.10.101.1, 00:04:35, Ethernet0/0
      6.0.0.0/32 is subnetted, 1 subnets
O IA     6.6.6.6 [110/51] via 10.10.101.1, 00:04:35, Ethernet0/0
      7.0.0.0/32 is subnetted, 1 subnets
O IA     7.7.7.7 [110/61] via 10.10.101.1, 00:04:35, Ethernet0/0
      8.0.0.0/32 is subnetted, 1 subnets
O IA     8.8.8.8 [110/21] via 10.10.101.1, 00:04:35, Ethernet0/0
      9.0.0.0/32 is subnetted, 1 subnets
O IA     9.9.9.9 [110/21] via 10.10.101.1, 00:04:35, Ethernet0/0
      10.0.0.0/8 is variably subnetted, 12 subnets, 3 masks
O IA     10.10.1.0/24 [110/40] via 10.10.101.1, 00:04:35, Ethernet0/0
O IA     10.10.2.0/24 [110/50] via 10.10.101.1, 00:04:35, Ethernet0/0
O IA     10.10.3.0/24 [110/20] via 10.10.101.1, 00:04:35, Ethernet0/0
O IA     10.10.10.10/32 [110/11] via 10.10.101.1, 00:04:35, Ethernet0/0
O IA     10.10.14.0/30 [110/40] via 10.10.101.1, 00:04:35, Ethernet0/0
O IA     10.10.18.0/30 [110/30] via 10.10.101.1, 00:04:35, Ethernet0/0
O IA     10.10.24.0/30 [110/50] via 10.10.101.1, 00:04:35, Ethernet0/0
O IA     10.10.39.0/30 [110/30] via 10.10.101.1, 00:04:35, Ethernet0/0
O IA     10.10.67.0/30 [110/60] via 10.10.101.1, 00:04:35, Ethernet0/0
O IA     10.10.100.0/30 [110/70] via 10.10.101.1, 00:04:35, Ethernet0/0
      11.0.0.0/24 is subnetted, 1 subnets
O IA     11.11.11.0 [110/31] via 10.10.101.1, 00:04:35, Ethernet0/0
      22.0.0.0/24 is subnetted, 1 subnets
O IA     22.22.22.0 [110/41] via 10.10.101.1, 00:04:35, Ethernet0/0
      33.0.0.0/24 is subnetted, 1 subnets
O IA     33.33.33.0 [110/31] via 10.10.101.1, 00:04:35, Ethernet0/0
      44.0.0.0/24 is subnetted, 1 subnets
O E2     44.44.44.0 [110/20] via 10.10.101.1, 00:04:30, Ethernet0/0
      55.0.0.0/24 is subnetted, 1 subnets
O E2     55.55.55.0 [110/20] via 10.10.101.1, 00:04:30, Ethernet0/0
      66.0.0.0/24 is subnetted, 1 subnets
O E2     66.66.66.0 [110/20] via 10.10.101.1, 00:04:35, Ethernet0/0
      77.0.0.0/24 is subnetted, 1 subnets
O IA     77.77.77.0 [110/61] via 10.10.101.1, 00:04:35, Ethernet0/0
      88.0.0.0/24 is subnetted, 1 subnets
O IA     88.88.88.0 [110/21] via 10.10.101.1, 00:04:35, Ethernet0/0
      99.0.0.0/24 is subnetted, 1 subnets
O IA     99.99.99.0 [110/21] via 10.10.101.1, 00:04:35, Ethernet0/0
      100.0.0.0/24 is subnetted, 1 subnets
O IA     100.100.100.0 [110/11] via 10.10.101.1, 00:04:35, Ethernet0/0

We can see OSPF routes (O), OSPF intra-area routes (O IA) and OSPF external-area routes (O E2).

 

Let’s check the OSPF database for the external route 44.44.44.0/24 (to R4).

R11#sh ip ospf database external 44.44.44.0

            OSPF Router with ID (111.111.111.111) (Process ID 10)

                Type-5 AS External Link States

  Routing Bit Set on this LSA in topology Base with MTID 0
  LS age: 1714
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 44.44.44.0 (External Network Number )
  Advertising Router: 44.44.44.44
  LS Seq Number: 80000001
  Checksum: 0x84E6
  Length: 36
  Network Mask: /24
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 20
        Forward Address: 0.0.0.0
        External Route Tag: 0  Network Mask: /24
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 20
        Forward Address: 0.0.0.0
        External Route Tag: 0

 

We are now configuring area 100 as a stub area, note that all routers in the stub area have to be configured as stub router.

R10(config)#router ospf 10
R10(config-router)#area 100 stub

R11(config)#router ospf 10
R11(config-router)#area 100 stub

 

Area 100 is now configure as a stub area.

R10#sh ip ospf
 Routing Process "ospf 10" with ID 100.100.100.100
SNIP
    Area 100
        Number of interfaces in this area is 1
        It is a stub area
        Generates stub default route with cost 1

R11#sh ip ospf
 Routing Process "ospf 10" with ID 111.111.111.111
SNIP
    Area 100
        Number of interfaces in this area is 1
        It is a stub area

 

How is looking the routing table and the OSPF database?

R11#sh ip route ospf | i E2
       E1 - OSPF external type 1, E2 - OSPF external type 2
R11#

R11#sh ip ospf database external 44.44.44.0

            OSPF Router with ID (111.111.111.111) (Process ID 10)

                Type-5 AS External Link States

  LS age: 1834
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 44.44.44.0 (External Network Number )
  Advertising Router: 44.44.44.44
  LS Seq Number: 80000001
  Checksum: 0x84E6
  Length: 36
  Network Mask: /24
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 20
        Forward Address: 0.0.0.0
        External Route Tag: 0

There is no more external routes in the routing table and the OSPF database is not showing that the routing bit is set.

 

Instead, we now have a default route entry in the routing table and a type-3 summary LSA via the ABR.

R11#sh ip route

Gateway of last resort is 10.10.101.1 to network 0.0.0.0

O*IA  0.0.0.0/0 [110/11] via 10.10.101.1, 00:02:27, Ethernet0/0

R11#sh ip ospf database summary 0.0.0.0

            OSPF Router with ID (111.111.111.111) (Process ID 10)

                Summary Net Link States (Area 100)

  Routing Bit Set on this LSA in topology Base with MTID 0
  LS age: 232
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 0.0.0.0 (summary Network Number)
  Advertising Router: 100.100.100.100
  LS Seq Number: 80000001
  Checksum: 0xEDBE
  Length: 28
  Network Mask: /0
        MTID: 0         Metric: 1

 

What could go wrong with OSPF stub area?

 

The stub area is not configured on all the routers in the same area:

Let’s try this by removing the stub command on R10.

R10(config-router)#no area 100 stub

*Sep  4 09:08:29.151: %OSPF-5-ADJCHG: Process 10, Nbr 111.111.111.111 on Ethernet0/2 from FULL to DOWN, Neighbor Down: Adjacency forced to reset

 

The adjacency with R11 is now down and it is not coming back up.

R10#sh ip ospf neigh
111.111.111.111   0   DOWN/  -           -        10.10.101.2     Ethernet0/2

 

 

In the next post we will see another type of OSPF area, the totally stubby area.

 

Thank you for reading.

 

 

OSPF – Implement and troubleshoot area types – Stub area