31 Days Before Your CCNA Exam

Day 12OSPF RoutingCCNA 640-802 Exam Topics■Configure, verify, and troubleshoot OSPF.Key TopicsOpen Shortest Path First (OSPF) is a link-state routing protocol that was developed as a replacementfor Routing Information Protocol (RIP). OSPF’s major advantages over RIP are its fast convergenceand its scalability to much larger network implementations. Today we review the operation,configuration, verification, and troubleshooting of basic OSPF.OSPF OperationIETF chose OSPF over Intermediate System-to-Intermediate System (IS-IS) as its recommendedInterior Gateway Protocol (IGP). In 1998, the OSPFv2 specification was updated in RFC 2328 andis the current RFC for OSPF. RFC 2328, OSPF Version 2, is on the IETF website athttp://www.ietf.org/rfc/rfc2328. Cisco IOS Software will choose OSPF routes over RIP routesbecause OSPF has an administrative distance of 110 versus RIP’s AD of 120.OSPF Message FormatThe data portion of an OSPF message is encapsulated in a packet. This data field can include one offive OSPF packet types. Figure 12-1 shows an encapsulated OSPF message in an Ethernet frame.Figure 12-1Encapsulated OSPF MessageData Link FrameHeaderIP PacketHeaderOSPF PacketHeaderOSPF Packet Type-Specific DataData Link Frame (Ethernet Fields Shown Here)MAC Source Address = Address of Sending InterfaceMAC Destination Address = Multicast: 01-00-5E-00-00-05 or 01-00-5E-00-00-06IP PacketIP Source Address = Address of Sending InterfaceIP Destination Address = Multicast: 224.0.0.5 or 224.0.0.6Protocol Field = 89 for OSPFOSPF Packet HeaderType Code for OSPF Packet TypeRouter ID and Area IDOSPF Packet Types0x01 Hello0x02 Database Description0x03 Link State Request0x04 Link State Update0x05 Link State Acknowledgment