A hello packet contains the following information: If a router ID has been manually configured using the router-id command, it is recorded in the field. If a router ID has not been configured, the highest loopback IP address is used. If a loopback address has not been configured, the router chooses the highest IP address of any physical interface. The interface does not have to run OSPF. For example, IP address 172.16.12.1 would be chosen over 172.16.1.1. On broadcast OSPF networks, the default hello interval is ten seconds, and the default dead interval is 40 seconds. On nonbroadcast networks, the default hello interval is 30 seconds, and the default dead interval is two minutes or 120 seconds.
Content 3.2 Review of OSPF Operation 3.2.4 Exchanging and Synchronizing LSDBs Once a bidirectional adjacency is formed, OSPF must exchange and synchronize the LSDBs between routers. When routers running OSPF initialize, an exchange process using the hello protocol is performed first, as illustrated in Figure :
  1. When router A is enabled on the network, it is initially in a down state because it has not exchanged information with any other router. It begins by sending a hello packet through each of its interfaces participating in OSPF, even though it does not know the identity of the DR or of any other routers.
On point-to-point and multiaccess broadcast networks, the hello packet is sent out using the multicast address 224.0.0.5. On nonbroadcast multiaccess (NBMA), point-to-multipoint, and virtual links, the hello packets are sent in unicast packets.
  1. All directly connected routers running OSPF receive the hello packet from router A and add router A to their list of neighbors. This state is the initial state (init).
  2. All routers that received the hello packet send a unicast reply hello packet to router A with their corresponding information. The neighbor field in the hello packet includes all neighboring routers and router A.
  3. When router A receives these hello packets, it adds all the routers that had its router ID in their hello packets to its own neighbor relationship database. This state is the two-way state. At this point, all routers that have each other in their list of neighbors have established bidirectional communication.
  4. If the link type is a broadcast network, generally a LAN link like Ethernet, a DR and BDR must first be elected. The DR forms bidirectional adjacencies with all other routers on the LAN link. This process must occur before the routers can begin exchanging link-state information.
  5. Periodically (every 10 seconds by default on broadcast networks), the routers within a network exchange hello packets to ensure that communication is still working. The hello updates include the DR, BDR, and the list of routers whose hello packets have been received by the router, where received means that the receiving router recognizes its router ID as one of the entries in the received hello packet.
Note
After a DR and BDR are selected, any router added to the network establishes adjacencies with the DR and BDR only.
Content 3.2 Review of OSPF Operation 3.2.5 Discovering the Network Routes After the DR and BDR have been selected, the routers are in the exstart state, and they are ready to discover the link-state information about the internetwork and create their LSDBs. The process used to discover the network routes is the exchange protocol, and it gets the routers to a full state of communication. The first step in this process is for the DR and BDR to establish adjacencies with each of the other routers. When adjacent routers are in a full state, they do not repeat the exchange protocol unless the full state changes. As shown in Figure , the exchange protocol operates as follows: Step 1 In the exstart state, the DR and BDR establish adjacencies with each router in the network. During this process, a master-slave relationship is created between each router and its adjacent DR and BDR. The router with the higher router ID acts as the master during the exchange process.
Note
Only the DR exchanges and synchronizes link-state information with the routers to which it has established adjacencies. Having the DR represent the network in this capacity reduces the amount of routing update traffic. Step 2 The master and slave routers exchange one or more DBD packets. The routers are in the exchange state. A DBD includes information about the LSA entry header that appears in the LSDB of the router. The entries can be about a link or a network. Each LSA entry header includes information about the link-state type, the address of the advertising router, the cost of the link, and the sequence number. The router uses the sequence number to determine the “newness” of the received link-state information. Step 3 When the router receives the DBD, it performs these actions, as shown in Figure :
  1. It acknowledges the receipt of the DBD using the LSAck packet.
  2. It compares the information it received with the information it has. If the DBD has a more up-to-date link-state entry, the router sends an LSR to the other router. The process of sending LSRs is called the loading state.
  3. The other router responds with the complete information about the requested entry in an LSU packet. Again, when the router receives an LSU, it sends an LSAck.
Step 4 The router adds the new link-state entries to its LSDB. When all LSRs have been satisfied for a given router, the adjacent routers are considered synchronized and in a full state. The routers must be in a full state before they can route traffic. At this point, all the routers in the area should have identical LSDBs. Interactive Media Activity Drag and Drop: OSPF States Upon completion of this activity, the student will be able to list in order the different states of OSPF.
Interactive Media Activity Drag and Drop: OSPF Neighbor States Upon completion of this activity, the student will be able to identify the different OSPF neighbor states.
Content 3.2 Review of OSPF Operation 3.2.6