OSPF

Open Shortest Path First (OSPF) is a hierarchical link state protocol. OSPF is an interior gateway protocol (IGP) used within large autonomous systems (ASs). OSPF routers exchange state, cost, and other relevant interface information with neighbors. The information exchange enables all participating routers to establish a network topology map. Each router applies the Dijkstra algorithm to calculate the shortest path to each destination in the network. The resulting OSPF forwarding table is submitted to the routing table manager to calculate the routing table.

When a router is started with OSPF configured, OSPF, along with the routing-protocol data structures, is initialized and waits for indications from lower-layer protocols that its interfaces are functional.

The hierarchical design of OSPF allows a collection of networks to be grouped into a logical area. An area’s topology is concealed from the rest of the AS which significantly reduces OSPF protocol traffic. With the correct network design and area route aggregation, the size of the route table can be greatly reduced, resulting in decreased OSPF route calculation time and topological database size.

Routers that belong to more than one area are called area border routers (ABRs). An ABR maintains a separate topological database for each area it is connected to. Every router that belongs to the same area has an identical topological database for that area.

Key OSPF areas are:

  • Backbone Areas – The backbone distributes routing information between areas.

  • Stub areas – A designated area that does not allow external route advertisements. Routers in a stub area do not maintain external routes. A single default route to an ABR replaces all external routes.

  • Not-So-Stubby Areas (NSSAs) – NSSAs are similar to stub areas in that no external routes are imported into the area from other OSPF areas. External routes learned by OSPF routers in the NSSA area are advertised as type-7 LSAs within the NSSA area and are translated by ABRs into type-5 external route advertisements for distribution into other areas of the OSPF domain.

OSPF global configuration

The minimal OSPF parameters that should be configured to deploy OSPF are:

  • OSPF version

    SR Linux supports OSPF version 2 and version 3. The OSPF version number must be specified in the configuration. If configuring OSPFv3, you must also specify the address family to be used, either IPv4 or IPv6.

  • OSPF instance ID (when configuring multiple instances)

    An OSPF instance ID must be defined when configuring multiple instances or the instance being configured is not the base instance. If an instance ID is not configured, the default instance IDs are as follows:

    • 0 for OSPFv2

    • 0 for OSPF v3 with address family IPv6 unicast

    • 64 for OSPF v3 with address family IPv4 unicast

  • Router ID

    Each router running OSPF must be configured with a unique router ID. The router ID is used by both OSPF and BGP routing protocols in the routing table manager.

    When you configure a new router ID, OSPF is automatically disabled and re-enabled to initialize the new router ID.

  • An area

    At least one OSPF area must be created. An interface must be assigned to each OSPF area.

  • Interfaces

    An interface is the connection between a router and one of its attached networks. An interface has state information associated with it, which is obtained from the underlying lower level protocols and the routing protocol itself. An interface to a network has associated with it a single IP address and mask (unless the network is an unnumbered point-to-point network). An interface is sometimes also referred to as a link.

Configuring basic OSPF parameters

To create a basic OSPF configuration, the minimal OSPF parameters required are the following:

  • OSPF version number, either v2 or v3. If configuring OSPFv3, you must specify the address family, either IPv4 or IPv6.

  • A router ID

  • One or more areas

  • Interfaces

The following is an example of a basic OSPFv2 configuration:

--{ * candidate shared default }--[ network-instance default protocols ]--
# info ospf
    ospf {
        instance default {
            admin-state enable
            version ospf-v2
            router-id 1.1.1.1
            area 0.0.0.1 {
                interface ethernet-1/1.1 {
                    interface-type broadcast
                }
                interface ethernet-1/2.1 {
                    interface-type broadcast
                }
                interface ethernet-1/16.1 {
                    interface-type broadcast
                }
                interface lo0.1 {
                    interface-type broadcast
                }
            }
        }
    }

The following is an example of a basic OSPFv3 configuration.

--{ * candidate shared default }--[ network-instance default protocols ]--
# info ospf
    ospf {
        instance default {
            admin-state enable
            version ospf-v3
            address-family ipv6-unicast
            router-id 1.1.1.1
            area 0.0.0.1 {
                interface ethernet-1/1.1 {
                    interface-type broadcast
                }
                interface ethernet-1/2.1 {
                    interface-type broadcast
                }
                interface ethernet-1/16.1 {
                    interface-type broadcast
                }
                interface lo0.1 {
                    interface-type broadcast
                }
            }
        }
    }

Configuring the router ID

The router ID, expressed like an IP address, uniquely identifies the router within an AS. In OSPF, routing information is exchanged between ASs, groups of networks that share routing information. It can be set to be the same as the loopback (system interface) address. Subscriber services also use this address as far-end router identifiers when service distribution paths (SDPs) are created. The router ID is used by both OSPF and BGP routing protocols.

When you configure a new router ID, OSPF is automatically disabled and re-enabled to initialize the new router ID.

The router ID can be configured either at the network-instance level or at the OSPF protocol level. If a router ID is configured at the OSPF protocol level, OSPF uses it instead of the router ID configured at the network-instance level.

The following example configures a router ID at the network-instance level. OSPF uses this router ID unless a different router ID is configured at the OSPF protocol level.

--{ * candidate shared default }--[  ]--
# info network-instance default
    network-instance default {
        router-id 10.10.10.104{
        }
    }

The following example configures a router ID for the OSPF instance at the OSPF protocol level:

--{ * candidate shared default }--[  ]--
# info network-instance default
    network-instance default {
        protocols {
            ospf {
                instance default {
                    version ospf-v2
                    router-id 2.2.2.2
                }
            }
        }
    }

Configuring an area

An OSPF area consists of routers configured with the same area ID. To include a router in a specific area, the common area ID must be assigned and an interface identified.

If your network consists of multiple areas, you must also configure a backbone area (0.0.0.0) on at least one router. The backbone comprises the area border routers and other routers not included in other areas. The backbone distributes routing information between areas. The backbone is considered to be a participating area within the autonomous system. To maintain backbone connectivity, there must be at least one interface in the backbone area.

The minimal configuration must include an area ID and an interface. Modifying other command parameters are optional.

The following example configures an area at the OSPF level:

--{ * candidate shared default }--[  ]--
# info network-instance default
    network-instance default {
    protocols {
        ospf {
            instance default {
                version ospf-v2
                area 1.1.1.1 {
                }
            }
        }
    }

Configuring a stub area

You can configure stub areas to control external advertisement flooding and to minimize the size of the topological databases on an area's routers. A stub area cannot also be configured as an NSSA. By default, summary route advertisements are sent into stub areas.

The following example configures an OSPF stub area: 

--{ * candidate shared default }--[  ]--
# info network-instance default
    network-instance default {
    protocols {
        ospf {
            instance default {
                version ospf-v2
                area 1.1.1.1 {
                    stub {
                    }
                }
            }
        }
    }

Configuring a Not-So-Stubby area

You can explicitly configure an area to be a Not-So-Stubby Area (NSSA). NSSAs are similar to stub areas in that no external routes are imported into the area from other OSPF areas. An NSSA has the capability to flood external routes it learns throughout its area and by an area border router to the entire OSPF domain. An area cannot be both a stub area and an NSSA.

The following is an OSPF NSSA configuration example:

--{ * candidate shared default }--[  ]--
# info network-instance default
    network-instance default {
    protocols {
        ospf {
            instance default {
                version ospf-v2
                area 1.1.1.1 {
                    nssa {
                    }
                }
            }
        }
    }

Configuring an interface

You can configure an interface to act as a connection between a router and one of its attached networks. An interface includes state information that was obtained from underlying lower level protocols and from the routing protocol itself. An interface to a network is associated with a single IP address and mask (unless the network is an unnumbered point-to-point network). If the address is merely changed, then the OSPF configuration is preserved.

The following is an OSPF interface configuration example:

--{ * candidate shared default }--[  ]--
# info network-instance default
    network-instance default {
    protocols {
        ospf {
            instance default {
                version ospf-v2
                area 1.1.1.1 {
                    interface ethernet-1/2 {
                    }
                }
            }
        }
    }