OSPF is a complex protocol. To understand their beginner abstract concepts, is a different approach - Visualization OSPF simulation. This tutorial simulates basic OSPF, OSPF basic activities on a simple topology: R1-R2-H1-H2. R1 and R2 routers. H1 and H2 are the host. This simulation shows 4 stages:
1) R1 and R2 not running OSPF. H1 H2 ping. It fails.
2) Run OSPF on two routers. R1 and R2 see each other as neighbors by flooding Hello.
3) R1 and R2 develop full relations and synchronize their LSA (Link State Advertisement) database by DD, and update request. R1 and R2 update their routing tables. H1 and H2 become available.
4) H1 H2 pings again. It is possible this time.
Q1. What is OSPF?
Answer: OSPF is a link state routing protocol. OSPF routers exchange their Link State Advertisement (OSPF LSA), interface to IP addresses, to learn from each other. Each router stores its LSAs in its link stateDatabase (LSDB.) adjacent nodes synchronize their LSDBs. With consistent LSDBs OSPF routers are able to compute shortest paths to achieve goals.
Q2. What does OSPF to do?
Answer: Roughly speaking OSPF is to go through the following stages:
1) OSPF node created to describe a router LSA its interface IP addresses. The LSA is the easiest LSA.
2) discover neighbor routers by flooding Hello. If a couple find each other, they are in 2-way state.
3) neighborssynchronize their LSDBs in three steps:
i. sending DD LSDB get catalog today.
ii. Send a request to ask for missing LSAs.
iii. Send Update to transfer LSAs requested.
If two neighbors have the same LSDB, they are in Full Adjacency state.
4) consistent with the same LSDB routers have knowledge of network topology and interface IP addresses. You can calculate routing paths and update their own routing tables. The collection of routing tables of OSPFRouter provides the shortest route between destinations.
Q3. What is the router LSA?
Answer: Each OSPF router creates a router LSA at the interfaces of IP addresses to describe. In a very simple OSPF network are only router LSAs are needed to compute shortest paths.
Q4. What do mean router LSA?
Answer: In a simple OSPF network (no LAN, no area), neighboring nodes exchange their router LSAs and learn how many links has its neighbor and its interface IP addresses. According to neighborsReproduction, all OSPF nodes the same set of router LSAs in their LSDBs. Now they have the same knowledge of network topology.
Q5. How to use a router LSDB to calculate routing paths?
Answer: LSDB, an OSPF node learns the entire topology of the network: the number of routers are connected; individual router interfaces and their IP addresses; link Interface cost (OSPF metric). charged with such detailed information to reach OSPF shortest path to all destinationsfound in LSDB.
For example, [a], in the OSPF LSDB's R1 simulation contains two routers LSA:
1) originated LSA R1 contains two IP addresses: 192.168.1.0/24, 192.168.3.0/30.
2) R2 were LSA contains two IP addresses: 192.168.2.0/24, 192.168.3.0/30.
After LSDB synchronization, R1 and R2 both have these two LSAs. And they know that R1 and R2 are connected by the link 192.168.3.0. Now R1 calculate routing path to the remote destination 192.11.68.2.2 reach and adds an entry(192.168.2.0/24, S1 / 0) to its routing table.
Note [a]: This article is the FAQ for OSPF simulation. After reading, you can play the interactive simulation in external links listed. It OSPF visualize work flow with moving packets are changing tables.
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