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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.

OSPF_initial_route_discovery.jpg

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

OSPF_initial_route_discovery_2.jpg

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.

OSPF_send_DBD.jpg

OSPF_send_DBD_2.jpg

R1 or R2 can send Request to get missing LSA from its neighbors

OSPF_send_request.jpg

OSPF_send_update.jpg

R2 sends back an LSAck packet to acknowledge the packet

OSPF_send_ack.jpg

Comments (8) Comments
  1. alpholove75
    December 28th, 2019

    Hey Dex,

    Did you take your ICND1 exam on November 16, 2019 ? If yes, how was it ?

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    December 30th, 2019

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    February 23rd, 2020

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    February 23rd, 2020

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  6. Hasan Ghassan
    November 7th, 2020

    Nice Explanation, but I think you forgot to add OSPF configuration commands in this tutorial, it is only theoretical without commands.
    Thanks

  7. Stef
    May 10th, 2022

    “Is the only link-state routing protocol you learn in CCNA” why there are EIGRP questions on the exam? You really should put configuration chapter in here

  8. Liza Soberano
    January 9th, 2023

    @Stef “Is the only link-state routing protocol you learn in CCNA” why there are EIGRP questions on the exam? – EIGRP is not a link state protocol

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