Configuring OSPF in multiple areas

The need for using multiple areas is dictated by scalability issues. A single area OSPF network with many routers implies frequent SPF calculations, large routing tables, large link-state tables, and so on…

The design of the OSPF protocol is hierarchical, that is why OSPF scales well. OSPF v2 achieves this through the use of many areas.

OSPF operation across multiple areas

In an OSPF v2 multiple area environnement the route to a specified destination is calculated as follows:

  • If the destination is in the same area, the normal SPF calculation is performed

  • If the destination is a network in another area, the route to the destination will be the route to the best ABR. Thus, packets addressed to the network will be received by an ABR, which will route them through the backbone area up to an ABR of the remote area. Finally, the remote ABR will forward the packets within the remote area up to the destination.

Configuration procedure

Below drawing illustrates how to configure a backbone network with 2 devices. At each side of the 2 devices, other area are defined. As you can see, all areas have one direct link connection to area 0.

../../../../_images/multiple-areas.svg

OSPF v2 router configuration in multi-area environment

rt1

vrf main
  routing ospf
    network 10.1.1.0/30 area 1
    network 172.16.1.0/24 area 1
    ..
    ..
  interface
    physical eth0_0
      ipv4 address 172.16.1.1/24
      ..
    physical eth1_0
      ipv4 address 10.1.1.1/30
      ..
    ..

rt2 (ABR between the areas 1 and 0)

vrf main
  routing ospf
    network 10.1.1.0/30 area 1
    network 172.16.1.4/30 area 0
    ..
    ..
  interface
    physical eth0_0
      ipv4 address 10.1.1.5/30
      ..
    physical eth1_0
      ipv4 address 10.1.1.2/30
      ..
    ..

rt3 (ABR between the areas 0 and 2)

vrf main
  routing ospf
    network 10.1.1.4/30 area 0
    network 192.168.1.0/24 area 2
    ..
    ..
  interface
    physical eth0_0
      ipv4 address 10.1.1.6/30
      ..
    physical eth1_0
      ipv4 address 192.168.1.1/24
      ..
    ..

rt4

vrf main
  routing ospf
    network 192.168.1.0/24 area 2
    ..
    ..
  interface
    physical eth1_0
      ipv4 address 192.168.1.2/24
      ..
    ..

Verifying OSPF multi-area operation

In this type of configuration, the most important thing to check is the OSPF v2 database.

Area 1 ABR

rt2> show ospf database default

      OSPF Router with ID (10.1.1.5)

              Router Link States (Area 0.0.0.0)

Link State ID  ADV Router  Age  Seq#        CkSum   Link count
10.1.1.5       10.1.1.5     53 0x80000004  0x7d84  1
192.168.1.1    192.168.1.1  53 0x80000004  0xfe4d  1

               Net Link States (Area 0.0.0.0)

Link State ID  ADV Router   Age  Seq#        CkSum
10.1.1.6       192.168.1.1  54 0x80000001  0x550e

               Summary Link States (Area 0.0.0.0)

Link State ID  ADV Router   Age  Seq#        CkSum  Route
10.1.1.0       10.1.1.5     62 0x80000001  0x9c9c 10.1.1.0/30
172.16.1.0     10.1.1.5     62 0x80000001  0x1c5e 172.16.1.0/24
192.168.1.0    192.168.1.1  75 0x80000001  0xf983 192.168.1.0/24

               Router Link States (Area 0.0.0.1)
Link State ID  ADV Router   Age  Seq#        CkSum  Link count
10.1.1.5       10.1.1.5     62 0x80000003  0x21e8 1
172.16.1.1     172.16.1.1   75 0x80000003  0xeceb 2

               Net Link States (Area 0.0.0.1)
Link State ID  ADV Router   Age  Seq#        CkSum
10.1.1.1       172.16.1.1   77 0x80000001  0x467b

               Summary Link States (Area 0.0.0.1)
Link State ID  ADV Router   Age  Seq#        CkSum  Route
10.1.1.4       10.1.1.5     53 0x80000001  0x74c0 10.1.1.4/30
192.168.1.0    10.1.1.5     43 0x80000001  0xefdd 192.168.1.0/24

rt2 has two databases: one in area 1, the other in area 0.

rt1

rt1> show ospf database default

       OSPF Router with ID (172.16.1.1)

                Router Link States (Area 0.0.0.1)

Link State ID  ADV Router  Age  Seq#        CkSum  Link count
10.1.1.5       10.1.1.5    100 0x8000000a  0x1fe9 1
172.16.1.1     172.16.1.1  200 0x8000000b  0xeaec 2

               Net Link States (Area 0.0.0.1)

Link State ID  ADV Router  Age  Seq#        CkSum
10.1.1.1       172.16.1.1  200 0x80000005  0x447c

               Summary Link States (Area 0.0.0.1)

Link State ID  ADV Router  Age  Seq#        CkSum   Route
10.1.1.4       10.1.1.5    96 0x80000002  0x72c1  10.1.1.4/30
192.168.1.0    10.1.1.5    93 0x80000001  0xefdd  192.168.1.0/24

Route summarization

Summarization is the aggregation of multiple routes into one advertisement. The functionality of route summarization has the obvious advantage of reducing routing tables, and positively affects the amount of bandwidth and CPU consumed, but proper summarization operation requires a contiguous network address space.

There are two types of summarization:

Inter-area route summarization

Done on ABR routers.

External route summarization

Done on ASBR routers, this type of summarization is specific to external routes redistributed from BGP, static, or other external routing information.

Inter-area Route summarization configuration

Example: inter-area route summarization configuration

Above figure 6 example (Figure 6 - OSPF v2 router configuration in multi-area environment) illustrates an inter-area configuration example. Assuming that prefix 10.2.1.0/24 has been delegated to area 1, then the area 1 administrator may want to advertise a summarized route to all sub-networks of this prefix.

In the previous example, the ABR router rt2 is now configured to advertise the aggregated prefix 10.2.1.0/24, and rt1 is configured to announce network 10.2.1.0/28.

Added configuration lines are written below:

rt1

vrf main
  routing ospf
    network 10.1.1.0/30 area 1
    network 172.16.1.0/24 area 1
    network 10.2.1.0/30 area 1
    ..
    ..
  interface
    physical eth0_0
      ipv4 address 172.16.1.1/24
      ..
    physical eth1_0
      ipv4 address 10.1.1.1/30
      ipv4 address 10.2.1.1/28
      ..
    ..

rt2

ABR between the areas 1 and 0:

vrf main
  routing ospf
    network 10.1.1.0/30 area 1
    network 172.16.1.4/30 area 0
    area 1 range 10.2.1.0/24
    ..
    ..
  interface
    physical eth0_0
      ipv4 address 10.1.1.5/30
      ..
    physical eth1_0
      ipv4 address 10.1.1.2/30
      ..
    ..

Check OSPF v2 routes.

rt1

rt1> show ospf route
============ OSPF network routing table ============
N    10.1.1.0/30           [100] area: 0.0.0.1
                           directly attached to eth1_0
N IA 10.1.1.4/30           [200] area: 0.0.0.1
                           via 10.1.1.2, eth1_0
N    10.2.1.0/28           [100] area: 0.0.0.1
                           directly attached to eth0_0
N    172.16.1.0/24         [100] area: 0.0.0.1
                           directly attached to eth0_0
N IA 192.168.1.0/24        [300] area: 0.0.0.1
                           via 10.1.1.2, eth1_0

============ OSPF router routing table =============
R    10.1.1.5              [100] area: 0.0.0.1, ABR
                           via 10.1.1.2, eth1_0

============ OSPF external routing table ===========

On rt1, which is in area 1, the new route to the 10.2.1.0/28 prefix has appeared in the OSPF RIB.

rt2

rt2> show ospf route
============ OSPF network routing table ============
N    10.1.1.0/30           [100] area: 0.0.0.1
                           directly attached to eth1_0
N    10.1.1.4/30           [100] area: 0.0.0.0
                           directly attached to eth0_0
D IA 10.2.1.0/24           Discard entry
N    10.2.1.0/28           [200] area: 0.0.0.1
                           via 10.1.1.1, eth1_0
N    172.16.1.0/24         [200] area: 0.0.0.1
                           via 10.1.1.1, eth1_0
N IA 192.168.1.0/24        [200] area: 0.0.0.0
                           via 10.1.1.6, eth0_0

============ OSPF router routing table =============
R    192.168.1.1           [100] area: 0.0.0.0, ABR
                           via 10.1.1.6, eth0_0

============ OSPF external routing table ===========

On rt2, which is the ABR of area 1, the new route to the 10.2.1.0/28 prefix has appeared in the OSPF RIB. This route will not be advertised beyond area 1. The summary route will instead be advertised. To avoid routing loops (since the 10.2.1.0/24 address space has not be entirely assigned to networks), a reject route will be injected in the ABR forwarding table (hence a discard entry appears in the OSPF RIB).

rt3

rt3> show ospf route
============ OSPF network routing table ============
N IA 10.1.1.0/30           [200] area: 0.0.0.0
                           via 10.1.1.5, eth0_0
N    10.1.1.4/30           [100] area: 0.0.0.0
                           directly attached to eth0_0
N IA 10.2.1.0/24           [300] area: 0.0.0.0
                           via 10.1.1.5, eth0_0
N IA 172.16.1.0/24         [300] area: 0.0.0.0
                           via 10.1.1.5, eth0_0
N    192.168.1.0/24        [100] area: 0.0.0.2
                           directly attached to eth1_0

============ OSPF router routing table =============
R 10.1.1.5                 [100] area: 0.0.0.0, ABR
                           via 10.1.1.5, eth0_0

============ OSPF external routing table ===========

The rt3 router, does not belong to area 1. Its OSPF RIB only contains a route to the summary route 10.2.1.0/24.

OSPF stub area overview

In some ASes, the majority of the link-state database may consist of AS-external-LSAs. An OSPF AS-external-LSA is usually flooded throughout the entire AS. However, OSPF allows certain areas to be configured as “stub areas”. AS-external-LSAs are not flooded into/throughout stub areas; routing to AS external destinations in these areas is based on a default route. This reduces the link-state database size, and therefore the memory requirements, for a stub area’s internal routers.

To configure a stub area, enter for example:

routing ospf
   area 1 stub

Totally stubby area overview

This feature prevents the ospf ABR from injecting inter-area summary into the considered area.

A Stub Area restricts the LSA types being injected into a stub area from other areas to Type 3 Summary LSA’s. Type 4’s and 5’s are represented by a default route to the Area Border Router. A totally stubby area takes this further by restricting Type 3’s as well, so all traffic being injected into a totally stubby area are represented by a default route.

To sum up, this means that the AS-external-LSAs (Type-5 LSA) and ASBR-Summary-LSA (Type-4 LSA) and Network summary LSA (Type-3 LSA) are not flooded into a totally stub areas.

Example

vrf main
  routing ospf
    area 1 stub summary false

OSPF NSSA overview

Virtual Service Router software supports the OSPF NSSA. This concept was first described in RFC 1587. An OSPF area is said to be NSSA if it can send some external links to other areas. These routes are said to be LSA type 7, which carry essentially type 5 LSA. Then, at the ASBR, it is converted in LSA type 5, which can flood the information to the rest of other areas networks.

Example

vrf main
  routing ospf
    area 1 nssa

OSPF options configuration example

../../../../_images/virtual-link.svg

OSPF v2 options configuration example

In this example, the routers will be configured so that rt1 and rt2 will have a virtual-link. Route summarization will be configured on rt1. rt2 and rt3 will be ABRs. Also, OSPF priority on rt2 will be changed. The last device, rt3, will be configured in area 2. It will be checked how routes announced by rt1 will be propagated.

rt11

vrf main
  routing ospf
    network 172.16.0.0/22 area 3
    ..
    ..
  interface
    physical eth0_0
      ipv4 address 172.16.1.2/24
      ..
    physical eth1_0
      ipv4 address 172.16.0.2/24
      ..
    ..

rt1

vrf main
  routing ospf
    area 1 virtual-link 10.1.1.5
    area 3 range 172.16.0.0/22
    network 172.16.0.0/22 area 3
    network 10.1.1.0/24 area 1
    ..
    ..
  interface
    physical eth0_0
      ipv4 address 172.16.1.1/24
      ..
    physical eth1_0
      ipv4 address 10.1.1.1/30
      ..
    ..

rt2

vrf main
  routing ospf
    network 10.1.1.0/30 area 1
    network 10.1.1.4/30 area 0
    area 1 virtual-link 172.16.1.1
    ..
    ..
  routing interface eth1_0
    ip ospf priority 3
    ..
    ..
  interface
    physical eth0_0
      ipv4 address 10.1.1.5/30
       ..
    physical eth1_0
      ipv4 address 10.1.1.2/30
      ..
    ..

rt3

vrf main
  routing ospf
    network 10.1.1.4/30 area 0
    network 192.168.1.0/24 area 2
    ..
  interface
    physical eth0_0
      ipv4 address 10.1.1.6/30
      ..
    physical eth1_0
      ipv4 address 192.168.1.1/24
      ..
    ..

rt4

vrf main
  routing ospf
    network 192.168.1.0/24 area 2
    ..
    ..
  interface
    physical eth1_0
      ipv4 address 192.168.1.2/24
      ..
    ..

Check the state of the multi-area OSPF domain.

rt1

Check the OSPF neighbors’ status:

rt1> show ospf neighbor

Neighbor ID  Pri  State        Dead Time  Address     Interface
   RXmtL RqstL DBsmL
10.1.1.5 3       Full/DR       35.432s   10.1.1.2    eth1_0:10.1.1.1
      0     0     0
10.1.1.5       1 Full/DROther  34.433s   10.1.1.2    VLINK0
      0     0     0
172.16.1.2 1     Full/DR       31.642   172.16.1.2  eth0_0:172.16.1.1
      0     0     0

rt2

Check the OSPF neighbors’ status:

rt2> show ospf neighbor

Neighbor ID  Pri  State        Dead Time  Address     Interface    RXmtL RqstL DBsmL

172.16.1.1     1  Full/Backup  38.325s   10.1.1.1    eth1_0:10.1.1.2  0     0     0
192.168.1.1    1  Full/DR      38.635s   10.1.1.6    eth0_0:10.1.1.5   0     0     0
172.16.1.1     1  Full/DROther 38.405s   10.1.1.1    VLINK0           0     0     0

rt3

Check the OSPF neighbors’ status:

rt3> show ospf neighbor
Neighbor ID  Pri  State        Dead Time  Address     Interface          RXmtL RqstL DBsmL
192.168.1.2    1  Full/DR      36.257s   192.168.1.2 eth1_0:192.168.1.1      0     0     0
10.1.1.5       1  Full/Backup  32.532s   10.1.1.5    eth0_0:10.1.1.6         0     0     0

rt2

Display the OSPF database:

rt2> show ospf database default

       OSPF Router with ID (10.1.1.5)

                Router Link States (Area 0.0.0.0)

Link State      ID ADV Router  Age Seq#        CkSum  Link count
10.1.1.5        10.1.1.5       601 0x8000001b  0x827f 2
172.16.1.1      172.16.1.1     598 0x80000010  0x5844 1
192.168.1.1     192.168.1.1    649 0x80000010  0xe45a 1

        Net Link States (Area 0.0.0.0)

Link State      ID ADV Router  Age Seq#        CkSum
10.1.1.6        192.168.1.1    653 0x80000001  0x550e

         Summary Link States (Area 0.0.0.0)
Link State      ID ADV Router  Age Seq#        CkSum  Route
10.1.1.0        10.1.1.5       990 0x80000005  0x94a0 10.1.1.0/30
10.1.1.0        172.16.1.1     979 0x80000005  0x7c0b 10.1.1.0/30
172.16.0.0      172.16.1.1     657 0x80000001  0x9b3f 172.16.0.0/22
192.168.1.0     192.168.1.1    626 0x80000006  0xef88 192.168.1.0/24

          Router Link States (Area 0.0.0.1)

Link State      ID ADV Router  Age Seq#        CkSum  Link count
10.1.1.5        10.1.1.5       602 0x80000004  0x35ce 1
172.16.1.1      172.16.1.1     603 0x80000005  0x3e6a 1

          Net Link States (Area 0.0.0.1)

Link State      ID ADV Router  Age Seq#        CkSum
10.1.1.2        10.1.1.5       611 0x80000001  0x541a

          Summary Link States (Area 0.0.0.1)

Link State      ID ADV Router  Age Seq#        CkSum  Route
10.1.1.4        10.1.1.5       649 0x80000001  0x74c0 10.1.1.4/30
172.16.0.0      172.16.1.1     657 0x80000001  0x9b3f 172.16.0.0/22
172.16.0.255    172.16.1.1     596 0x80000001  0x0fbe 172.16.0.0/24
172.16.1.0      172.16.1.1     596 0x80000001  0x9f37 172.16.1.0/24
192.168.1.0     10.1.1.5       639 0x80000001  0xefdd 192.168.1.0/24

On above show command, a summary LSA exists for networks 172.16.0.0/24 and 172.16.0.1/24 in area 1 (although these networks are in area 3), thanks to the virtual link between rt1 and rt2. The LSAs for these two networks are aggregated in area 0 as a summary link state, thanks to route summarization on router rt1, hence only a route to network 172.16.0.0/22 is advertised on the backbone area.

rt3

Display the OSPF routes received by rt3:

rt3> show ospf route
============ OSPF network routing table ============
N IA 10.1.1.0/30           [200] area: 0.0.0.0
                           via 10.1.1.5, eth0_0
N    10.1.1.4/30           [100] area: 0.0.0.0
                           directly attached to eth0_0
N IA 172.16.0.0/22         [310] area: 0.0.0.0
                           via 10.1.1.5, eth0_0
N    192.168.1.0/24        [100] area: 0.0.0.2
                           directly attached to eth1_0

============ OSPF router routing table =============
R    10.1.1.5              [100] area: 0.0.0.0, ABR
                           via 10.1.1.5, eth0_0
R    172.16.1.1            [200] area: 0.0.0.0, ABR
                           via 10.1.1.5, eth0_0

=========== OSPF external routing table ===========

The aggregated route to network 172.16.0.0/22 is received by rt3 thanks to the virtual link and route summarization.

rt1

On rt1, the OSPF routes are as follows:

rt1> show ospf route
============ OSPF network routing table ============
N    10.1.1.0/30           [100] area: 0.0.0.1
                           directly attached to eth1_0
N    10.1.1.4/30           [200] area: 0.0.0.0
                           via 10.1.1.2, eth1_0
D IA 172.16.0.0/22         Discard entry
N    172.16.0.0/24         [110] area: 0.0.0.3
                           via 172.16.1.2, eth0_0
N    172.16.1.0/24         [100] area: 0.0.0.3
                           directly attached to eth0_0
N IA 192.168.1.0/24        [300] area: 0.0.0.0
                           via 10.1.1.2, eth1_0

============ OSPF router routing table =============
R    10.1.1.5              [100] area: 0.0.0.1, ABR
                           via 10.1.1.2, eth1_0
                           [100] area: 0.0.0.0, ABR
                           via 10.1.1.2, eth1_0
R    192.168.1.1           [200] area: 0.0.0.0, ABR
                           via 10.1.1.2, eth1_0

============ OSPF external routing table ===========

The routes to area 3 networks (172.16.0.0/24 and 172.16.1.0/24) appear in the RIB, as well as a reject route to the aggregated network (172.16.0.0/22), to avoid routing loops. Only the aggregated route will be advertised to other areas. Routes to networks in remote areas have also been received by rt1.

Routes are now installed on all routers, so that packets can flow from rt11 to rt4.