This chapter contains basic OSPFv2 (Open Shortest Path First) configuration examples.

The diagram shows the minimum configuration required to enable OSPF on an interface. R1 and R2 are two routers in Area 0 connecting to network 10.10.10.0/24.

This example shows how to set the priority for an interface. Set a high priority for a router to make it the Designated Router (DR). Router R3 is configured to have a priority of 10, which is higher than the default priority (1) of R1 and R2; making it the DR.

No authentication

This example shows configuration for an Area Border Router. R2 is an Area Border Router (ABR). On R2, Interface eth0 is in Area 0, and Interface eth1 is in Area 1.

In this example, the configuration causes BGP routes to be imported into the OSPF routing table, and advertised as Type 5 External LSAs into Area 0.

A route can be made the preferred route by changing its cost. In this example, cost has been configured to make R2 the next hop for R1.

The default cost for each interface is 1. Interface eth2 on R2 has a cost of 100, and Interface eth2 on R3 has a cost of 150. The total cost to reach 10.10.14.0/24 (R4) through R2 and R3 is computed as follows:

R2: 1+100 = 101

R3: 1+150 = 151

Therefore, R1 chooses R2 as its next hop to destination 10.10.14.0/24 because it has the lower cost.

In IPv4, path MTU discovery enables a host to actively identify and adapt to variations in the MTU size across different links along a data path. In contrast, IPv6 adopts an approach where fragmentation is managed by the packet's source when the path MTU of a specific link along the data path cannot accommodate the packet's size. This approach, where IPv6 hosts handle packet fragmentation, conserves processing resources in IPv6 devices and enhances the overall efficiency of IPv6 networks.

Virtual links are used to connect a temporarily-disjointed non-backbone area to the backbone area, or to repair a non-contiguous backbone area. In this example, the ABR R3 has temporarily lost connection to Area 0, in turn, disconnecting Area 2 from the backbone area. The virtual link between ABR R1 and ABR R2 connects Area 2 to Area 0. Area 1 is used as a transit area.

There are three types of OSPF authentications--Null (Type 0), Simple Text (Type 1), and MD5 (Type 2). With Null authentication, routing exchanges over the network are not authenticated. In Simple Text authentication, the authentication type is the same for all routers that communicate using OSPF in a network. For MD5 authentication, configure a key and a key ID on each router. The router generates a message digest on the basis of the key, key ID, and OSPF packet, and adds it to the OSPF packet.

The authentication type can be configured on a per-interface basis or a per-area basis. Additionally, Interface and Area authentication can be used together. Area authentication is used for an area, and interface authentication is used for a specific interface in the area. If the Interface authentication type is different from the Area authentication type, the Interface authentication type overrides the Area authentication type. If the Authentication type is not specified for an interface, the Authentication type for the area is used. The authentication command descriptions contain details of each type of authentication.

In the example below, R1 and R2 are configured for both the interface and area authentications. The authentication type of interface eth1 on R1 and interface eth0 on R2 is MD5 mode, and is defined by the area authentication command; however, the authentication type of interface eth2 on R1 and interface eth1 on R2 is plain text mode, and is defined by the ip ospf authentication command. This interface command overrides the area authentication command.

By using multiple OSPF instances, OSPF routes can be segregated, based on their instance number. Routes of one instance are stored differently from routes of another instance running in the same router.

To configure multiple OSPF instances, perform the following procedures referring to the topology diagram below:

In this example, routers R1, R2, and R3 are in Area 0, and all run OSPF.

In this example, routes of one instance are redistributed to another instance to enable ping from R1 to R3 or vice versa; and R2 redistributes routes from one instance to another.

In this example, on R3, R1 and R2 have each other’s routes with a metric of 100.

In this example, on R3, R1 has R3 routes as type 2, and R2 has R1 routes as type 1.

Multiple OSPF instances can be configured on the same subnet. The OSPF instance ID supports separate OSPFv2 protocol instances. With this feature, an adjacency is formed only if the received packet’s instance ID is the same as the instance ID configured for that interface.

Multiple OSPF areas for a same subnet can be configured between two routers. In the diagram below, OSPF is enabled between R2 and R3 under area 0 and area 1, though there is only one link available between these two routers. Multi-area adjacency allows establishing adjacency on multiple areas between the Area Border Routers (ABRs). The specified interface of the ABR is associated with multiple areas.

Each multi-area-adjacency internally implements point-to-point functionality, once the adjacency reaches the FULL state. This point-to-point link provides a topological path for that area. Like a virtual link, there is no restriction for multi‑area adjacency that the packets always go through the backbone.

show ip ospf multi-area-adjacencies, show ip ospf neighbor, show ip ospf route, show ip route

This section contains basic OSPF LSA throttling configuration examples.

The OSPF Link-State Advertisement (LSA) throttling feature provides a mechanism to dynamically slow down link-state advertisement (LSA) updates in OSPF during times of network instability. It also allows faster OSPF convergence by providing LSA rate limiting in milliseconds, when the network is stable.

The timers throttle lsa all command controls the generation (sending) of LSAs. The first LSA is always generated immediately upon an OSPF topology change, and the next LSA generated is controlled by the minimum start interval. The subsequent LSAs generated for the same LSA are rate-limited until the maximum interval is reached. The “same LSA” is defined as an LSA instance that contains the same LSA ID number, LSA type, and advertising router ID.

The timers lsa arrival command controls the minimum interval for accepting the same LSA. If an instance of the same LSA arrives sooner than the interval that is set, the LSA is dropped. It is recommended that the arrival interval be less than or equal to the hold-time interval of the timers throttle lsa all command.

The diagram shows the minimum configuration required to enable OSPF LSA Throttling Timers feature. R1 and R2 are two routers in Area 0 connecting to network 10.10.10.0/24.

Check the output of show ip ospf and verify the initial throttle delay, minimum hold time for LSA throttle and maximum wait time for LSA throttle.

Here, we administratively shutdown and then bring up the loopback interface to generate Rate Limit Timer events for OSPF debugging to capture.

Check the output of “show ip ospf neighbor” and verify that OSPF adjacency is up.

Check the output of show ip ospf database and verify that LSA (router LSA in this example) is updated according to the configured LSA throttling timers configured on its neighbor.

The diagram shows the minimum configuration required to enable OSPF Minimum LSA Arrival Timers feature. R1 and R2 are two routers in Area 0 connecting to network 10.10.10.0/24.

Check the output of show ip ospf and verify that the minimum LSA arrival timer by default is set to 1 sec.

Check the output of show ip ospf and verify that the minimum LSA arrival timer is set to 100 sec.

Check the output of show ip ospf neighbor and verify that OSPF adjacency is up.

Check the output of “show ip ospf database” and verify that LSA is accepted only after a time difference of 100 sec between two consecutive LSAs.

This section contains basic OSPF Loop-Free Alternate Fast Reroute (LFA-FRR) configuration examples.

The goal of (LFA-FRR) is to reduce failure reaction time to 10s of milliseconds by using a pre-computed alternate next- hop in the event that the currently selected primary next-hop fails, so that the alternate can be rapidly used when the failure is detected. A network with this feature experiences less traffic loss and less micro-looping of packets than a network without LFA-FRR.

After enabling LFA-FRR on routers, routers calculate a backup path for each primary path to reach the destination.The backup path is calculated based on the attributes such as node protecting, link protecting, broadcast-link protecting and secondary path.

The diagram shows the configuration required to enable the OSPF LFA feature.

R1

 

 

 

 

 

 

 

Check OSPF neighborship.

Check the OSPF route installation and LFA-FRR backup path for the primary path.

Not mandatory that for all primary path, there exists an LFA backup path only if inequality equation satisfies according to attributes configured on routers, backup path will be calculated.

Not mandatory that for all primary path, there exists an LFA backup path only if inequality equation satisfies according to attributes configured on routers, backup path will be calculated.

To prohibit an interface from being used as a repair path, disable fast reroute calculation on the interface.

Verify that the eth3 interface is not used for backup path calculation.

 

 

Based on the index values configured, if inequalities are satisfied, protections will be provided:

Verify show ip route and show ip route fast-reroute for backup path calculated according to attributes configured above.

A router MUST limit the amount of time an alternate next-hop is used after the primary next-hop has become unavailable. This ensures that the router will start using the new primary next-hops.

LFA termination avoids a micro looping in topology, when particular network goes down, LFA backup path will be installed and if termination interval is configured, LFA backup will be still used till the interval and it is used in order to verify new primary path is loop free.

Configure termination interval on R1 in router mode:

 

If you check "show ip ospf" you can see the configured termination-hold on interval value along with ospf output:

 

Shut down one of the primary nexthops, here eth2 of rtr1:

 

SNMP operation by default are tied to a specific OID which is unique. However protocol like OSPF can have multiple instances, and have different values of same parameters for different OSPF instances. To be able to support SNMP for each of these instances, it is needed that each instance of the protocol has its own instance of the MIBs. It is aimed to achieve that with mapping each instance to a context. Each context will point to a different copy of the same OID for the protocol.

In this example, routers R1 & R2 are in Area 0, and all run OSPF. SNMPv2 user is created and Mapping of user with group and context for SNMPwalk /SNMP get operation on context.

Perform snmpwalk as mentioned below with IPv4 address using SNMPv2

Perform snmpwalk as mentioned below with IPv4 address using SNMPv2 for R2

In this example, routers R1 & R2 are in Area 0, and all run OSPF. SNMPv3 user is created and Mapping of user with group and context for SNMPwalk /SNMP get operation on context.

Perform snmpwalk as mentioned below with IPv4 address using SNMPv3 of R2

In this example, routers R1, R2 & R3 are in Area 0, and all run OSPF. SNMPv2/v3 user is created and Mapping of user with group and context for SNMPwalk /SNMP get operation on context.