Figure 14-2: Network Setup for OSPF Multiarea
- OSPF hierarchical and multiarea design adds complexity to the router configuration. However, OSPF multiarea configuration is simple once understood OSPF single-area configuration. The difficult part would be network design (deciding which subnets should be placed in which areas).
After the area design is complete, the configuration would be straightforward and easy.
- OSPF Multiarea configuration on RT1:
- OSPF Multiarea configuration on RT2:
- OSPF Multiarea configuration on RT3:
RT1#router ospf 1
RT1(config-router)#network 10.1.1.1 0.0.0.0 area 0 RT1(config-router)#network 10.1.2.1 0.0.0.0 area 1 RT1(config-router)#network 10.1.3.1 0.0.0.0 area 0 RT1(config-router)#^Z
RT1#
RT2#router ospf 1
RT2(config-router)#network 10.0.0.0 0.255.255.255 area 1 RT2(config-router)#^Z
RT2#
RT3#router ospf 1
RT3(config-router)#network 10.1.3.2 0.0.0.0 area 0 RT3(config-router)#network 10.1.5.2 0.0.0.0 area 1 RT3(config-router)#network 10.1.6.1 0.0.0.0 area 0 RT3(config-router)#^Z
RT3#
RT2 RT3
10.1.1.1 Fa1/0
10.1.2.1 S0/0
RT1
Fa1/0 10.1.4.1
Fa1/0 10.1.6.1 10.1.3.1
S0/1
S0/0 10.1.2.2
S0/0 10.1.3.2
S0/1 10.1.5.1
10.1.5.2 S0/1 Area 0
Area 1
- RT1 and RT3 cannot match all their 3 interfaces with a single network router subcommand, as both routers have an interface that resides in a different area than the other 2 interfaces.
However, this is possible for RT2 since all its interfaces are in the same area.
- Verify the OSPF Multiarea configuration on RT1:
- Verify the OSPF Multiarea configuration on RT2:
- Verify the OSPF Multiarea configuration on RT3:
- Note: The routes learned by RT2 from the other 2 routers are shown as OSPF interarea (IA) routes, as those subnets are reside in Area 0, while RT2 resides in Area 1.
RT1#sh ip route
Gateway of last resort is not set
10.0.0.0/24 is subnetted, 6 subnets
C 10.1.1.0 is directly connected, FastEthernet10 C 10.1.2.0 is directly connected, Serial0/0 C 10.1.3.0 is directly connected, Serial0/1
O 10.1.4.0 [110/65] via 10.1.2.2, 00:02:04, Serial0/0 O 10.1.5.0 [110/128] via 10.1.2.2, 00:02:04, Serial0/0 O 10.1.6.0 [110/65] via 10.1.3.2, 00:07:41, Serial0/1 RT1#
RT2#sh ip route
Gateway of last resort is not set
10.0.0.0/24 is subnetted, 6 subnets
O IA 10.1.1.0 [110/65] via 10.1.2.1, 00:03:05, Serial0/0 C 10.1.2.0 is directly connected, Serial0/0
O IA 10.1.3.0 [110/128] via 10.1.2.1, 00:03:05, Serial0/0 [110/128] via 10.1.5.2, 00:03:05, Serial0/1 C 10.1.4.0 is directly connected, FastEthernet1/0 C 10.1.5.0 is directly connected, Serial0/1
O IA 10.1.6.0 [110/65] via 10.1.5.2, 00:03:05, Serial/01 RT2#
RT3#sh ip route
Gateway of last resort is not set
10.0.0.0/24 is subnetted, 6 subnets
O 10.1.1.0 [110/74] via 10.1.3.1, 00:09:48, Serial0/0 O 10.1.2.0 [110/128] via 10.1.5.1, 00:04:11, Serial0/1 C 10.1.3.0 is directly connected, Serial0/0
O 10.1.4.0 [110/74] via 10.1.5.1, 00:04:11, Serial0/1 C 10.1.5.0 is directly connected, Serial/01
C 10.1.6.0 is directly connected, FastEthernet1/0 RT3#
- Below lists the different terminology used for the OSPF tables:
OSPF neighbor table OSPF adjacency database
OSPF topology table OSPF topology database of link-state database OSPF routing table OSPF forwarding database
- A router requires an operational interface (line up, protocol up) with an IP address when initializing the OSPF process; or else the can’t allocate router-id error message will be generated and the OSPF process will not be started.
- Loopback interfaces are logical interfaces (not real router interfaces). They are important in OSPF configuration. Without using loopback interfaces, the highest physical IP address on a router will become the RID, which is used for DR election and link-state database exchange.
When using loopback interfaces in OSPF configuration, the RID of a router will never change as they never go down (always active) and hence can produce a stable network. Cisco suggests to use logical interfaces (eventually become the RIDs) as the best practice for OSPF configuration.
- MISC OSPF Router ID notes:
i) Use a private IP addressing scheme for loopback interface IP address configuration.
ii) A newly configured and enabled loopback interface with an IP address higher than the current RID will only take effect as the new RID when after a router reboot or set the loopback interface address as the RID with the router-id router subcommand followed by issuing the clear ip ospf process privileged command. This prevents a newly enabled loopback interface with an IP address higher than the current RID from restarting the OSPF process which will trigger SPF calculation and network service interruption.
Note: The clear ip ospf process privileged command is unable to make the IP address of a loopback interface to take effect as the new RID.
iii) RID is selected at the moment when the OSPF process starts. It will remain even if the interface with the IP address that is selected as the RID is disabled and even removed.
A router reboot or the router-id router subcommand is required to change the RID.
iv) Loopback interface address overrides any physical interface address even if there is a physical interface address that is numerically higher than the loopback interface address.
Additionally, the highest IP address of any logical interface will be selected as the RID.
v) RID can also be set persistently with the router-id {rid-ip-addr} router subcommand.
The RID set with this command overrides loopback interface address, which means a physical interface address can be set as the RID even it is numerically lower than the current RID selected from a loopback interface address. The RID set with the OSPF
router-id router subcommand requires a router reboot or issue the clear ip ospf process privileged command for it to take effect as the new RID.
Note: The OSPF router-id router subcommand behaves differently on stand-alone routers and routers with established OSPF neighbors. It takes effect immediately on stand-alone routers – the RID is changed immediately upon issuing the command; while the clear ip ospf process
privileged command must be issued on latter situation for a newly configured RID to take effect.
- The formula for calculating the number of adjacencies will to be formed between DROTHER routers with the DR and BDR routers on a multi-access network is 2(n – 1), where n is the number of routers on the network.
- OSPF does not support unequal-cost load balancing. If OSPF has 2 unequal cost links to a destination network, only the lowest-cost link will be used, and the other link will remain idle.
One may manually change the cost of the interfaces with the ip ospf cost {cost} interface