Foundation Summary

     

The "Foundation Summary" section of each chapter lists the most important facts from the chapter. Although this section does not list every fact from the chapter that will be on your exam, a well-prepared candidate should, at a minimum, know all the details in each "Foundation Summary" before going to take the exam.

Table 6-5 explains common OSPF terms.

Table 6-5. OSPF Terms

Term	Description
Adjacency	Formed when two neighboring routers have exchanged information and have the same topology table. The databases are synchronized, and they both see the same networks.
Area	A group of routers that share the same area ID. Each router in the area has the same topology table. The area is defined on an interface basis in the configuration of OSPF.
Autonomous system	Routers that share the same routing protocol within the organization.
Backup designated router (BDR)	The backup to the designated router (DR), in case the DR fails. The BDR performs none of the DR functions while the DR is operating correctly.
Cost	The metric for OSPF. It is not defined in the standard with a value. Cisco use the default of the inverse of bandwidth so that the higher the speed of the link, the lower the cost, and, therefore, the more attractive the path . This default can be overridden by a manual configuration. This should be done only if you have a full knowledge of the network.
Database descriptor	Referred to as DBDs or database descriptor packets (DDPs). These are packets exchanged between neighbors during the exchange state. The DDPs contain a summary of the LSA, which describe the links of every router in the neighbor's topology table.
Designated router (DR)	Router responsible for making adjacencies with all neighbors on a multiaccess network, such as Ethernet or FDDI. The DR represents the multiaccess network, in that it ensures that every router on the link has the same topology database.
Dijkstra algorithm	A complex algorithm used by routers running link-state routing protocols to find the shortest path to the destination.
Exchange state	Method by which two neighboring routers discover the map of the network. When these routers become adjacent, they must first exchange DDPs to ensure that they have the same topology table.
Exstart state	State in which the neighboring routers determine the sequence number of the DDPs and establish the master/slave relationship.
Flood	A term that refers to network information. When network information is flooded, it is sent to every network device in the domain.
Fully adjacent	When the routing tables of the two neighbors are fully synchronized, with exactly the same view of the network.
Init state	State in which a hello packet has been sent from the router, which is waiting for a reply to establish two-way communication.
Internal router	A router that has all its interfaces in the same area.
Link-state advertisement (LSA)	A packet describing a router's links and the state of those links. There are different types of LSAs to describe the different types of links.
Link-state database	Otherwise known as the topology map, the link-state database has a map of every router, its links, and the state of the links. It also has a map of every network and every path to each network.
Link-state request (LSR)	When the router receives a DDP complete with summary of the LSA information, it compares the LSA against the topological database. If either the LSA entry is not present or the entry is older than the DDP, it will request further information.
Link-state update (LSU)	Update sent in response to the LSR. It is the LSA that was requested .
Loading state	State in which, if the receiving router requires more information during the process in which two routers are creating an adjacency, it will request that particular link in more detail using the LSR packet. The LSR will prompt the master router to send the LSU packet. This is the same as an LSA used to flood the network with routing information. While the receiving router is awaiting the LSUs from its neighbor, it is in the loading state.
Neighbor	A router on the same link with whom routing information is exchanged.
Neighbor table	A table built from the hello messages received from the neighbors. The hello message also carries a list of the neighbors.
Priority	The means by which the DR can be manually electedor, conversely, prevented from taking part in the DR/BDR election.
Shortest Path First (SPF)	The same as the Dijkstra algorithm, which is the algorithm used to find the shortest path
SPF tree	A tree of the topological network. It can be drawn after the SPF algorithm has been run. The algorithm prunes the database of alternative paths and creates a loop-free shortest path to all networks. The router is at the root of the network, which is perceived from its perspective.
Topology table	The same as a link-state database. The table contains every link in the wider network.
Two-way state	State during the process in which two routers are creating an adjacency. The new router sees its own router ID in the list of neighbors, and a neighbor relationship is established. This is the stage before routing information is exchanged.

Table 6-6 describes the Hello packet.

Table 6-6. The Hello Packet

Field	Characteristics	Function
Router ID	This is a 32-bit number. The highest IP address on the router is used as the ID. If a loopback address is configured, this will be used, even if it is not the highest address. If there are multiple loopback addresses, the highest IP is chosen .	This field identifies the router within the autonomous system. It is the ID of the originating router.
Hello/Dead Intervals	Used on broadcast, multiaccess networks: Dead Interval=40 Hello=10 sec Used on nonbroadcast networks: Dead Interval=120 Hello = 30	Hello maintains the presence of the router in its neighbor's databases. It is a keepalive. The dead interval is how long the router waits before it determines that a neighbor is unavailable.
Neighbors	The router ID of a neighbor is entered in the neighbor table when a two-way (bidirectional) communication is established within the RouterDeadInterval. The communication is established when the router sees itself listed as a neighbor in the hello packet generated by another router on the same physical segment.	A neighbor is another router with which updates will be exchanged to synchronize databases.
Area ID	This is the area ID of the originating router's interface.	The hello packet must come from a router within the same area to be valid.
Router Priority	This is the priority of the source router interface. The higher the priority, the higher the likelihood of the router being selected as a DR or BDR.	This field is used to select the DR and the BDR manually.
DR IP address	This is the address of the existing DR.	This field is used to allow the router to create unicast traffic to the DR router.
BDR IP Address	This is the address of the existing BDR.	This field is used to allow the router to create unicast traffic to the BDR router.
Authentication Password	This is the authentication type and information. If this field is set, the password must match the password stated on the router.	This field is used as security.
Stub Area Flag	This field is set if the area is a stub area. All routers in the area must have this flag set.	This field identifies which type of LSAs will be transmitted and accepted.

Five packets are used to build the routing table for the first time:

• s Hello protocol

• Database descriptor

• Link-state request

• Link-state update

• Link-state acknowledgement

Figure 6-9 is a flowchart illustrating the updating of the topological database

Figure 6-9. Updating the Topological Database

Table 6-7 indicates the characteristics and options for OSPF over NBMA.

Table 6-7. OSPF over NBMA

	Point-to-Multipoint Nonbroadcast	Point-to-Point	Broadcast	NBMA	Point-to-Multipoint
Addressing	Unicast	Multicast	Multicast	Unicast	Multicast
DR/BDR	No	No	Yes	Manual Yes	No
Manual Configuration of Neighbors	Yes	No	No	Yes	No
Hello	30 seconds Dead=120	10 seconds Dead=40	10 seconds Dead=40	30 seconds Dead=120	30 seconds Dead=120
RFC/Cisco	Cisco	Cisco	Cisco	RFC 2328	RFC 2328
Network Supported	Star Partial mesh	Star Partial mesh, using subinterfaces	Full mesh	Full mesh	Star Partial mesh (Seen as point-to-point)
Replicates Packets	Yes	Yes	Yes	Yes	Yes
Number of Subnets	1	Many (1 per circuit)	1	1	1