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...

Comparing OSPF with distance vector routing protocols 2.2.3  This page will explain how OSPF compares to distance vector protocols such as RIP. Link-state routers maintain a common picture of the network and exchange link information upon initial discovery or network changes. Link-state routers do not broadcast routing tables periodically as distance vector protocols do. Therefore, link-state routers use less bandwidth for routing table maintenance. RIP is appropriate for small networks, and the best path is based on the lowest number of hops. OSPF is appropriate for large, scalable internetworks, and the best path is determined by the speed of the link. RIP and other distance vector protocols use simple algorithms to compute best paths. The SPF algorithm is complex. Routers that implement distance vector protocols need less memory and less powerful processors than those that implement OSPF. OSPF selects routes based on cost, which is related to speed. The higher the spee...

OSPF terminology 2.2.2 There are many words and concepts for students in this TI and the figures should help to explain them. Use the interactive media activity to reinforce the terms and their abbreviations. Instructors might like to hold an acronym competition to see who can explain the words and concepts in the following table: Link A link is a physical and electrical connection between two network devices. Link-state (LS) Link-state is the status of a link between two routers. This status includes information about a router interface and its relationship to neighboring routers. Cost Cost is the value assigned to a link. Link-state protocols assign a cost to a link, which is based on the speed of the network connection. Area An area is a collection of networks and routers that has the same area identification. Each router within an area has the same link-state information. A router w...

Single-Area OSPF Concepts OSPF overview 2.2.1  This page will introduce OSPF. OSPF is a link-state routing protocol that is based on open standards. It is described in several standards of the Internet Engineering Task Force (IETF). The Open in OSPF means that it is open to the public and is non-proprietary. OSPF, when compared to RIP v1 and v2, is the preferred IGP because it is scalable. RIP is limited to 15 hops, it converges slowly, and it sometimes chooses slow routes because it ignores critical factors such as bandwidth in route determination. A drawback to using OSPF is that it only supports the TCP/IP protocol suite. OSPF has overcome these limitations and is a robust and scalable routing protocol that is suitable for modern networks. OSPF can be used and configured as a single area for small networks. It can also be used for large networks. As shown in Figure , large OSPF networks use a hierarchical design. Multiple areas connect to a distribution area, o...