Introduction
This document shows how Open Shortest Path First (OSPF) injects a default route into a stub or totally stub area.
Prerequisites
Requirements
There are no specific requirements for this document.
Components Used
This document is not restricted to specific software and hardware versions.
Conventions
Configure
In this section, you are presented with the information to configure the features described in this document.
Note: Use the Command Lookup Tool (registered customers only) to find more information on the commands used in this document.
Network Diagram
This document uses the network setup shown in this diagram.
Configurations
This document uses the configurations shown here.
| Router 1.1.1.1 |
|---|
Current configuration: hostname r1.1.1.1 interface Loopback0 ip address 1.1.1.1 255.0.0.0 interface Serial2/1/0 ip address 5.0.0.1 255.0.0.0 router ospf 2 network 5.0.0.0 0.255.255.255 area 1 area 1 stub end |
| Router 2.2.2.2 |
|---|
Current configuration: hostname r2.2.2.2 interface Loopback0 ip address 2.2.2.2 255.0.0.0 interface Serial0/1/0 ip address 5.0.0.2 255.0.0.0 interface ATM1/0.20 ip address 6.0.0.2 255.0.0.0 router ospf 2 network 5.0.0.0 0.255.255.255 area 1 network 6.0.0.0 0.255.255.255 area 0 area 1 stub end |
Verify
This section provides information you can use to confirm your configuration is working properly.
The Output Interpreter Tool (registered customers only) (OIT) supports certain show commands. Use the OIT to view an analysis of show command output.
- show ip ospf database—Displays a list of the Link State Advertisements (LSAs) and types them into a link state database. This list shows only the information in the LSA header.
- show ip ospf database summary <link-state id> —Displays the area border router (ABR) summary links.
- show ip route—Displays the current status of the routing table.
Examine the OSPF Database in a Stub Area
The ABR for the stub area originates a summary LSA with a link ID of 0.0.0.0. It does this even if it does not have a default route. You can see this happen with the show ip ospf database command.
r2.2.2.2#show ip ospf database
OSPF Router with ID (2.2.2.2) (Process ID 2)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
2.2.2.2 2.2.2.2 19 0x80000001 0x8F8B 1
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
5.0.0.0 2.2.2.2 9 0x80000001 0x8E61
Router Link States (Area 1)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 1335 0x80000059 0x56DA 2
2.2.2.2 2.2.2.2 4 0x80000013 0x7FF3 2
Summary Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
0.0.0.0 2.2.2.2 20 0x80000001 0x75C0
6.0.0.0 2.2.2.2 13 0x80000001 0x2709
r2.2.2.2#show ip ospf database summary 0.0.0.0
OSPF Router with ID (2.2.2.2) (Process ID 2)
Summary Net Link States (Area 1)
LS age: 184
Options: (No TOS-capability, DC)
LS Type: Summary Links(Network)
Link State ID: 0.0.0.0 (summary Network Number)
!--- The ABR (Router 2.2.2.2) injects a default route
!--- into the stub area.
Advertising Router: 2.2.2.2
LS Seq Number: 80000001
Checksum: 0x75C0
Length: 28
Network Mask: /0
TOS: 0 Metric: 1
r2.2.2.2#show ip route 0.0.0.0
% Network not in table
!--- The ABR (Router 2.2.2.2) does not have a default route
!--- in its routing table.
r1.1.1.1#show ip route ospf
O IA 6.0.0.0/8 [110/65] via 5.0.0.2, 00:04:23, Serial2/1/0
O*IA 0.0.0.0/0 [110/65] via 5.0.0.2, 00:04:23, Serial2/1/0
Examine the OSPF Database in a Totally Stub Area
If you change area 1 in the stub area example from a stub area to a totally stub area, the ABR still injects the 0.0.0.0 summary LSA into area 1. The only difference is that other summary LSAs are not sent into the totally stub area.
Note: The only configuration change made was to the ABR. The no-summary statement was added to its OSPF configuration:area 1 stub no-summary.
This command output shows what the OSPF database looks like in a totally stub area.
r2.2.2.2#show ip ospf database
OSPF Router with ID (2.2.2.2) (Process ID 2)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
2.2.2.2 2.2.2.2 617 0x80000001 0x8F8B 1
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
5.0.0.0 2.2.2.2 608 0x80000001 0x8E61
Summary ASB Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
1.1.1.1 2.2.2.2 243 0x80000003 0x8F5E
Router Link States (Area 1)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 1934 0x80000059 0x56DA 2
2.2.2.2 2.2.2.2 247 0x80000015 0x7BF5 2
Summary Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
0.0.0.0 2.2.2.2 249 0x80000003 0x71C2
!--- Notice that this is the only summary LSA
!--- in the totally stub area.
r1.1.1.1#show ip route ospf
O*IA 0.0.0.0/0 [110/65] via 5.0.0.2, 00:04:11, Serial2/1/0
The ABR does not originate a summary LSA for 6.0.0.0/8. As a result, Router 1.1.1.1 no longer has a route for 6.0.0.0/8. The only inter-area route this router has is the default route.
