Terminology will prevent you from getting lost in the future study. The following terms are important before you proceed.OSPF02.jpg

Link A link is a network or router interface assigned to any given network. When an interface is added to the process, it’s considered by to be a link. This link, or interface, will have state information related to it (up or down) as well as one or more IP addresses.

Router ID The Router ID (RID) is an IP address used to identify the router. Cisco selects the Router ID by use of the highest IP address of all configured loopback interfaces. If no loopback interfaces are configured with addresses, will choose the highest IP address of all active physical interfaces.

Neighbors Neighbors are two or more routers that have an interface on a common network, such as two routers connected on a point-to-point serial link.

Adjacency An adjacency is a relationship between two routers that permits the direct exchange of route updates. is really picky about sharing routing information-unlike EIGRP,which directly shares routes with all of its neighbors that have also established adjacencies. And not all neighbors will become adjacent-this depends upon both the type of network and the configuration of the routers.

Hello protocol The Hello protocol provides dynamic neighbpr discovery and maintains neighbor relationships. Hello packets and Link State Advertisments (LSAs) build and maintain the topological database. Hello packets are addressed to 244.0.0.5.

Neighborship database The neighborship database is a list of all routers for which Hello packets have been seen. A variety of details, including the Router ID and state, are maintained on each router in the neighborship database.

Topology database The topology database contains information from all of the Link State Advertisement packets that have been received for an area. The router uses the information from the topology database as input into the Dijkstra algorithm that computes the shortest path to every network.

Note: LSA packets are used to update and maintain the topology database.

Link state Advertisement A Link State Advertisement (LSA) is an data packet containing link-state and routing information that’s shared among routers. There are different types of LSA packets, and I’ll go into these shortly. An router will exchange LSA packets only with routers to which it has established adjacencies.

Designated router A designated router(DR) is selected whenever routers are connected to the same muti-access network. Cisco likes to call these broadcast networks, but really, they are networks that have multiple recipients. Try not to confuse multi-access with multipoint, which can be easy to do sometimes.

A prime example is an Ethernet LAN. To minimize the number of adjacencies formed, a DR is chosen to disseminate/receive routing information to/from the remaining routers on the broadcast network or link. This ensures that their topology tables are synchronized. All routers on the shared network will establish adjacencies with the DR and backup designated router (BDR)- I’ll define this next. The election is won by the router with the highest priority, and the Router ID is used as a tiebreaker if the priority of more than one router turns out to be the same.

Backup designated router A backup designated router (BDR) is a hot standby for the DR on multi-access links(remember that Cisco sometimes likes to call these “broadcast” networks). The BDR receives all routing updates from adjacent routers, but does’t flood LSA updates.

areas An area is a grouping of contiguous networks and routers. All routers in the same area share a common Area ID. Because a router can be a member of more than one area at a time, the Area ID is associated with specific interfaces on the router. This would allow some interfaces to belong to area 1 while the remaining interfaces can belong to area 0. All of the routers within the same area have the same topology table. When configuring , you must remember that there must be an area 0, and that this is typically configured on the routers that connect to the backbone of the network. Areas also play a role in establishing a hierarchical network organization-something that really enhances the scalability of !

Broadcast (multi-access) Broadcast (multi-access) networks such as Ethernet allow multiple devices to connect to (or access) the same network, as well as provide a braodcast ability in which a single packet is delivered to all nodes on the network. In , a DR and a BDR must be elected for each broadcast multi-access network.

Non-broadcast multi-access Non-broadcast multi-access (NBMA) networks are types such as Frame Relay,X.25, and Asynchronous Transfer Mode (ATM). These networks allow for multi-access, but have no broadcast ability like Ethernet. So,NBMA networks require special configuration to function properly and neighbor relationships must be defined.

Point-to-point Point-to-point refers to a type of network topology consisting of a direct connection between two routers that provides a single communication path. The point-to-point connection can be physical, as in a serial cable directly connection two routers, or it can be logical, as in two routers that are thousands of miles apart yet connected by a circuit in a Frame Relay network. In either case, this type of configuration eliminates the need for DRs or BDRs-but neighbors are discovered automatically.

Point-to-multipoint Point-to-multipoint refers to a type of network topology consisting of a series of connections between a single interface on one router and multiple destination routers. All of the interfaces on all of the routers sharing the point-to-multipoint connection belong to the same network. As with point-to-point, no DRs or BDRs are needed.

All of these terms play an important role in understanding the operation of . , so, again, make sure you are familiar with each of them.

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