Example for Configuring an OSPF Stub Area

This section describes how to configure a stub area that imports static routes. Such configuration can reduce the number of LSAs advertised to this area without affecting route reachability.

Networking Requirements

On the network shown in Figure 1, all routers run OSPF, and the entire AS is divided into three areas. Device A and Device B function as ABRs to advertise inter-area routes; Device D functions as the ASBR to import external routes (static routes).

Configure Area 1 as a stub area to reduce the LSAs advertised to this area without affecting route reachability.

Figure 1 Networking for configuring an OSPF stub area

Interfaces 1 and 2 in this example represent GE 0/1/0 and GE 0/1/8, respectively.

Precautions

When configuring an OSPF stub area, note the following rules:

The backbone area cannot be configured as a stub area.
An ASBR cannot exist in a stub area. That is, AS-external routes are not flooded in the stub area.
A virtual link cannot pass through a stub area.

Configuration Roadmap

The configuration roadmap is as follows:

Configure basic OSPF functions on each router for interconnection.
Configure a static route on Device D and import it into OSPF.
Configure Area 1 as a stub area by running the stub command on all routers in Area 1 and check the OSPF routing information on Device C.
Prevent Device A from advertising Type 3 LSAs to the stub area, and check the OSPF routing information on Device C.

Data Preparation

To complete the configuration, you need the following data:

Device Name	Router ID	Process ID	IP Address
Device A	1.1.1.1	1	Area 0: 192.168.0.0/24 Area 1: 192.168.1.0/24
Device B	2.2.2.2	1	Area 0: 192.168.0.0/24 Area 2: 192.168.2.0/24
Device C	3.3.3.3	1	Area 1: 192.168.1.0/24 and 172.16.1.0/24
Device D	4.4.4.4	1	Area 2: 192.168.2.0/24 and 172.17.1.0/24
Device E	5.5.5.5	1	Area 1: 172.16.1.0/24
Device F	6.6.6.6	1	Area 2: 172.17.1.0/24

Procedure

Assign an IP address to each interface. For configuration details, see Configuration Files in this section.
Configure basic OSPF functions. For details, see Example for Configuring Basic OSPF Functions.

Configure Device D to import static routes.

[*DeviceD] ip route-static 10.0.0.0 8 null 0
[*DeviceD] ospf 1
[*DeviceD-ospf-1] import-route static type 1
[*DeviceD-ospf-1] commit
[~DeviceD-ospf-1] quit

# Display the ABR and ASBR information on Device C.

[~DeviceC] display ospf abr-asbr
          OSPF Process 1 with Router ID 3.3.3.3
                  Routing Table to ABR and ASBR
 Type        Destination       Area       Cost  NextHop         RtType
 Intra-area  1.1.1.1           0.0.0.1    1     192.168.1.1     ABR
 Inter-area  4.4.4.4           0.0.0.1    3     192.168.1.1     ASBR

# Display the OSPF routing table of Device C.

If the area where Device C resides is a common area, external routes exist in the routing table.

[~DeviceC] display ospf routing
          OSPF Process 1 with Router ID 3.3.3.3

                   Routing Tables

 Routing for Network
 Destination        Cost       Type       NextHop         AdvRouter       Area

 172.17.1.0/24      4          Inter-area 192.168.1.1     1.1.1.1         0.0.0.1
 192.168.0.0/24     2          Inter-area 192.168.1.1     1.1.1.1         0.0.0.1
 192.168.2.0/24     3          Inter-area 192.168.1.1     1.1.1.1         0.0.0.1

 Routing for ASEs
 Destination        Cost       Type       Tag        NextHop         AdvRouter

 10.0.0.0/8        4         Type1      1         192.168.1.1     4.4.4.4


 Total Nets: 4
 Intra Area: 0  Inter Area: 3  ASE: 1  NSSA: 0

Configure Area 1 as a stub area.

# Configure Device A.

[~DeviceA] ospf 1
[*DeviceA-ospf-1] area 1
[*DeviceA-ospf-1-area-0.0.0.1] stub
[*DeviceA-ospf-1-area-0.0.0.1] commit
[~DeviceA-ospf-1-area-0.0.0.1] quit

# Configure Device C.

[~DeviceC] ospf 1
[*DeviceC-ospf-1] area 1
[*DeviceC-ospf-1-area-0.0.0.1] stub
[*DeviceC-ospf-1-area-0.0.0.1] commit
[~DeviceC-ospf-1-area-0.0.0.1] quit

# Configure Device E.

[~DeviceE] ospf 1
[*DeviceE-ospf-1] area 1
[*DeviceE-ospf-1-area-0.0.0.1] stub
[*DeviceE-ospf-1-area-0.0.0.1] commit
[~DeviceE-ospf-1-area-0.0.0.1] quit

# Display the routing table on Device C.

After the area where Device C resides is configured as a stub area, a default route rather than AS external routes is displayed in the routing table.

[~DeviceC] display ospf routing
          OSPF Process 1 with Router ID 3.3.3.3

                   Routing Tables

 Routing for Network
 Destination        Cost       Type       NextHop         AdvRouter       Area
 0.0.0.0/0          2          Inter-area 192.168.1.1     1.1.1.1         0.0.0.1
 172.17.1.0/24      4          Inter-area 192.168.1.1     1.1.1.1         0.0.0.1
 192.168.0.0/24     2          Inter-area 192.168.1.1     1.1.1.1         0.0.0.1
 192.168.2.0/24     3          Inter-area 192.168.1.1     1.1.1.1         0.0.0.1

 Total Nets: 4
 Intra Area: 0  Inter Area: 4  ASE: 0  NSSA: 0

# Prevent Device A from advertising Type 3 LSAs to the stub area.

[~DeviceA] ospf
[*DeviceA-ospf-1] area 1
[*DeviceA-ospf-1-area-0.0.0.1] stub no-summary
[*DeviceA-ospf-1-area-0.0.0.1] commit
[~DeviceA-ospf-1-area-0.0.0.1] quit

Verify the configuration.

# Display the OSPF routing table on Device C.

[~DeviceC] display ospf routing
          OSPF Process 1 with Router ID 3.3.3.3

                   Routing Tables

 Routing for Network
 Destination        Cost       Type       NextHop         AdvRouter       Area
 0.0.0.0/0          2          Inter-area 192.168.1.1     1.1.1.1         0.0.0.1

 Total Nets: 1
 Intra Area: 0  Inter Area: 1  ASE: 0  NSSA: 0

After the advertisement of summary LSAs to the stub area is disabled, the number of routing entries on the routers in the stub area further decreases, and only the default route to a destination beyond the stub area is reserved.

Configuration Files

Device A configuration file

#
 sysname DeviceA
#
router id 1.1.1.1
#
interface GigabitEthernet0/1/0
 undo shutdown 
 ip address 192.168.0.1 255.255.255.0
#
interface GigabitEthernet0/1/8
 undo shutdown
 ip address 192.168.1.1 255.255.255.0
#
ospf 1
 area 0.0.0.0
  network 192.168.0.0 0.0.0.255
 area 0.0.0.1
  network 192.168.1.0 0.0.0.255
  stub no-summary
#
return

Device B configuration file

#
 sysname DeviceB
#
router id 2.2.2.2
#
interface GigabitEthernet0/1/0
 undo shutdown
ip address 192.168.0.2 255.255.255.0
#
interface GigabitEthernet0/1/8
 undo shutdown
 ip address 192.168.2.1 255.255.255.0
#
ospf 1
 area 0.0.0.0
  network 192.168.0.0 0.0.0.255
 area 0.0.0.2
  network 192.168.2.0 0.0.0.255
#
return

Device C configuration file

#
 sysname DeviceC
#
router id 3.3.3.3
#
interface GigabitEthernet0/1/0
 undo shutdown
 ip address 192.168.1.2 255.255.255.0
#
interface GigabitEthernet0/1/8
 undo shutdown
 ip address 172.16.1.1 255.255.255.0
#
ospf 1
 area 0.0.0.1
  network 192.168.1.0 0.0.0.255
  network 172.16.1.0 0.0.0.255
  stub
#
return

Device D configuration file

#
 sysname DeviceD
#
router id 4.4.4.4
#
interface GigabitEthernet0/1/0
 undo shutdown 
 ip address 192.168.2.2 255.255.255.0
#
interface GigabitEthernet0/1/8
 undo shutdown
 ip address 172.17.1.1 255.255.255.0
#
ospf 1
 import-route static type 1
 area 0.0.0.2
  network 192.168.2.0 0.0.0.255
  network 172.17.1.0 0.0.0.255
#
ip route-static 10.0.0.0 255.0.0.0 NULL0
#
return

Device E configuration file

#
 sysname DeviceE
#
router id 5.5.5.5
#
interface GigabitEthernet0/1/8
 undo shutdown
 ip address 172.16.1.2 255.255.255.0
#
ospf 1
 area 0.0.0.1
  network 172.16.1.0 0.0.0.255
  stub
#
return

Device F configuration file

#
 sysname DeviceF
#
router id 6.6.6.6
#
interface GigabitEthernet0/1/8
 undo shutdown
 ip address 172.17.1.2 255.255.255.0
#
ospf 1
 area 0.0.0.2
  network 172.17.1.0 0.0.0.255
#
return