OSPF Tutorial
There are 3 type of tables
+ Neighbor
+ Topology
+ Routing
Neighbor table
+ Contain information about the neighbors
+ Neighbor is a router which shares a link on same network
+ Another relationship is adjacency
+ Not necessarily all neighbors
+ LSA updates are only when adjacency is established
Topology table
+ Contain information about all network and path to reach any network
+ All LSA’s are entered into the topology table
+ When topology changes LSA’s are generated and send new LSA’s
+ On topology table an algorithm is run to create a shortest path, this algorithm is known as SPF or dijkstra algorithm
Routing Table
+ Also knows as forwarding database
+ Generated when an algorithm is run on the topology database
+ Routing table for each router is unique
D: Exchange LSDB’s list
Neighbors use DD (Data Description) to exchange their LSDB catalogs. In this scenario, R1 sends DD to R2 first. It says: I have a Route LSA from R1. R2 also sends DD to R1: I have a Route LSA from R2.
Note: DD works like table fo content. It lists what LSDB has, but not details. By reading DD, the receiving router can determine what it is missing and them ask the sender to transmit required LSAs..
R1 Request, R2 Update
R1 has learned that R2 has a R2 Router LSA that it does not have.
R1 sends a LS Request to R2. When R2 receives this request, it sends an Update to transmit this LSA to R1.
R2 Request, R1 Update
R2 also sends request to R1. R1 replies an Update. Upon receiving Update, R2 adds R1 Router LSA to its LSDB, calculates its routes, and add a new entry (192.168.1.0, S1/0) to its routing tabe.
Note: OSPF works distributely. After routers have synchronized their LSDB, they use the same data (LSDB) to calculate shortest paths, and updates their routing tables independently.
Ack update : LSAs are received
In order to assure reliable transmission, when a router receives an Update, it sends an Ack to the Update sender. If the sender does not receivie Ack within a specific peried, it times out and retransmits Update.
Note: OSPF uses Update-Ack to implemnet relaible transmission. It does not use TCP.
H1 ping H2: succeeded.
Each OSPF router creates a Router LSA to describe its interfaces’ IP addresses and floods its Router LSA to its neighbors. After a few rounds of flooding, all OSPF routers have the same set of Router LSAs in their LSDBs. Now routers can use the same LSDB to calculate routes and update routing tables.
From LSDB, a router learns the entire topology: the number of routers being connected. Router interfaces and their IP addresses, interface link costs (OSPF metric). With such detail information, routers are able to calculate routing paths to reach all destinations found in LSDB. For example, in the OSPF basic simulation (see External links), R1′s LSDB contains two Router LSAs: – A Router LSA from R1. R1 has two links. Their IP addresses are 192.168.1.0/24,192.168.3.0/30. – A Router LSA from R2. R2 has two links. Their IP addresses are 192.168.2.0/24,192.168.3.0/30. From these LSA, R1 can calculate the routing path to reach remote destination 192.11.68.2.2 and adds an entry (192.168.2.0/24, S1/0) to its routing table.
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Wonderful, a concise way for me to review. Straight and to-the-point thanks!!
thanks buddy…great job done..thanks once again.
Hello,
I have a question. There is a reference to R1, R2, with 192.168.x.x IP addresses. It will be nice if the topology diagram is listed somewhere, I will be able to relate better.
Thanks
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To 9tut: New OSPF question about the process of building the adjacency.You must provide the good stage position between Exstart, Exchange, Loading and Full. You have to put these steps in the right postion.
Thank you, nice article.
It would be nice if you could append it and explain the elusive reasons for DR and BDR routers. I’m still not crystal on that.
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hello, what about multi area ospf, v2 v3 is this covered here sorry if it is and i didn’t see it
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that should have been CCNA -200-120.
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thanks for this type of information about ospf
Anyone seen any OSPFv3 questions on the 200-120 exams? Specifically, any involving OSPF configurations using IPv6?
Note: DD works like table fo content.
fo should be of !
cheers…
By reading DD, the receiving router can determine what it is missing and them ask the sender to transmit required LSAs..
them should be then :)
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Question No. 8 for summarization is not understood properly, can someone explain how ?
Question 6:
Refer to the exhibit. What is the most efficient summarization that R1 can use to advertise its networks to R2?
autosummarize
A. 172.1.0.0/22
B. 172.1.0.0/21
C. 172.1.4.0/22
D. 172.1.4.0/24
172.1.5.0/24
172.1.6.0/24
172.1.7.0/24
E. 172.1.4.0/25
172.1.4.128/25
172.1.5.0/24
172.1.6.0/24
172.1.7.0/24
dear Anonymous
the answer is C 172.1.4.0/22
because that’s prefix 22 have 1022 ip active and then the range ip is 172.1.4.1 until 172.1.7.254 and have network-id = 172.1.4.0 broadcast = 172.1.7.255 , i hope my answere help you CMIIW
Use sign R2 In place of R1 in d second router figure. U have named R1 at 2 places but with different interfaces
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@9tut EIGRP is also now open-standard. Kindly make correction above in OSPF tutorial’s 1st page.
@ImmiRavian: Yes, thanks for your detection. We updated it.
could you modify the name of the Router at the end of the first page ?
When you are talking about the router 2, please write router R2 and not R1 as in your example.
@Anonymous: Thanks for your detection. We have just updated it.
thanks 9tut…..its very helpful to me.