Example for Configuring an OSPF NSSA

This section describes how to configure an OSPF not-so-stubby area (NSSA).

Networking Requirements

An excessive number of entries in a routing table wastes network resources and causes high CPU usage. To solve this problem, a non-backbone area on the border of an AS can be configured as an NSSA to reduce the amount of routing information to be transmitted. An NSSA in an AS does not transmit routes learned from other areas in the AS but imports AS external routes. This reduces bandwidth and storage resource consumption.

On the network shown in Figure 1, OSPF runs on all routers and the entire AS is divided into two areas. Device A and Device B function as Area Border Routers (ABRs) to forward inter-area routes; Device D functions as an Autonomous System Boundary Router (ASBR) and imports the external static route 10.0.0.0/8. To import AS-external routes but reduce the number of Link State Advertisements (LSAs) advertised to area 1 without affecting route reachability, configure area 1 as an NSSA and configure Device A as an LSA translator in the NSSA.

Figure 1 Networking for configuring an OSPF NSSA

Interfaces 1 through 3 in this example represent GE 0/1/0, GE 0/1/8, and GE 0/1/16, respectively.

Configuration Roadmap

The configuration roadmap is as follows:

Enable OSPF on all routers and configure basic OSPF functions to ensure that routers communicate with each other using OSPF. For details, see Configuring Basic OSPF Functions.
Configure area 1 as an NSSA.
Configure Device D to import the static route 10.0.0.0/8.
Configure Device A in the NSSA as an LSA translator.

Data Preparation

To complete the configuration, you need the following data:

Router ID 1.1.1.1 of Device A; OSPF process ID 1; network segment 192.168.0.0/24 of area 0; network segments 192.168.1.0/24 and 192.168.3.0/24 of area 1
Router ID 2.2.2.2 of Device B; OSPF process ID 1; network segment 192.168.2.0/24 of area 0; network segments 192.168.1.0/24 and 192.168.4.0/24 of area 1
Router ID 3.3.3.3 of Device C; OSPF process ID 1; network segments 192.168.0.0/24 and 192.168.2.0/24 of area 0
Router ID 4.4.4.4 of Device D; OSPF process ID 1; network segments 192.168.3.0/24 and 192.168.4.0/24 of area 1

Procedure

Configure an IP address for each interface.
Configure an IP address for each interface as shown in Figure 1. For configuration details, see Configuration Files in this section.
Configure basic OSPF functions.
Configuring Basic OSPF Functions shows how to configure basic OSPF functions. For details about the configuration, see Configuration Files in this section.

Configure area 1 as an NSSA.

# Configure Device A.

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

# Configure Device B.

[~DeviceB] ospf
[*DeviceB-ospf-1] area 1
[*DeviceB-ospf-1-area-0.0.0.1] nssa
[*DeviceB-ospf-1-area-0.0.0.1] commit
[~DeviceB-ospf-1-area-0.0.0.1] quit

# Configure Device D.

[~DeviceD] ospf
[*DeviceD-ospf-1] area 1
[*DeviceD-ospf-1-area-0.0.0.1] nssa
[*DeviceD-ospf-1-area-0.0.0.1] commit
[~DeviceD-ospf-1-area-0.0.0.1] quit

NSSA attributes must be configured on all routers in the NSSA using the nssa command.

Configure Device D to import the static route 10.0.0.0/8.

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

# 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
 192.168.3.0/24     2     Inter-area 192.168.0.1     1.1.1.1       0.0.0.0
 192.168.4.0/24     2     Inter-area 192.168.2.1     2.2.2.2       0.0.0.0
 192.168.0.0/24     1     Stub       192.168.0.2     3.3.3.3       0.0.0.0
 192.168.1.0/24     2     Inter-area 192.168.0.1     1.1.1.1       0.0.0.0
 192.168.1.0/24     2     Inter-area 192.168.2.1     2.2.2.2       0.0.0.0
 192.168.2.0/24     1     Stub       192.168.2.2     3.3.3.3       0.0.0.0

 Routing for ASEs
 Destination      Cost      Type       Tag    NextHop         AdvRouter
 10.0.0.0/8       1         Type2      1      192.168.2.1     2.2.2.2

 Total Nets: 7
 Intra Area: 2  Inter Area: 4  ASE: 1  NSSA: 0

The command output shows that Device B has imported an AS external route and that the router ID of the router that advertises the imported AS external route is 2.2.2.2. Device B functions as the type-5 LSA translator because OSPF selects the ABR with a larger router ID as an LSA translator.

# Configure Device A as an LSA translator.

[~DeviceA] ospf
[*DeviceA-ospf-1] area 1
[*DeviceA-ospf-1-area-0.0.0.1] nssa default-route-advertise no-summary translator-always
[*DeviceA-ospf-1-area-0.0.0.1] commit
[~DeviceA-ospf-1-area-0.0.0.1] quit

Verify the configuration.

# View 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
 192.168.3.0/24     2     Inter-area 192.168.0.1     1.1.1.1       0.0.0.0
 192.168.4.0/24     2     Inter-area 192.168.2.1     2.2.2.2       0.0.0.0
 192.168.0.0/24     1     Stub       192.168.0.2     3.3.3.3       0.0.0.0
 192.168.1.0/24     2     Inter-area 192.168.2.1     2.2.2.2       0.0.0.0
 192.168.1.0/24     2     Inter-area 192.168.0.1     1.1.1.1       0.0.0.0
 192.168.2.0/24     1     Stub       192.168.2.2     3.3.3.3       0.0.0.0

 Routing for ASEs
 Destination      Cost      Type       Tag    NextHop         AdvRouter
 10.0.0.0/8       1         Type2      1      192.168.0.1     1.1.1.1

 Total Nets: 7
 Intra Area: 2  Inter Area: 4  ASE: 1  NSSA: 0

The command output shows that Device C has imported an AS external route and that the router ID of the router that advertises the imported AS external route is 1.1.1.1. Device A functions as the type-5 LSA translator.

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 192.168.3.1 255.255.255.0
#
interface GigabitEthernet0/1/16
 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
  network 192.168.3.0 0.0.0.255
  nssa default-route-advertise no-summary translator-always
#
return

Device B configuration file

#
 sysname DeviceB
#
router id 2.2.2.2
#
interface GigabitEthernet0/1/0
 undo shutdown
 ip address 192.168.1.2 255.255.255.0
#
interface GigabitEthernet0/1/8
 undo shutdown
 ip address 192.168.2.1 255.255.255.0
#
interface GigabitEthernet0/1/16
 undo shutdown
 ip address 192.168.4.2 255.255.255.0
#
ospf 1
 area 0.0.0.0
  network 192.168.2.0 0.0.0.255
 area 0.0.0.1
  network 192.168.1.0 0.0.0.255
  network 192.168.4.0 0.0.0.255
  nssa
#
return

Device C configuration file

#
 sysname DeviceC
#
router id 3.3.3.3
#
interface GigabitEthernet0/1/0
 undo shutdown
 ip address 192.168.0.2 255.255.255.0
#
interface GigabitEthernet0/1/8
 undo shutdown
 ip address 192.168.2.2 255.255.255.0
#
ospf 1
 area 0.0.0.0
  network 192.168.0.0 0.0.0.255
  network 192.168.2.0 0.0.0.255
#
return

Device D configuration file

#
 sysname DeviceD
#
router id 4.4.4.4
#
interface GigabitEthernet0/1/0
 undo shutdown
 ip address 192.168.3.2 255.255.255.0
#
interface GigabitEthernet0/1/8
 undo shutdown
 ip address 192.168.4.1 255.255.255.0
#
ospf 1
import-route static
 area 0.0.0.1
  network 192.168.3.0 0.0.0.255
  network 192.168.4.0 0.0.0.255
  nssa
#
ip route-static 10.0.0.0 255.0.0.0 NULL0
#
return