OSPF Tutorial
For example, R1 wants to find out if it has any neighbor running OSPF it sends a Hello message to the multicast address 224.0.0.5. This is the multicast address for all OSPF routers and all routers running OSPF will proceed this message.
If an OSPF router receives an OSPF Hello packet that satisfied all its requirement then it will establish adjacency with the router that sent the Hello packet. In this example, if R1 meet R2′s requirements, meaning it has the same Hello interval, Dead interval and AREA number, R2 will add R1 to its neighbor table.
+ Hello interval: indicates how often it sends Hello packets. By default, OSPF routers send Hello packets every 10 seconds on multiaccess and point-to-point segments and every 30 seconds on non-broadcast multiaccess (NBMA) segments (like Frame Relay, X.25, ATM)
+ Dead interval: number of seconds this router should wait between receiving hello packets from a neighbor before declaring the adjacency to that neighbor down
+ AREA number: the area it belongs to
Now R1 and R2 are neighbors but they don’t exchange LSAs immediately. Instead, they sends Database Description (DD or DBD) packets which contain an abbreviated list of the sending router’s link-state database.
The neighbors also determine who will be the master and who will be the slave. The router which higher router-id will become master and initiates the database exchange. The receiver acknowledges a received DD packet by sending an identical DD packet back to the sender. Each DD packet has a sequence number and only the master can increment sequence numbers.
R1 or R2 can send Request to get missing LSA from its neighbors
R2 sends back an LSAck packet to acknowledge the packet
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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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Can someone also send my the latest VCE CCNA 200-120 please?
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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.