Hello Packets

Routers periodically send hello packets on all interfaces, including virtual links, to establish and maintain neighbor relationships. Hello packets are multicast on physical networks that have a multicast or broadcast capability, which enables dynamic discovery of neighboring routers. (On nonbroadcast networks, dynamic neighbor discovery is not possible, so you must configure all neighbors statically using the neighbor statement.)

Hello packets consist of the OSPF header plus the following fields:

• Network mask—Network mask associated with the interface.
• Hello interval—How often the router sends hello packets. All routers on a shared network must use the same hello interval. You configure this interval with the hello-interval statement.
• Options—Optional capabilities of the router.
• Router priority—The router’s priority to become the designated router. You can configure this value with the priority statement.
• Router dead interval—How long the router waits without receiving any OSPF packets from a router before declaring that router to be down. All routers on a shared network must use the same router dead interval. You can configure this value with the dead-interval statement.
• Designated router—IP address of the designated router.
• Backup designated router—IP address of the backup designated router.
• Neighbor—IP addresses of the routers from which valid hello packets have been received within the time specified by the router dead interval.

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