Link-state routing protocols take a different approach. Link-state routing protocols are more like a road map because they create a topological map of the network and each router uses this map to determine the shortest path to each network. Just as you refer to a map to find the route to another town, link-state routers use a map to determine the preferred path to reach another destination.
Routers running a link-state routing protocol send information about the state of its links to other routers in the routing domain. The state of those links refers to its directly connected networks and includes information about the type of network and any neighboring routers on those networks-hence the name link-state routing protocol.
The ultimate objective is that every router receives all of the link-state information about all other routers in the routing area. With this link-state information, each router can create its own topological map of the network and independently calculate the shortest path to every network.
Link-state routing protocols have the reputation of being much more complex than their distance vector counterparts. However, the basic functionality and configuration of link-state routing protocols is not complex at all. Even the algorithm itself can be easily understood, as you will see in the next topic. Basic OSPF operations can be configured with a router ospf process-id command and a network statement, similar to other routing protocols like RIP and EIGRP.
Open Shortest Path First (OSPF) Basics
Open Shortest Path First (OSPF) is an open standard routing protocol that’s been implemented by a wide variety of network vendors, including Cisco.OSPF works by using the Dijkstra algorithm. First, a shortest path tree is constructed, and then the routing table is populated with the resulting best paths. OSPF converges quickly, although perhaps not as quickly as EIGRP, and it supports multiple, equal-cost routes to the same destination. Like EIGRP, it does support both IP and IPv6 routed protocols.
OSPF provides the following features:
- Consists of areas and autonomous systems
- Minimizes routing update traffic
- Allows scalability
- Supports VLSM/CIDR
- Has unlimited hop count
- Allows multi-vendor deployment (open standard.
The following are reasons for creating OSPF in a hierarchical design:
- To decrease routing overhead
- To speed up convergence
- To confine network instability to single areas of the network
OSPF Terminology
Imagine how challenging it would be if you were given a map and compass but had no knowledge of east or west, north or south, river or mountain, lake or desert. You’d probably not get very far putting your new tools to good use without knowing about this stuff. For this reason, you’ll begin your exploration of OSPF with a long list of terms that will prevent you from getting lost in the later sections.The following are important OSPF terms to familiarize yourself with before you proceed:
Link: A link is a network or router interface assigned to any given network. When an interface is added to the OSPF process, it’s considered by OSPF to be a link. This link, or interface, will have state information associated with 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 chooses the Router ID by using the highest IP address of all configured loopback interfaces. If no loopback interfaces are configured with addresses, OSPF will choose the highest IP address of all active physical interfaces.
Neighbor: 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 OSPF routers that permits the direct exchange of route updates. OSPF is really picky about sharing routing information—unlike EIGRP, which directly shares routes with all of its neighbors. Instead, OSPF directly shares routes only with 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 OSPF Hello protocol provides dynamic neighbor discovery and maintains neighbor relationships. Hello packets and Link State Advertisements (LSAs) build and maintain the topological database. Hello packets are addressed to 224.0.0.5.
Neighborship database: The neighborship database is a list of all OSPF 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.
Topological database: The topological 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.
Link State Advertisement: A Link State Advertisement (LSA) is an OSPF data packet containing link-state and routing information that’s shared among OSPF routers. There are different types of LSA packets, and I’ll go into these shortly. An OSPF router will exchange LSA packets only with routers to which it has established adjacencies.
Designated router: A Designated Router (DR) is elected whenever OSPF routers are connected to the same multi-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.
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 OSPF adjacent routers but doesn’t flood LSA updates.
OSPF areas: An OSPF 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 bwhile the remaining interfaces can belong to area 0. All of the routers within the same area have the same topology table. When configuring OSPF, you’ve got to 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 OSPF!
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 broadcast ability in which a single packet is delivered to all nodes on the network. In OSPF, 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 OSPF 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 connecting 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.