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multi area ospf

Multiarea OSPF

How to Configure Multiarea OSPF

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OSPF can be implemented as single-area OSPF or multiarea OSPF. The basic steps for a single area or multiarea OSPF implementation are the following:-

Gather the network requirements and parameters – The first step for implementing multiarea OSPF is the collecting network requirement and important parameters. The requirement includes the number of hosts and required network devices, the IP addressing plan, the routing domain size including, the size of the routing tables. It is also important to consider the risk of topology changes, and other network characteristics.

OSPF parameters – The first step is the base of the second step, the network administrator must find out if single-area or multiarea OSPF is the ideal implementation. If it is multiarea OSPF, there are several considerations to determine the OSPF parameters including IP addressing plan, OSPF areas, and Network topology.

Configure multiarea OSPF – Configure the multiarea OSPF implementation based on the parameters.

Verifies the Multiarea OSPF – Verify the multiarea OSPF implementation based on the parameters.

Configuring Multiarea OSPFv2

Figure 1 displays the reference multiarea OSPF topology. In this example:

  • R1 is an internal router for area 10 because all of its interfaces are in area 1.
  • R2 is an ABR because it has interfaces in area 10 and also an interface in area 0.
  • R3 is also an ABR because it has interfaces in area 20 and an interface in area 0.
  • R4 is an internal router for area 20 because all of its interfaces are in area 20.
Multiarea OSPF

There are no unique commands for multiarea OSPF configuration. A router simply becomes an Area Border Router (ABR) when it has two “network” statements in different areas.

  • R1 is the member of area 10, and we have assigned the router ID 1.1.1.1 to R1. The configuration on R1 is the following:
  • R1(config)#router ospf 10
  • R1(config-router)router-id 1.1.1.1
  • R1(config-router)network 192.168.0.0 0.0.0.255 area 10
  • R1(config-router)network 192.168.2.0 0.0.0.255 area 10
  • R1(config-router)network 10.10.10.8 0.0.0.3 area 10
  • R1(config-router) do wr

Both network 192.168.1.0/24 and 10.10.10.8/30 are configured in the same area because this is an internal router for area 10. The autonomous number for the OSPF is 10. Now let’s configure the R2, which is the ABR for Area 10 and Area 0 (backbone area). The configuration of R2 is the following:

  • R2(config)#router ospf 10
  • R2(config-router)router-id 2.1.1.1
  • R2(config-router)network 10.10.10.0 0.0.0.3 area 0
  • R2(config-router)network 10.10.10.8 0.0.0.3 area 10
  • R2(config-router) do wr

The above configuration enables OSPF in the two different areas. The GigabitEthernet 0/2 interface is configured as part of OSPF area 10 and GigabitEthernet 0/0 as part of OSPF area 0. Because R2 has interfaces connected to two different areas, so it is an ABR between area 10 and area 0. Now I am going to configure R3, which is also an ABR. The configuration is:

  • R3(config)#router ospf 10
  • R3(config-router)router-id 3.1.1.1
  • R3(config-router)network 10.10.10.0 0.0.0.3 area 0
  • R3(config-router)network 172.16.0.0 0.0.0.3 area 20
  • R3(config-router) do wr

You can see the configuration of R3, GigbitEthernet0/0 is the part of area 0 and GigabitEthernet 0/2 is the member of area 20. The R2 is a member of both area 0 and area 20, therefore it is an ABR for both areas. R4 is the internal router of area 20, the router-id of the R4 is 4.1.1.1. the configuration is simple, just like R1:

  • R4(config)#router ospf 10
  • R4(config-router)router-id 4.1.1.1
  • R4(config-router)network 192.168.2.0 0.0.0.255 area 20
  • R4(config-router)network 192.168.3.0 0.0.0.255 area 20
  • R4(config-router)network 172.16.0.0 0.0.0.3 area 20
  • R4(config-router) do wr

Notice that we have used the wildcard mask in the network statement. It is the inverse of subnet masks.

Configuring Multiarea OSPFv3

The OSPFv3 is not more different than the OSPFv2, we are using the same topology for multiarea OSPFv3 configuration. The figure-2 illustrates the topology with the IP addressing scheme. A router simply becomes an ABR when it has two interfaces in different areas just like OSPFv2 and there are no special commands required.

Multiarea OSPF

Now lest come first of all configure the R1, which is the member of area 10 because all interfaces are connected to area 10. We should assign the same router IDs as OSPFv2.

Multiarea OSPFv3 Configuration on R1

  • R1(config)#ipv6 router ospf 10
  • R1(config-rtr)#router-id 1.1.1.1
  • R1(config-rtr)#exit
  • R1(config)#interface GigabitEthernet 0/0
  • R1(config-if)# ipv6 ospf 10 area 10
  • R1(config-if)#exit
  • R1(config)#interface GigabitEthernet 0/2
  • R1(config-if)# ipv6 ospf 10 area 10
  • R1(config-if)#exit
  • R1(config)#interface GigabitEthernet 0/1
  • R1(config-if)# ipv6 ospf 10 area 10
  • R1(config-if)#do wr
  • R1(config-if)#end
  • R1#
Multiarea OSPFv3 Configuration on R2

  • R2(config)#ipv6 router ospf 10
  • R2(config-rtr)#router-id 2.1.1.1
  • R2(config-rtr)#exit
  • R2(config)#interface GigabitEthernet 0/0
  • R2(config-if)# ipv6 ospf 10 area 0
  • R2(config-if)#exit
  • R2(config)#interface GigabitEthernet 0/2
  • R2(config-if)# ipv6 ospf 10 area 10
  • R2(config-if)#do wr
  • R2(config-if)#end
  • R2#
Multiarea OSPFv3 Configuration on R3

  • R3(config)#ipv6 router ospf 10
  • R3(config-rtr)#router-id 3.1.1.1
  • R3(config-rtr)#exit
  • R3(config)#interface GigabitEthernet 0/0
  • R3(config-if)# ipv6 ospf 10 area 0
  • R3(config-if)#exit
  • R3(config)#interface GigabitEthernet 0/2
  • R3(config-if)# ipv6 ospf 10 area 20
  • R3(config-if)#do wr
  • R3(config-if)#end
  • R3#
Multiarea OSPFv3 Configuration on R4

  • R4(config)#ipv6 router ospf 10
  • R4(config-rtr)#router-id 4.1.1.1
  • R4(config-rtr)#exit
  • R4(config)#interface GigabitEthernet 0/0
  • R4(config-if)# ipv6 ospf 10 area 20
  • R4(config-if)#exit
  • R4(config)#interface GigabitEthernet 0/2
  • R4(config-if)# ipv6 ospf 10 area 20
  • R4(config-if)#exit
  • R4(config)#interface GigabitEthernet 0/1
  • R4(config-if)# ipv6 ospf 10 area 20
  • R4(config-if)#do wr
  • R4(config-if)#end
  • R4#
Introduction to Single-Area OSPF and Multi-Area OSPF

Introduction to Single-Area OSPF and Multi-Area OSPF

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Because of its hierarchical design, OSPF is more efficient and scalable. The OSPF segments the network into different areas. An area is a group of routers sharing the same link-state information in their Link-State Databases (LSDBs). We can implement the OSPF in two ways.

  • Single Area OSPF
  • Multi-Area OSPF

Single Area OSPF (Area 0)

Area 0 is also known as the backbone area for OSPF, which links all other smaller areas within the hierarchy. The single area OSPF is helpful in smaller networks where only a few routers are working, the web of router links is not complex, and paths to individual destinations are easy. If an area becomes too big, the network will be affected.

  • Large routing table
  • Extensive link-state database (LSDB)
  • Frequent SPF algorithm calculations

OSPF supports hierarchical routing using areas, which makes OSPF more efficient and scalable. An area is a group of routers sharing the same link-state information in their databases. The figure below illustrates the single-area OSPF network.

single area

Multi-Area OSPF

When a large OSPF area is separated into several smaller areas, this is called multi-area OSPF. It is helpful in larger network deployments to decrease processing and memory overhead. The network contains OSPF areas in a hierarchal design. All areas are connected to area 0, known as the backbone area—the interconnecting routers between the backbone area and other areas are known as area border routers (ABR).

The figure shows that multi-area OSPF segments one large network into several networks. One autonomous system ( AS) has been divided into multiple areas to support hierarchical routing. The routing occurs between the multiple areas. The more operations that require system resources, such as recalculating the database, are done only within a relevant area.

For example, when changes occur in the topology, like addition, deletion, or modification of the link, the router must rerun the SPD algorithm to create a new SPF tree and update the routing table in the same area where changes occur.  However, changes have been shared with routers in other areas in a distance-vector format to update their routing tables, and these routers and areas do not need to rerun the SPF algorithm. As illustrated in Figure 1, the hierarchical-topology possibilities of multi-area OSPF have these advantages:

  • Smaller routing tables—Fewer routing table entries as network addresses can be summarized between areas. For example, R1 summarizes the routes from Area 1 to Area 0, and R2 summarizes the routes from Area 51 to Area 0. R1 and R2 also propagate a default static route to Area 1 and 51.
  • Reduced link-state update overhead– Minimizes processing and memory requirements because fewer routers exchange LSAs.
  • Reduced frequency of SPF calculations—This Minimizes the impact of a topology change within an area. For instance, it minimizes the impact of routing updates because LSA flooding stops at the area boundary.
  • Small Link-State Data Base. Multi-areas OSPF efficiently partitions a potentially large database into smaller, more manageable ones.
Multi-Area OSPF

OSPF Two-Layer Area Hierarchy

Multiarea OSPF is implemented in a two-layer area hierarchy:

  • Backbone (Transit) area—An OSPF area transfers IP packets fast and efficiently. Backbone areas interconnect multiple OSPF areas. End users are usually not found within a backbone area.
  • Regular (Non-backbone) area –This area connects users and other resources. The areas are usually set up along functional or geographical groupings. By default, a regular area does not allow traffic from another area. Traffic from other areas must forward through a transit area.  A regular can contain several subtypes, including a standard area, stub area, totally stubby area, and not-so-stubby area (NSSA). The Cisco recommends the following guidelines for areas:
  1. The maximum number of routers per area is 50.
  2. One router can join a maximum of three areas.
  3. The maximum number of neighbors for a single router is 60.