OSPF FAQ - Frequently Asked Questions

In this post, some of the frequently asked OSPF questions will be answered. Some of the answers will be from a design point of view and we will try to provide enough justification for the answer. Questions are selected randomly, not based on the order of importance.

How many routers should be placed in one OSPF area?

We don't give any numerical number as an answer to this question. Depending on the number of routers, links, prefixes, and the topology of the network also depends on the hardware capabilities and the performance of the routers, the number change from Network to network. In some networks, you can place only a couple of hundreds, and in some networks, you can place thousands of Routers in a single OSPF Area.

What command would you use to only see the OSPF routes in the routing table?

On Cisco devices, sh ip route ospf is used to see only the OSPF routes in the routing table.

What will an OSPF router prefer to use first as a router-id?

The first preference for an OSPF router ID is an explicitly configured 32-bit address. This address is not included in the routing table and is not defined by the network command. If it is not configured manually, the highest loopback IP address is used.

In which OSI model layer will you find the OSPF protocol?

OSPF is an IP-based protocol. Not like IS-IS or BGP. IS-IS is Layer protocol and BGP runs on top of Layer 4, TCP protocol. OSPF is Layer 3, IP Based protocol based on OSI Layer.

What is one reason to use the ip ospf priority command when the OSPF routing protocol is in use?

In DR and BDR selection in OSPF, a priority command is used for the election process. A higher priority router is selected as a DR. If the purpose is not to allow one of the routers in the election process, then ip ospf priority command is used.

With multiarea OSPF, which OSPF router type has all interfaces in the same area?

In Multi-area OSPF, which is also commonly known as Hierarchical OSPF design, Internal routers have all interfaces in the same area. ABR Routers need to have more than one Area, and one of the OSPF Areas, has to be OSPF Area 0, OSPF Backbone Area.

Why would you want to change the OSPF default reference bandwidth?

By default, the reference bandwidth is 100 Mbps. However, if you have faster links in your network, such as gigabit Ethernet or higher speed, OSPF can't give these links a better cost than 1. So you should set the reference bandwidth to at least as high as the fastest link in your network. Overall you can manipulate the cost, so you can do better Traffic Engineering by using OSPF cost.

What is the format of the router id on an OSPF-enabled router?

OSPF router ID is a 32-bit number formatted like an IPv4 address and assigned in order to uniquely identify a router.

What is a benefit of multiarea OSPF routing?

Multi-Area OSPF routing is done for scalability purposes. When routers are in place in different OSPF Areas, the purpose is to be able to send only summary routes or default route into the OSPF Non-Bachone Areas. Multiarea OSPF Routing is known as Hierarchical OSPF Routing and it is done to grow a number of routers and the prefixes in the OSPF network.

What function is performed by the OSPF designated router?

OSPF Designated Router also provides scalability. When it is used, routers don't have a full-mesh adjacency with each other in OSPF networks. Thus it reduces the number of control plane exchanges and LSA advertisements. Reducing flooding in case of newly added networks or in case of failure allows scalability. If there is a p2p network, we should avoid having OSPF DR.

Which command will provide information specific to ospfv3 routes in the routing table?

show ipv6 route ospf command provides information specific to ospfv3 routes only.

Which command is used to display the collection of OSPF link states?

show ip ospf database provides all the entries in the OSPF Link state database.

What is a characteristic of a single-area OSPF network?

Single Area OSPF network is by default placing routers in a single OSPF Area. It can be OSPF Backbone Area, which is OSPF Area 0. It is the most common deployment model. Or, if all the routers are placed in an OSPF Non-backbone area, which is any number other than Area 0, it is also called Single Area OSPF Routing design. Single-Area OSPF is known as a Flat OSPF network as well.

What does an OSPF area contain?

The OSPF area contains OSPF internal routers. Their connections, which mean links, and through these links, OSPF neighborship allows OSPF Routers to exchange IP prefixes. In a given OSPF Area, every router has the same Link state database.

What command will display all connected ospfv2 routers?

With show ip ospf neighbor command you can view the connected OSPFv2 routes.

How to configure OSPF on the Cisco router?

To enable OSPF on a Cisco router and advertise interfaces, the following tasks are required:

Use the command router ospf process ID to start OSPF.
Use the network command to enable the interfaces.
Identify OSPF area assignments.
(Optional) Assign the router ID.

Which type of OSPF router will generate type 3 lsas?

OSPF Type3 LSA is generated by the OSPF ABR to summarize OSPF Type 1 and OSPF Type2 LSA. Because between the OSPF Areas, Type 1 and Type 2 LSAs cannot be sent, Type 3 LSA is generated by the ABR to teach a given OSPF Area prefixes to another.

What is the easiest and preferred method of manipulating OSPF route costs?

OSPF uses cost to calculate the metric, based on the bandwidth of the link. Higher bandwidth means lower cost, and it is preferred over higher cost. So increasing bandwidth is one way to influence/manipulate the path in OSPF.

What does the Cisco IOS use to calculate the OSPF cost to a destination network?

Cisco and many other vendor Routers use Bandwidth to calculate OSPF cost to a destination network. Although cost can be manipulated manually and manual cost can be assigned based on other criteria such as delay/latency, fiber route mile, monetary cost, and so on, many networks in real-life OSPF design, keep the default cost parameter, which is bandwidth.

In OSPF, hellos are sent to what IP address?

OSPF neighbors are dynamically discovered by sending Hello packets out to each OSPF-enabled interface on a router. Hello packets are sent to the multicast IP address of 224.0.0.5.

Which command can be used to verify the contents of the lsdb in an OSPF area?

show ip ospf database is used to verify all the contents of the OSPF link-state database. We have two different databases in OSPF. One of them is LSDB, a link-state database and another is TED, a Traffic Engineering Database.

Which type of OSPF LSA is generated by an ASBR and includes external routes?

Type5 LSA is generated in OSPF by the ASBR and sent into the network. Single-Area OSPF and Multi-Area OSPF networks Type5 LSA is sent everywhere. Except, if the Area is Stub, Totally Stub, NSSA, or Totally NSSA, then Type 5 LSAs are not allowed into those OSPF Area Types.

What are two reasons for creating an OSPF network with multiple areas?

The main reason we create Multiple Areas in OSPF is Scalability. Scalability means allowing for growth. So, the number of Routers, links, and prefixes, in time, can grow and our network design shouldn't stop growing. When Multi-area OSPF is designed, into the Non-backbone areas topology information is not sent, thus it provides more stability in those areas. Also, convergence time gets faster and troubleshooting becomes easier in Multi-area network design.

What default length is used for the OSPF dead interval?

The default values are 10 seconds for the hello time, and 40 seconds for the dead time. The usual rule of thumb with OSPF is to keep the dead time value four times the hello interval. If OSPF fast convergence is the requirement, then these values can be reduced to provide fast failure detection. BFD is another method for fast failure detection.

What are the enhancements that ospfv3 supports over ospfv2?

In OSPFv3, similar to IS-IS protocol, reachability information and topology information are separated. So. when we add a new loopback interface (not topology change), in OSPFv2, it triggers full-SPF calculation, which is a resource-intensive task. Also, OSPFv3 supports both IPv6 and IPv4, but OSPFv2 only supports IPv4.

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