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Day 12 229Figure 12-2OSPF Packet Header and Hello PacketData LinkFrame HeaderIP Packet Header OSPF Packet Header OSPF Packet Type-Specific DataHello PacketBits07 8 15 16 23 24 31VersionType = 1Packet LengthRouter IDOSPFPacketHeadersChecksumArea IDAuTypeAuthenticationAuthenticationNetwork MaskHello Interval Option Router PriorityOSPFHelloPacketsRouter Dead IntervalDesignated Router (DR)Backup Designated Router (BDR)List of Neighbor(s)Receiving an OSPF Hello packet on an interface confirms for a router that another OSPF routerexists on this link. OSPF then establishes adjacency with the neighbor. To establish adjacency, twoOSPF routers must have the following matching interface values:■■■Hello IntervalDead IntervalNetwork TypeBefore both routers can establish adjacency, both interfaces must be part of the same network,including the same subnet mask. Then full adjacency will happen after both routers haveexchanged any necessary LSUs and have identical link-state databases. By default, OSPF Hellopackets are sent to the multicast address 224.0.0.5 (ALLSPFRouters) every 10 seconds on multiaccessand point-to-point segments and every 30 seconds on nonbroadcast multiaccess (NBMA) segments(Frame Relay, X.25, ATM). The default dead interval is four times the Hello interval.Link-State AdvertisementsLink-state updates (LSUs) are the packets used for OSPF routing updates. An LSU packet cancontain 11 types of link-state advertisements (LSAs), as shown in Figure 12-3.