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What is OSPF Routing Protocol? OSPF Hidden FACTS Revealed

What is OSPF Routing Protocol? OSPF Hidden FACTS Revealed 

Author: - Amarjit Singh

Why are loopbacks advertised as /32 host routes in OSPF? Loopbacks are considered host routes in OSPF, and they are advertised as /32. If the ip ospf network point−to−point command is configured under loopbacks, OSPF advertises the loopback subnet as the actual subnet configured on loopbacks

All the OSPF packet types share a common 20-byte OSPF protocol header.

Common OSPF Protocol Header Format is as shown below


The current version number of OSPF is 2. Version 1 is not compatible with Version 2

The router ID is used to uniquely identify the router throughout the autonomous system. After the router ID is chosen, it will not change unless the router is restarted, the inter-face that is selected as a router ID is shut down, or the IP address has been removed or replaced on that interface

The value of Area ID must be the same on both sides to form neighbor relationships. There are two ways to write this: Area 1 or Area There is no difference between the two

 The type code for the authentication:

- 0 means that there is a null authentication

- 1 means that the authentication type is plain text

- 2 means that the authentication type is MD5

Hello packets are the first type of packets in OSPF

Hello Packet Format is as shown below


Hello packets are used to form a neighbor relationship between two routers. In environments that include broadcast/nonbroadcast media, Hello packets are used to elect the designated (DR) and backup designated (BDR) routers. On broadcast media, the destination address of the Hello packets is On nonbroadcast media, the destination address is unicast

The network mask is checked only on broadcast media

Hello interval must be the same for the two routers that are trying to form an adjacency. The Hello interval is 10 seconds on broadcast and point-to-point media, and 30 seconds on all other media

 Optional capabilities supported by the router are as follows:

E, when set, means that external LSA are allowed in this area

MC designates multicast OSPF

N/P is used for not-so-stubby area (NSSA) option

EA is the external attribute

O bit is used for opaque LSAs

By default router's priority value is set to 1. A higher priority increases the chances that the router will become the DR. A priority of 0 means that this router will not take part in DR election

By default, the dead interval is four times the Hello interval

The DR is elected through the Hello protocol. If there is no DR, this field has a value of No DRs/BDRs exist on point-to-point or point-to-multipoint segments

The second type of OSPF packet is database description (DBD) packet. The first DBD packet is used to elect the master and slave relationship and to set the initial sequence number elected by the master. The router with the highest router ID becomes the master and initiates the database synchronization. The master sends the sequence number, and the slave acknowledges it.

Database Description Packet Format is as shown below
 Interface MTU contains the largest data size, in bytes, that can be sending through the associated interface. This field must be set to 0 when sending the packet over a virtual link
MS Bit is used for master and slave. When this bit is set, it means that the router is a master in the DBD exchange process. If this bit is set to 0, it means that the router is the slave.

Only a master can increment the sequence number
The Type 3 OSPF packet is a link-state request packet. It is sent if part of the database is missing or out-of-date
Link-State Request Packet Format is as shown below:
OSF packet Type 4 is the link-state update packet, and it implements flooding. Several LSAs are included in a single packet. Link-state update packets are also sent in response to link-state request packets
If an LSA is not acknowledged, it is retransmitted every retransmit interval (5 seconds, by default)
The last type of OSPF packet is the link-state acknowledgment packet, is used to acknowledge each LSA. Multiple LSAs can be acknowledged in a single link-state acknowledgment packet. This packet is responsible for the reliable delivery of link-state update packets
Link-state acknowledgment packets are sent as multicasts. If the state of the router is DR or BDR, the acknowledgment is sent to the OSPF router multicast address of If the state of the router is not DR or BDR, the acknowledgment is sent to the all DR router multicast address of

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