Router OSPF Overview Router OSPF-Part1 Router OSPF is the command used to enable OSPF routing on a Cisco router. But before we get into the command syntax I want to give you a brief summary of some important OSPF details for the ICND2 and CCNA Exam. Dynamic Routing Protocol OSPF is a Dynamic Routing Protocol that can be used across many different brands of routers and not just Cisco. For a little refresher, a dynamic routing protocol is able to adapt quickly to networking changes in case a link breaks or a router ceases to function. It will be able to find an alternate route automatically so your whole network doesnâ€™t come crashing down as opposed to static routes where the network administrator would have to update all the new routes manually. Interior Gateway Protocol There are interior gateway protocols and exterior gateway protocols. An interior gateway protocol works with a single group of networks and an exterior gateway protocol connects different groups of networks together. OSPF is an interior gateway protocol. Interior gateway protocols have three different types of ways to determine the best route between routers. There are distance vector, link-state, and a mixture of the two. OSPF uses link-state. Many different routing protocols can be configured on a Cisco router at a time. In order for the routers to determine which routing protocol should take priority they use administrative distance. The lower the administrative distance the better. A directly connected route has an AD of 0 and a statically configure route has an AD of 1. OSPF has an AD of 110. As a comparison to other routing protocols EIGRP has an AD of 90 and RIP has an AD of 120. This means that if you have OSPF and RIP configured in a network, the routers will choose the routes created by OSPF over RIP. Link-State The main difference between link-state and distance vector routing protocols is that link state will only send updates when there is a topology change. Distance vector however send updates at regular intervals, such as every 30 seconds. As a comparison OSPF sends its topology table out every 30 minutes. Which is quite a bit longer time to wait to here about updates. However, OSPF makes up for its infrequent topology exchanges by using link-state advertisements which are tiny http://www.router-switch.com/
updates that get sent out immediately after a change is detected. So, the benefit of link-state is that it can use less networking resources with fewer updates and can converge much faster. Convergence means that all the routers in the network have finished discussing the changes that have occurred and are no longer sending updates back and forth. Shortest Path First OSPF uses what is called the SPF algorithm to calculate the shortest paths between routers. Even though each OSPF router has the exact same topology, they each will have a different point of view because the SPF algorithm gets calculated on each router individually. Areas OSPF is able to scale well with large networks because it uses areas to break up an autonomous system so that it can minimize all the routing updates that occur during convergence. This is necessary because if you have a network with 50 different routers and there are frequent changes, the SPF algorithm will have to be run every time. And with many routers this can take a lot of time. If you split the 50 router network into 5 different areas the topology tables are only propagated within each area individually. By splitting it up this way the SPF algorithm can run much faster. OSPF Routers have different roles depending on their location in an area. There are Area Border Routers, Autonomous System Boundary Routers, Internal Routers, and Backbone Routers. More to come… This article was meant to be a brief introduction to OSPF and I plan on talking more in depth about different areas of OSPF such as LSA’s, SPF, areas, and configuration. If you have any specific OSPF questions you would like to have answered please comment below and let me know. I hope your ICND2 and CCNA preparation is coming a long great. Keep up the hard work!
Router OSPF-Part2 This is Part 2 of the Router OSPF Serieswhich will cover topics about OSPF Areasand OSPF LSA’s. Before continuing on I recommend reading Part 1 of the Router OSPF Series which gives an introduction on the topic. For the ICND2 and CCNA exam you will only need to configure a single area OSPF implementation, but you will still be required to know many different things about a full multi-area OSPF network.
OSPF is a link-state routing protocol that stores a complete topology map of the network it is in. In order for OSPF to know where the shortest routes are to each network it has to calculate it off of the topology map using the Dijkstra algorithm every time there is a change in the network. This process can take up a lot of CPU cycles and slow down the routing process if there are many routers. In order for OSPF to be able to scale well in larger network situations with a lot of routers it uses Areas to logically break up the network. OSPF Router Types Backbone Router – Every OSPF implementation must have a backbone area. The backbone area will always be area 0. A Backbone router is any router that has an interface connected to area 0. Area Border Router (ABR) – Are routers that have on interface in area 0 and another interface in a different area. ABR’s sit between the two areas. All ABR’s are backbone routers because they have an interface connected to area 0. Autonomous System Boundary Router (ASBR) – Share information with other routers that are not running OSPF. Internal Router – Are routers that are not area border routers. They simply are routers running OSPF, but are not passing information between areas. Here is an example of a network with multiple OSPF areas so that you can visually see the different OSPF router roles amongst the areas.
Types of OSPF Areas Backbone Area – This area is also known as the standard area. Every OSPF implementation has to have one. Backbone areas accept all types of LSA’s. Stub Area – Will only receive summary LSA’s. Routing LSA’s are NOT allowed. Totally Stubby Area – Absolutely no LSA’s are allowed. Not So Stubby Area (NSSA) – This is a stub area that has a ASBR router that receives information from another routing protocol other than OSPF and passes it into the OSPF network. http://www.router-switch.com/
OSPF LSA Types There are seven different types of Link-State advertisements that OSPF routers pass around to check up on each other. When you type “show ip route” on a Cisco router to see the routing table, ospf entries will show up with the letter “o” in front. There are also many different variations that will show up in the routing table depending on the LSA type. LSA Type 1: “O” — Intra-Area — Passed around inside an area LSA Type 2: “O 1A — Inter-area — Pass through an area LSA Type 3: Summary LSA by an ABR LSA Type 4: Summary LSA to an ASBR LSA Type 5: “O E1″ or “O E2″ — From an ASBR about external links LSA Type 6: This is a Groupe Membership Link LSA that is sent out by multicast OSPF Routers LSA Type 7: “O N1″ or “O N2″ — These are NSSA external routes from an ASBR Well, I Think that is quite a bit of OSPF information for today. Tomorrow I’m actually going to show you how to configure OSPF since we still haven’t gone over that yet. More OSPF Tips: OSPF Areas Types, OSPF Router Types & OSPF Route Types How to Fix OSPF Split Area with GRE Tunnel?