Configuring OSPF¶

Basic elements for configuration
Verifying OSPF configuration
OSPF configuration in single area example
OSPF configuration with BGP redistribution
OSPF per interface configuration

Basic elements for configuration ¶

Enable OSPF:

vrf main
    routing ospf
      router-id 10.125.0.1
      network 10.125.0.0/30 area 0
      ..
    ..

Above example shows an OSPF instance configured on main VRF. The configuration of the router-id is not mandatory, since an election process takes place inside OSPF: the router-id is first based on the IP given by manual configuration, followed by the highest IP available on loopback interface, then followed by the highest IP available on non loopback interface.

Network command permits enabling OSPF on the network interface whose address and network mask is included into this prefix, and will announce a link connected to a stub or transit network defined by the interface address and prefix. As the 10.100.0.0/24 network belongs to eth1 interface, then OSPF will establish adjacencies over that interface. The network entry passed as parameter will be passed in the Type 3 LSA.

The area identifier is a 32 bit id by which an area is identified. Note that area value is 0, usually reserved for backbone operations. Having multiple areas in a complex IGP topology permits simplifying the route calculation of OSPF. Only ABR routers will know both areas defined.

It is possible to use an alternative OSPF configuration by defining networks based on interface and area configurations. Below configuration relies on interface eth0_0 where the 10.125.0.0/30 network is configured. The below configuration assumes that there is only one IPv4 address under eth0_0 so that both configurations are the same.

vrf main
    routing
      ospf
        router-id 10.125.0.1
        ..
      interface eth0_0
        ip ospf area 0
        ..
      ..
    interface physical eth0_0
      ipv4 address 10.125.0.1/30

Using above configuration can simplify network deployments, since the address configuration is tightly linked with the IP provisioning done on the interfaces.

You can also disable OSPF, without having to remove the configuration, by using following command:

vrf main
    routing ospf
        enabled false

Nonetheless, it is always possible to supress OSPF configuration:

vrf main
    routing ospf
      del network 10.125.0.0/24
      ..
      ..
    del routing ospf
    ..

Verifying OSPF configuration ¶

The following commands can be used to verify OSPF operation.

Display the global OSPF parameters (timers, area, router-id, etc.):

vsr> show ospf
   OSPF Routing Process, Router ID: 10.125.0.1
   Supports only single TOS (TOS0) routes
   This implementation conforms to RFC2328
   RFC1583Compatibility flag is disabled
   OpaqueCapability flag is disabled
   Initial SPF scheduling delay 0 millisec(s)
   Minimum hold time between consecutive SPFs 50 millisec(s)
   Maximum hold time between consecutive SPFs 5000 millisec(s)
   Hold time multiplier is currently 1
   SPF algorithm last executed 55.670s ago
   Last SPF duration 62 usecs
   SPF timer is inactive
   LSA minimum interval 5000 msecs
   LSA minimum arrival 1000 msecs
   Write Multiplier set to 20
   Refresh timer 10 secs
   Number of external LSA 0. Checksum Sum 0x00000000
   Number of opaque AS LSA 0. Checksum Sum 0x00000000
   Number of areas attached to this router: 1
   Area ID: 0.0.0.0 (Backbone)
     Number of interfaces in this area: Total: 1, Active: 1
     Number of fully adjacent neighbors in this area: 1
     Area has no authentication
     SPF algorithm executed 4 times
     Number of LSA 3
     Number of router LSA 2. Checksum Sum 0x0001701c
     Number of network LSA 1. Checksum Sum 0x00005dd4
     Number of summary LSA 0. Checksum Sum 0x00000000
     Number of ASBR summary LSA 0. Checksum Sum 0x00000000
     Number of NSSA LSA 0. Checksum Sum 0x00000000
     Number of opaque link LSA 0. Checksum Sum 0x00000000
     Number of opaque area LSA 0. Checksum Sum 0x00000000

Display the OSPF v2 RIB:

vsr> show ospf route
   ============ OSPF network routing table ============
   N    10.125.0.0/24         [100] area: 0.0.0.0
                                directly attached to eth1

   ============ OSPF router routing table =============

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

Display the OSPF configuration for the specified interface:

vsr> show ospf interface eth2
   eth2 is up
     ifindex 4, MTU 1500 bytes, BW 1000 Mbit <UP,BROADCAST,RUNNING,MULTICAST>
     Internet Address 10.125.0.1/24, Area 0.0.0.0
     MTU mismatch detection: enabled
     Router ID 10.125.0.1, Network Type BROADCAST, Cost: 100
     Transmit Delay is 1 sec, State DR, Priority 1
     No backup designated router on this network
     Saved Network-LSA sequence number 0x80000002
     Multicast group memberships: OSPFAllRouters OSPFDesignatedRouters
     Timer intervals configured, Hello 10s, Dead 40s, Wait 40s, Retransmit 5
       Hello due in 5.118s
     Neighbor Count is 1, Adjacent neighbor count is 1

Display the state of the relations with the neighbors:

vsr> show ospf neighbor

   Neighbor ID     Pri State           Dead Time Address         Interface            RXmtL RqstL DBsmL
   10.125.0.3        1 Full/DR           30.833s 10.125.0.3      eth1:10.125.0.1         0     0     0

Display the OSPF v2 Link-State databases and information about LSAs (Link State Advertisements)

vsr> show ospf database default

         OSPF Router with ID (10.125.0.1)

              Router Link States (Area 0.0.0.0)

   Link ID         ADV Router      Age  Seq#       CkSum  Link count
   10.125.0.1      10.125.0.1      1171 0x80000007 0xb213 1
   10.125.0.3      10.125.0.3      1134 0x80000007 0xae11 1

              Net Link States (Area 0.0.0.0)

   Link ID         ADV Router      Age  Seq#       CkSum
   10.125.0.3      10.125.0.3      1174 0x80000004 0x57d7

OSPF configuration in single area example ¶

../../../../_images/first-configuration.svg

First OSPF v2 configuration¶

rt1

vrf main
  routing ospf
    network 10.1.1.0/28 area 0
    ..
    ..
  interface
    physical eth1_0
      ipv4 address 192.168.1.0/24
      ..
    physical eth0_0
      ipv4 address 10.1.1.1/28
      ..
    ..

rt2

vrf main
  routing ospf
    network 10.1.2.0/28 area 0
    ..
    ..
  interface
    physical eth1_0
      ipv4 address 192.168.2.0/24
      ..
    physical eth0_0
      ipv4 address 10.1.1.2/28
      ..
    ..

rt3

vrf main
  routing ospf
    network 10.1.3.0/28 area 0
    ..
    ..
  interface
    physical eth1_0
      ipv4 address 192.168.3.0/24
      ..
    physical eth0_0
      ipv4 address 10.1.1.3/28
      ..
    ..

The verification of the operation can be done with following command:

rt1> show ospf neighbor
Neighbor ID     Pri State           Dead Time Address         Interface            RXmtL RqstL DBsmL
192.168.2.0       1 Full/Backup       33.553s 10.1.1.2        eth0_0:10.1.1.1           0     0     0
192.168.3.0       1 Full/DROther      37.951s 10.1.1.3        eth0_0:10.1.1.1           0     0     0

Note

The state must be Full. In this state, routers are fully adjacent with each other. All the router and network LSAs are exchanged and the routers’ databases are fully synchronized.

When you get used with the semantic of the OSPF v2 database, it can be displayed with the following command. The details about these entries are out of the scope of this document.

rt1> show ospf database default

 OSPF Router with ID (192.168.2.0)

            Router Link States (Area 0.0.0.0)

 Link ID         ADV Router      Age  Seq#       CkSum  Link count
 192.168.1.0     192.168.1.0      214 0x80000004 0xeb12 1
 192.168.2.0     192.168.2.0      213 0x80000004 0xe713 1
 192.168.3.0     192.168.3.0      214 0x80000004 0xe314 1

 Net Link States (Area 0.0.0.0)

 Link ID         ADV Router      Age  Seq#       CkSum
 10.1.1.1        192.168.1.0      214 0x80000002 0x5d4d

OSPF configuration with BGP redistribution ¶

Following example illustrates how OSPF can be used to redistribute routes to BGP. The above drawing is reused, with some changes on rt1 and rt2.

rt1

vrf main
  routing bgp
    router-id 10.1.1.1
    address-family
      ipv4-unicast
        redistribute ospf
        ..
      ..
    as 55
      neighbor 192.168.1.10
        remote-as 55
        ..
      ..
    ..
  routing ospf
    network 10.100.0.0/24 area 0
      router-id 10.175.0.1
      ..
    ..
  interface
    physical eth1_0
      ipv4 address 192.168.1.0/24
      ..
    physical eth0_0
      ipv4 address 10.1.1.1/28
      ..
    ..

rt2

vrf main
  routing ospf
    network 10.1.2.0/28 area 0
    network 192.168.2.0/28 area 0
    ..
    ..
  interface
    physical eth1_0
      ipv4 address 192.168.2.0/24
      ..
    physical eth0_0
      ipv4 address 10.1.1.2/28
      ..
    ..

The BGP routing table of rt1 is updated with the information from rt2.

rt1> show bgp ipv4 unicast
 BGP table version is 4, local router ID is 10.1.1.1, vrf id 0
 Status codes:  s suppressed, d damped, h history, * valid, > best, = multipath,
                i internal, r RIB-failure, S Stale, R Removed
 Nexthop codes: @NNN nexthop's vrf id, < announce-nh-self
 Origin codes:  i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
 *> 192.168.2.0      10.1.1.2               200         32768 ?

OSPF per interface configuration ¶

There is a wide variety of per-interface OSPF configuration items. Using same parameters between 2 OSPF instances is mandatory, and it is often useful to rely on that. Below example shows it is possible to change the network type of an interface. The OSPF network of interface eth0_0 is defined as a non broadcast type. Adding to that configuration, the retransmit interval timer has been changed.

vrf main
  routing ospf
    router-id 10.125.0.1
    ..
    ..
  routing interface eth1_0
    ip ospf network non-broadcast
    ip ospf area 0
    ip ospf retransmit-interval 6
    ..
    ..

Configuring OSPF¶

Basic elements for configuration¶

Verifying OSPF configuration¶

OSPF configuration in single area example¶

OSPF configuration with BGP redistribution¶

OSPF per interface configuration¶