31 Days Before Your CCNA Exam

228 31 Days Before Your CCNA ExamThe OSPF packet header is included with every OSPF packet, regardless of its type. The OSPF packetheader and packet type-specific data are then encapsulated in an IP packet. In the IP packet header, theprotocol field is set to 89 to indicate OSPF, and the destination address is typically set to one of twomulticast addresses: 224.0.0.5 or 224.0.0.6. If the OSPF packet is encapsulated in an Ethernet frame,the destination MAC address is also a multicast address: 01-00-5E-00-00-05 or 01-00-5E-00-00-06.OSPF Packet TypesThese five OSPF packet types each serve a specific purpose in the routing process:■■■■■Hello: Hello packets are used to establish and maintain adjacency with other OSPF routers.DBD: The database description (DBD) packet contains an abbreviated list of the sendingrouter’s link-state database and is used by receiving routers to check against the local linkstatedatabase.LSR: Receiving routers can then request more information about any entry in the DBD bysending a link-state request (LSR).LSU: Link-state update (LSU) packets are used to reply to LSRs and to announce new information.LSUs contain 11 types of link-state advertisements (LSA).LSAck: When an LSU is received, the router sends a link-state acknowledgment (LSAck) toconfirm receipt of the LSU.Neighbor EstablishmentHello packets are exchanged between OSPF neighbors to establish adjacency. Figure 12-2 showsthe OSPF header and Hello packet.Important fields shown in the figure include the following:■■■■■■■■■Type: OSPF packet type: Hello (Type 1), DBD (Type 2), LS Request (Type 3), LS Update(Type 4), LS ACK (Type 5)Router ID: ID of the originating routerArea ID: Area from which the packet originatedNetwork Mask: Subnet mask associated with the sending interfaceHello Interval: Number of seconds between the sending router’s HellosRouter Priority: Used in DR/BDR election (discussed later in the section “DR/BDR Election”)Designated Router (DR): Router ID of the DR, if anyBackup Designated Router (BDR): Router ID of the BDR, if anyList of Neighbors: Lists the OSPF Router ID of the neighboring router(s)Hello packets are used to do the following:■■■Discover OSPF neighbors and establish neighbor adjacenciesAdvertise parameters on which two routers must agree to become neighborsElect the DR and BDR on multiaccess networks such as Ethernet and Frame Relay