OSPF Hello Packet
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1. What is OSPF Hello Packet ?
An OSPF hello packet is a fundamental component of the Open Shortest Path First routing protocol. It serves as a means for OSPF routers to discover and establish neighbor relationships with other routers within the network. The hello packet plays a crucial role in maintaining the integrity and efficiency of OSPF routing.
When an OSPF router is powered on or an interface is enabled, it begins sending hello packets out of all its active interfaces. These packets are multicast to the "all OSPF routers" address (224.0.0.5) and are periodically transmitted to ensure continuous neighbor detection and maintenance.
The hello packet contains vital information that aids in the establishment of OSPF neighbor relationships. This information includes the router's OSPF interface IP address, the network mask associated with the interface, and the router priority. Additionally, the hello packet carries the router's dead interval, which specifies the maximum time allowed between the receipt of consecutive hello packets from a neighbor.
Upon receiving a hello packet, OSPF routers examine the information within it to determine if they can become neighbors. The routers compare their own interface IP addresses, network masks, and priorities with those provided in the hello packet. If the information matches and meets certain criteria, a neighbor relationship is established.
The hello packet also serves as a mechanism for monitoring the health of OSPF neighbors. By exchanging hello packets at regular intervals, routers can verify that their neighbors are still active and reachable. If a router fails to receive hello packets from a neighbor within a specified time (the dead interval), it assumes the neighbor is no longer available.
In summary, OSPF hello packets are essential for neighbor discovery, establishment, and maintenance in the OSPF routing protocol. They facilitate the exchange of crucial information between routers, ensuring the efficient and reliable operation of OSPF networks.
2. OSPF Hello Packet Format Details
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1. Network Mask: 32 bits, This field specifies the subnet mask associated with the interface through which the hello packet is being sent. It helps routers determine if they belong to the same network and can become OSPF neighbors.
2.Hello Interval: 16 bits, This field indicates the time interval between successive hello packets sent by the router. It defines how often the router will send hello packets to its neighbors to maintain neighbor relationships.
3.Options: 8 bits, Optional OSPF capabilities:
E: AS-external-LSAs can be flooded.
N/P: Type 7 LSAs can be processed.
DC: Demand circuits can be processed.
4. Priority: 8 bits, This field represents the priority of the router in the OSPF network. It is used in the process of electing the designated router (DR) and backup designated router (BDR) on multi-access networks.
5. Dead Interval: 32 bits, This field specifies the maximum time allowed between the receipt of consecutive hello packets from a neighbor. If a router fails to receive hello packets from a neighbor within this interval, it assumes the neighbor is no longer available. Dead interval is always 4 times of hello interval.
6. Designated Router (DR): 32 bits, This fields contains the IP addresses of the Designated Router on a multi-access network.
7. Backup Designated Router (BDR): 32 bits, This fields contains the IP addresses of the Backup Designated Router on a multi-access network.
8. Neighbor: 32 bits, This fields contains a list of OSPF neighbors seen on that network segment. A router must have received a hello from the neighbor within the dead interval.
Note:
There is another timer if OSPF need to select DR and BDR, this timer called wait Interval. We can not set wait Interval manually, the value of wait Interval is same as dead interval.
To ensure that all routers on a segment have fully initialized, OSPF initiates a wait timer when OSPF hello packets do not contain a DR/BDR router for a segment. The default value for the wait timer is the dead interval timer. When the wait timer has expired, a router participates in the DR election. The wait timer starts when OSPF first starts on an interface, so a router can still elect itself as the DR for a segment without other OSPF routers; it waits until the wait timer expires.
3. OSPF Timer For Different Network Type
Note:
To establish neighbor relationships between devices on the same network segment, set the same HelloInterval, DeadInterval, and WaitInterval values for the devices. By default these values are the same, if you need to change and check these values, you could use below command.
Router(config)#interface gigabitethernet 0/1
Router(config-if)#ip ospf hello-interval 11 (set hello timer)
Router(config-if)#ip ospf dead-interval 44 (set dead timer, wait timer use this value)
Router(config-if)#end
Router#show ip ospf interface gigabitethernet 0/1
#We only choose relevant output and post here#
Timer intervals configured, Hello 11, Dead 44, Wait 44
#We only choose relevant output and post here#