Example for Configuring an OSPF NSSA
Networking Requirements
Stub areas and NSSAs are described as follows:
Stub area
- Does not transmit external routes from other areas.
- Does not transmit AS external routes imported by ASBRs.
Type 5 LSAs are not transmitted in stub areas.
NSSA
Does not transmit external routes from other areas.
Transmits AS external routes imported by ASBRs.
Type 7 LSAs generated by ASBRs in an NSSA are transmitted only within the NSSA. After receiving the Type 7 LSAs, ABRs in the NSSA translate them into Type 5 LSAs to advertise external routes to other areas on the OSPF network.
Configuring an area as an NSSA prevents a large number of external routes from consuming the bandwidth and storage resources of the router.
In Figure 5-39, OSPF runs on all routers, and the entire AS is partitioned into two areas. Router A and Router B function as ABRs to forward inter-area routes; Router C and Router D function as ASBRs and import external static routes 10.0.0.0/8 and 20.0.0.0/8. To import AS external routes but reduce the number of LSAs to be advertised to area 1 without affecting route reachability, configure area 1 as an NSSA and configure Router A as an LSA translator in the NSSA.
Configuration Roadmap
The configuration roadmap is as follows:
- Enable OSPF and configure basic OSPF functions on each router
- Configure Area 1 as an NSSA.
- Configure Router D to import the static route 10.0.0.0/8.
- Configure Router C to import the static route 20.0.0.0/8.
- Configure Router A as an LSA translator in the NSSA.
- Disable Type 3 LSA advertisement to the NSSA to reduce the routing table size.
Data Preparation
To complete the configuration, you need the following data:
- Router ID 1.1.1.1 of Router 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 Router 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 Router 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 Router 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.
Assign an IP address to each interface according to Figure 5-39. 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 Router C to import the static route 20.0.0.0/8.
<Huawei> system-view [Huawei] sysname RouterC [RouterC] ip route-static 20.0.0.0 8 null 0 [RouterC] ospf 1 [RouterC-ospf-1] import-route static [RouterC-ospf-1] quit
- Configure Router D to import the static route 10.0.0.0/8.
[RouterD] ip route-static 10.0.0.0 8 null 0 [RouterD] ospf 1 [RouterD-ospf-1] import-route static [RouterD-ospf-1] quit
# Display the OSPF routing table of Router C.
[RouterC] display ospf routing OSPF Process 1 with Router ID 3.3.3.3 Routing Tables Routing for Network Destination Cost Type NextHop AdvRouter Area 192.168.0.0/24 1 Transit 192.168.0.2 3.3.3.3 0.0.0.0 192.168.2.0/24 1 Transit 192.168.2.2 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 0.0.0.0 192.168.3.0/24 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 Routing for ASEs Destination Cost Type Tag NextHop AdvRouter 10.0.0.0/8 1 Type2 1 192.168.0.1 2.2.2.2 Total Nets: 7 Intra Area: 2 Inter Area: 4 ASE: 1 NSSA: 0
After an NSSA is configured, the OSPF routing table of RouterRouter C contains the AS external static route 10.0.0.0/8 imported by Router D.
# Display the OSPF routing table of Router D.
[RouterD] display ospf routing OSPF Process 1 with Router ID 4.4.4.4 Routing Tables Routing for Network Destination Cost Type NextHop AdvRouter Area 192.168.3.0/24 1 Transit 192.168.3.2 4.4.4.4 0.0.0.1 192.168.4.0/24 1 Transit 192.168.4.1 4.4.4.4 0.0.0.1 192.168.0.0/24 2 Inter-area 192.168.3.1 1.1.1.1 0.0.0.1 192.168.1.0/24 2 Transit 192.168.4.2 2.2.2.2 0.0.0.1 192.168.1.0/24 2 Transit 192.168.3.1 2.2.2.2 0.0.0.1 192.168.2.0/24 2 Inter-area 192.168.4.2 2.2.2.2 0.0.0.1 Routing for ASEs Destination Cost Type Tag NextHop AdvRouter 20.0.0.0/8 1 Type2 1 192.168.3.1 1.1.1.1 Total Nets: 7 Intra Area: 2 Inter Area: 4 ASE: 1 NSSA: 0
After an NSSA is configured, the OSPF routing table of Router D contains the AS external static route 20.0.0.0/8 imported by Router C.
- Configure Area 1 as an NSSA.
# Configure Router A.
[RouterA] ospf 1 [RouterA-ospf-1] area 1 [RouterA-ospf-1-area-0.0.0.1] nssa
# Configure Router B.
[RouterB] ospf 1 [RouterB-ospf-1] area 1 [RouterB-ospf-1-area-0.0.0.1] nssa
# Configure Router D.
[RouterD] ospf 1 [RouterD-ospf-1] area 1 [RouterD-ospf-1-area-0.0.0.1] nssa
NSSA attributes must be configured on all routers in the NSSA using the nssa command.
# Display the OSPF routing table of Router C.
[RouterC] display ospf routing OSPF Process 1 with Router ID 3.3.3.3 Routing Tables Routing for Network Destination Cost Type NextHop AdvRouter Area 192.168.0.0/24 1 Transit 192.168.0.2 3.3.3.3 0.0.0.0 192.168.2.0/24 1 Transit 192.168.2.2 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 0.0.0.0 192.168.3.0/24 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 Routing for ASEs Destination Cost Type Tag NextHop AdvRouter 10.0.0.0/8 1 Type2 1 192.168.0.1 2.2.2.2 Total Nets: 7 Intra Area: 2 Inter Area: 4 ASE: 1 NSSA: 0
After an NSSA is configured, the OSPF routing table of Router C still contains the AS external static route 10.0.0.0/8 imported by Router D, indicating that the NSSA can transmit AS external routes imported by ASBRs.
The AdvRouter field shows that the router ID of the advertiser is 2.2.2.2. Router B functions as the LSA translator in the NSSA because OSPF selects the ABR with the largest router ID as an LSA translator by default.
# Display the OSPF routing table of Router D.
[RouterD] display ospf routing OSPF Process 1 with Router ID 4.4.4.4 Routing Tables Routing for Network Destination Cost Type NextHop AdvRouter Area 192.168.3.0/24 1 Transit 192.168.3.2 4.4.4.4 0.0.0.1 192.168.4.0/24 1 Transit 192.168.4.1 4.4.4.4 0.0.0.1 192.168.0.0/24 2 Inter-area 192.168.3.1 1.1.1.1 0.0.0.1 192.168.1.0/24 2 Transit 192.168.4.2 2.2.2.2 0.0.0.1 192.168.1.0/24 2 Transit 192.168.3.1 2.2.2.2 0.0.0.1 192.168.2.0/24 2 Inter-area 192.168.4.2 2.2.2.2 0.0.0.1 Routing for NSSAs Destination Cost Type Tag NextHop AdvRouter 0.0.0.0/0 1 Type2 1 192.168.4.2 2.2.2.2 0.0.0.0/0 1 Type2 1 192.168.3.1 1.1.1.1 Total Nets: 8 Intra Area: 4 Inter Area: 2 ASE: 0 NSSA: 2
After an NSSA is configured, the OSPF routing table of Router D does not contain the AS external static route 20.0.0.0/8 imported by Router C, indicating that the NSSA cannot transmit external routes from other areas.
- Configure Router A as an LSA translator in the NSSA.
[RouterA] ospf 1 [RouterA-ospf-1] area 1 [RouterA-ospf-1-area-0.0.0.1] nssa default-route-advertise translator-always
# Display the OSPF routing table of Router C.
[RouterC] display ospf routing OSPF Process 1 with Router ID 3.3.3.3 Routing Tables Routing for Network Destination Cost Type NextHop AdvRouter Area 192.168.0.0/24 1 Transit 192.168.0.2 3.3.3.3 0.0.0.0 192.168.2.0/24 1 Transit 192.168.2.2 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 0.0.0.0 192.168.3.0/24 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 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 AdvRouter field shows that the router ID of the advertiser has become 1.1.1.1, indicating that Router A functions an LSA translator in the NSSA.
- Disable Type 3 LSA advertisement to the NSSA.
# Display the OSPF routing table of Router D.
[RouterD] display ospf routing OSPF Process 1 with Router ID 4.4.4.4 Routing Tables Routing for Network Destination Cost Type NextHop AdvRouter Area 192.168.3.0/24 1 Transit 192.168.3.2 4.4.4.4 0.0.0.1 192.168.4.0/24 1 Transit 192.168.4.1 4.4.4.4 0.0.0.1 192.168.0.0/24 2 Inter-area 192.168.3.1 1.1.1.1 0.0.0.1 192.168.1.0/24 2 Transit 192.168.4.2 2.2.2.2 0.0.0.1 192.168.1.0/24 2 Transit 192.168.3.1 2.2.2.2 0.0.0.1 192.168.2.0/24 2 Inter-area 192.168.4.2 2.2.2.2 0.0.0.1 Routing for NSSAs Destination Cost Type Tag NextHop AdvRouter 0.0.0.0/0 1 Type2 1 192.168.4.2 2.2.2.2 0.0.0.0/0 1 Type2 1 192.168.3.1 1.1.1.1 Total Nets: 8 Intra Area: 4 Inter Area: 2 ASE: 0 NSSA: 2
# Disable Type 3 LSA advertisement to the NSSA to reduce the routing table size.
[RouterA] ospf [RouterA-ospf-1] area 1 [RouterA-ospf-1-area-0.0.0.1] nssa default-route-advertise no-summary
Configuring the no-summary parameter may result in route flapping.
- Verify the configuration.
# Display the OSPF routing table of Router D.
[RouterD] display ospf routing OSPF Process 1 with Router ID 4.4.4.4 Routing Tables Routing for Network Destination Cost Type NextHop AdvRouter Area 192.168.3.0/24 1 Transit 192.168.3.2 4.4.4.4 0.0.0.1 192.168.4.0/24 1 Transit 192.168.4.1 4.4.4.4 0.0.0.1 0.0.0.0/0 2 Inter-area 192.168.4.2 2.2.2.2 0.0.0.1 0.0.0.0/0 2 Inter-area 192.168.3.1 1.1.1.1 0.0.0.1 192.168.1.0/24 2 Transit 192.168.4.2 2.2.2.2 0.0.0.1 192.168.1.0/24 2 Transit 192.168.3.1 2.2.2.2 0.0.0.1 Total Nets: 6 Intra Area: 4 Inter Area: 2 ASE: 0 NSSA: 0
The command output shows that ABRs no longer transmit Type 3 LSAs to the NSSA and that the routing table size is reduced.
Configuration Files
Router A configuration file
# sysname RouterA # router id 1.1.1.1 # interface GigabitEthernet1/0/0 ip address 192.168.0.1 255.255.255.0 # interface GigabitEthernet2/0/0 ip address 192.168.3.1 255.255.255.0 # interface GigabitEthernet3/0/0 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 translator-always nssa default-route-advertise no-summary # return
Router B configuration file
# sysname RouterB # router id 2.2.2.2 # interface GigabitEthernet1/0/0 ip address 192.168.1.2 255.255.255.0 # interface GigabitEthernet2/0/0 ip address 192.168.2.1 255.255.255.0 # interface GigabitEthernet3/0/0 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
Router C configuration file
# sysname RouterC # router id 3.3.3.3 # interface GigabitEthernet1/0/0 ip address 192.168.0.2 255.255.255.0 # interface GigabitEthernet2/0/0 ip address 192.168.2.2 255.255.255.0 # ospf 1 import-route static area 0.0.0.0 network 192.168.0.0 0.0.0.255 network 192.168.2.0 0.0.0.255 # ip route-static 20.0.0.0 255.0.0.0 NULL0 # return
Router D configuration file
# sysname RouterD # router id 4.4.4.4 # interface GigabitEthernet1/0/0 ip address 192.168.3.2 255.255.255.0 # interface GigabitEthernet2/0/0 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