Link-state routing protocols address some of the limitations of distance-vector protocols. When running a link-state routing protocol, routers originate information about themselves (IP addresses), their connected links (the number and types of links), and the state of those links (up or down). The information is flooded to all routers in the network as changes in the link state occur. Each router makes a copy of the information received and forwards it without change. Each router independently calculates the best path to each destination network by using Dijkstra’s shortest-path algorithm, creating a shortest-path tree with itself as the root, and maintaining a map of the network.
After the initial exchange of information, link-state updates are not sent unless a change in the topology occurs. Routers do send small hello messages between neighbors to maintain neighbor relationships. If no updates have been sent, the link-state route database is refreshed after 30 minutes.
The following are link-state routing protocols:
- OSPFv2
- IS-IS
- OSPFv3
OSPFv2 and OSPFv3 are covered in Chapter 4, “OSPF, BGP, and Route Manipulation.”
Distance-Vector Routing Protocols Versus Link-State Protocols
When choosing a routing protocol, consider that distance-vector routing protocols use more network bandwidth than link-state protocols. Distance-vector protocols generate more bandwidth overhead because of the large periodic routing updates. Link-state routing protocols do not generate significant routing update overhead but do use more router CPU and memory resources than distance-vector protocols. This occurs because with link-state routing protocols (generally speaking), WAN bandwidth is a more expensive resource than router CPU and memory in modern devices.
Table 3-3 compares distance-vector and link-state routing protocols.
Table 3-3 Distance-Vector Versus Link-State Routing Protocols
| Characteristic | Distance Vector | Link State |
| Scalability | Limited | Good |
| Convergence | Slow | Fast |
| Routing overhead | More traffic | Less traffic |
| Implementation | Easy | More complex |
| Protocols | RIPv1, RIPv2, EIGRP, RIPng | OSPF, IS-IS, OSPFv3 |
EIGRP is a distance-vector protocol with link-state characteristics (hybrid) that give it high scalability, fast convergence, less routing overhead, and relatively easy configuration. If “distance-vector” is not an answer to a question on the ENSLD 300-420 exam, then “hybrid” would be a valid option.
Hierarchical Versus Flat Routing Protocols
Some routing protocols require a network topology that must have a backbone network defined. This network contains some or all of the routers in the internetwork. When the internetwork is defined hierarchically, the backbone consists of only some devices. Backbone routers service and coordinate the routes and traffic to or from routers not in the local internetwork. The supported hierarchy is relatively shallow. Two levels of hierarchy are generally sufficient to provide scalability. Selected routers forward routes into the backbone. OSPF and IS-IS are hierarchical routing protocols. By default, EIGRP is a flat routing protocol, but it can be configured with manual summarization to support hierarchical designs.
Flat routing protocols do not allow a hierarchical network organization. They propagate all routing information throughout the network without dividing or summarizing large networks into smaller areas. Carefully designing network addressing to naturally support aggregation within routing-protocol advertisements can provide many of the benefits offered by hierarchical routing protocols. Every router is a peer of every other router in flat routing protocols; no router has a special role in the internetwork. EIGRP, RIPv1, and RIPv2 are flat routing protocols.