The Hello protocol uses a multicast address of 18.104.22.168, and all routers periodically send hellos. On hearing hellos, the router creates a table of its neighbors. The continued receipt of these packets maintains the neighbor table. If a hello from a known neighbor is not heard within a predetermined amount of time, as stated in the holdtime, the router will decide that the neighbor is no longer operational and will take the appropriate action. The holdtime is set at the default of three times the Hello timer. Therefore, if the router misses three hellos, the neighbor is declared dead. The Hello timer on a LAN is set to 5 seconds; the holdtime, therefore, is 15 seconds. On a WAN link, the Hello timer is 60 seconds, and the holdtime correspondingly is 180 seconds.
To become a neighbor, the following conditions must be met:
• The router must hear a hello packet or an ACK from a neighbor.
• The AS number in the packet header must be the same as that of the receiving router.
• The neighbor's metric settings must be the same.
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