How to configure and verify OSPF

Certification: Cisco CCNA Routing and Switching - Cisco Certified Network Associate Routing and Switching

Open Shortest Path First is an interior gateway routing protocol that gathers information and constructs a topology map of a network with a single routing domain such as an Autonomous System. OSPF protocols support both IPv4 and IPv6, featuring both VLSM and CIDR address models. It measures using external metrics associated with each routing interface in order to create its tables using both distance and availability such to find the shortest path to the destination. OSPF also detects any changes in the network, such as new routes or link failures, and updates its topology accordingly using the Dijkstra's algorithm.

While the OSPF does work on its default settings, it is often recommended that you configure it to tweak it to your needs. This article will talk about such configuring and try to touch as many points as possible, but if you are a network technician and want an in-depth explanation you might be better off looking at this CISCO guide.

Things to know beforehand

While OSPF supports both IPv4 and 6, the security aspects on either differ. On IPv4 it can operate securely between routers to authenticate using the built in protection, while on the IPv6 it needs to rely on the IPv6 protection security.

The routes created by the OSPF can be redistributed to any other protocol. It can import routes from intradomain routes such as ABR, ASBR, and can export into IGRP, RIP, IS-IS. For interdomain it can export into both EGP and BGP.

It requires coordination within internal routers, both ABR and ASBR. The minimum default parameters include no authentication and area-assigned interfaces. During configuration, you can specify route distribution, can alter certain interface-specific OSPF parameters as needed, can configure the network either as broadcast or NBMA.

While the OSPF requires a fully meshed network, you can configure it as a point to multipoint network, in case it is not fully meshed. This route may prove more advantageous as it is easier to configure, since you don’t need to configure nearby routers and it consumes only one IP subnet. Also, since it doesn’t require a fully meshed network it is cheaper, and can maintain connectivity in case of FVc fail.

A designated router must be selected (along with an optional backup designated router). A designated router is a router with a non-zero router priority value. Youc an also assign a value to its neighbors, although that is optional.

Using NSSA can simplify network administration for ISPs and can simplify communication with an OSPF protocol. It is also recommended that you configure the sending interface to passive.

The OSPF uses the largest IP address configured on the interface as router ID, and its metric is calculated as ref-bw, by default 108, divided by bandwidth. The OSPF also uses an administrative distance metric counting from 0 to 255, to measure reliability of administrative data. A point with a 255 administrative distance metric is counted and unsecure and is not taken into account at all.

How to configure

To configure it, you must first enable the OSPF. That can be done by the privileged exec mode. (also enter password if prompted). After which you should enter the glob config mode. Then go on and enable the OSPF (# router ospf 109). After that you will be prompted to configure it, after which you should exit the router confi mode.

Next, to configure the interface parameters you need to enter the exec mode, go to configure terminal to enter global configuration, then go to the interface type number command (Router(config)# interface GigabitEthernet 0/0) this will let you configure the type and interface mode. Then you can set the ipospf cost, re-transmit interval, transmit delay, priority, hello/dead-interval, authentification password (for nearby routers). Don’t forget the end command when you are done.

To enable the NSSA, the steps are:enable the exec mode, enter to configure terminal, configrouter ospf process-id;“redistribute protocol [process-id] {level-1 | level-1-2 | level-2} [autonomous-system-number][metric {metric-value | transparent}][metric-type type-value][match {internal | external 1 | external 2}][tag tag-value][route-map map-tag][subnets][nssa-only]” (As said by CISCO);define the interface on which it runs (network ip-address wildcard-mask area area-id);configure a nss area (area-id nssa [no-redistribution][default-information-originate [metric][metric-type]][no-summary][nssa-only]), then you can configure the summary-address prefix mask ([not-advertise][tag tag][nssa-only]), and then don’t forget to exit.

You can also configure an NSSA ABR as a (forced) NSSA LSA Translator. Most steps are the same, except for: configuring a NSSA area border router, (also remember the always keyword to config. the NSSA ABR device as a forced NSSA LSA translator.)(area-idnssa translate type7 always). Also, don’t forget to allow ABR to suppress the forwarding address in translated Type-5 LSA.

You can also disable RFC 3101 Compatibility and Enabling RFC 1587 Compatibility, donfigure OSPF over Different PNs, configure OSPF for Nonbroadcast Networks, configure OSPF Area Parameters, configure OSPF NSSA Parameters (although you may need to view the prerequisites to doing such). There are quite a lot of options when it comes to configuring different aspects of the OSPF, so be sure to check them on the CISCO help-page. In case the explanations in this article seemed a bit blurry, then you will have no problems finding your way around the cisco documentation. You may feel pressured by the big block of text, but fret not. Just keep scrolling until you get to the actual configuration steps.   

There may also be times when you want to verify certain aspects of the configuration. When you need to do that, all you have to remember is the show ipospfandshow ipospf flood-listcommand.Also, remember the use of point-to-multipoint rather than the usual one, and the differences that need to be configured for a network that is not fully meshed.

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