CCNA :) Be a Good Network Administrator

OSPF Hello protocol 2.2.6  This page will introduce hello packets and the Hello protocol. When a router starts an OSPF routing process on an interface, it sends a hello packet and continues to send hellos at regular intervals. The rules that govern the exchange of OSPF hello packets are called the Hello protocol. At Layer 3 of the OSI model, the hello packets are addressed to the multicast address 224.0.0.5. This address is “all OSPF routers”. OSPF routers use hello packets to initiate new adjacencies and to ensure that neighbor routers are still functioning. Hellos are sent every 10 seconds by default on broadcast multi-access and point-to-point networks. On interfaces that connect to NBMA networks, such as Frame Relay, the default time is 30 seconds. On multi-access networks the Hello protocol elects a designated router (DR) and a backup designated router (BDR). Although the hello packet is small, it consists of the OSPF packet header. For the hello packet the typ...

OSPF network types 2.2.5  This page will introduce the three types of OSPF networks. A neighbor relationship is required for OSPF routers to share routing information. A router will try to become adjacent, or neighbor, to at least one other router on each IP network to which it is connected. OSPF routers determine which routers to become adjacent to based on the type of network they are connected to. Some routers may try to become adjacent to all neighbor routers. Other routers may try to become adjacent to only one or two neighbor routers. Once an adjacency is formed between neighbors, link-state information is exchanged. OSPF interfaces automatically recognize three types of networks: Broadcast multi-access, such as Ethernet Point-to-point networks Nonbroadcast multi-access (NBMA), such as Frame Relay A fourth type, point-to-multipoint, can be manually configured on an interface by an administrator.  In a multi-access network, it is not known in a...

Shortest path algorithm 2.2.4  This page will explain how OSPF uses the shortest-path algorithm to determine the best path to a destination. In this algorithm, the best path is the lowest cost path. Edsger Wybe Dijkstra, a Dutch computer scientist, formulated the shortest path-algorithm, also known as Dijkstra's algorithm. The algorithm considers a network to be a set of nodes connected by point-to-point links. Each link has a cost. Each node has a name. Each node has a complete database of all the links and so complete information about the physical topology is known. All router link-state databases, within a given area, are identical. The table in Figure shows the information that node D has received. For example, D received information that it was connected to node C with a link cost of 4 and to node E with a link cost of 1. The shortest path algorithm then calculates a loop-free topology using the node as the starting point and examining in turn information it has abo...