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...

Compare and contrast distance vector and link-state routing 2.1.6  This page will compare distance vector and link-state routing protocols. All distance vector protocols learn routes and then send these routes to directly connected neighbors. However, link-state routers advertise the states of their links to all other routers in the area so that each router can build a complete link-state database. These advertisements are called link-state advertisements or LSAs. Unlike distance vector routers, link-state routers can form special relationships with their neighbors and other link-state routers. This is to ensure that the LSA information is properly and efficiently exchanged. The initial flood of LSAs provides routers with the information that they need to build a link-state database. Routing updates occur only when the network changes. If there are no changes, the routing updates occur after a specific interval. If the network changes, a partial update is sent immediately. ...

Advantages and disadvantages of link-state routing 2.1.5  This page lists the advantages and disadvantages of link-state routing protocols. The following are advantages of link-state routing protocols:  Link-state protocols use cost metrics to choose paths through the network. The cost metric reflects the capacity of the links on those paths. Link-state protocols use triggered updates and LSA floods to immediately report changes in the network topology to all routers in the network. This leads to fast convergence times. Each router has a complete and synchronized picture of the network. Therefore, it is very difficult for routing loops to occur. Routers use the latest information to make the best routing decisions. The link-state database sizes can be minimized with careful network design. This leads to smaller Dijkstra calculations and faster convergence. Every router, at the very least, maps the topology of it...

Link-state routing algorithms 2.1.4  Link-state routing algorithms maintain a complex database of the network topology by exchanging link-state advertisements (LSAs) with other routers in a network. This page describes the link-state routing algorithm. Link-state routing algorithms have the following characteristics: They are known collectively as SPF protocols. They maintain a complex database of the network topology. They are based on the Dijkstra algorithm. Link-state protocols develop and maintain full knowledge of the network routers and how they interconnect. This is achieved through the exchange of LSAs with other routers in the network. Each router constructs a topological database from the LSAs that it receives. The SPF algorithm is then used to compute the reachability of destinations. This information is used to update the routing table. This process can discover changes in the network topology caused by component failure or network growt...

How routing information is maintained 2.1.3 This page will explain how link-state protocols use the following features: The LSAs A topological database The SPF algorithm The SPF tree A routing table of paths and ports to determine the best path for packets  Link-state routing protocols were designed to overcome the limitations of distance vector routing protocols. For example, distance vector protocols only exchange routing updates with immediate neighbors while link-state routing protocols exchange routing information across a much larger area. When a failure occurs in the network, such as a neighbor becomes unreachable, link-state protocols flood LSAs with a special multicast address throughout an area. This process sends information out all ports, except the port on which the information was received. Each link-state router takes a copy of the LSA and updates its link-state, or topological database. The link-state router then forwards the LSA to ...

Link-state routing protocol features 2.1.1  This page will explain how link-state protocols route data. Link-state routing protocols collect route information from all other routers in the network or within a defined area of the network. Once all of the information is collected, each router calculates the best paths to all destinations in the network. Since each router maintains its own view of the network, it is less likely to propagate incorrect information provided by any of its neighboring routers. The following are some link-state routing protocol functions: Respond quickly to network changes Send triggered updates only when a network change has occurred Send periodic updates known as link-state refreshes Use a hello mechanism to determine the reachability of neighbors  Each router multicasts hello packets to keep track of the state of the neighbor routers. Each router uses LSAs to keep track of all the routers in its area of the net...

Link-State Routing Protocol Overview of link-state routing 2.1.1 Link-state routing protocols perform differently than distance vector protocols. This page will explain the differences between distance vector and link-state protocols. This information is vital for network administrators. One essential difference is that distance vector protocols use a simpler method to exchange route information. Ooutlines the characteristics of both distance vector and link-state routing protocols. Link-state routing algorithms maintain a complex database of topology information. While the distance vector algorithm has nonspecific information about distant networks and no knowledge of distant routers, a link-state routing algorithm maintains full knowledge of distant routers and how they interconnect. The Interactive Media Activity will help students identify the different features of link-state and distance vector protocols. The next page will describe link-state routing protocols.

Overview The two main classes of IGPs are distance vector and link-state. Both types of routing protocols find routes through autonomous systems. Distance vector and link-state routing protocols use different methods to accomplish the same tasks. Link-state routing algorithms, also known as shortest path first (SPF) algorithms, maintain a complex database of topology information. A link-state routing algorithm maintains full knowledge of distant routers and how they interconnect. In contrast, distance vector algorithms provide nonspecific information about distant networks and no knowledge of distant routers. It is important to understand how link-state routing protocols operate in order to configure, verify, and troubleshoot them. This module explains how link-state routing protocols work, outlines their features, describes the algorithm they use, and points out the advantages and disadvantages of link-state routing. Early routing protocols such as RIP v1 were all distance...

Summary This page summarizes the topics discussed in this module. Variable-Length Subnet Masks (VLSM), often referred to as "subnetting a subnet", is used to maximize addressing efficiency. It is a feature that allows a single autonomous system to have networks with different subnet masks. The network administrator is able to use a long mask on networks with few hosts, and a short mask on subnets with many hosts.   It is important to design an addressing scheme that allows for growth and does not involve wasting addresses. To apply VLSM to the addressing problem, large subnets are created for addressing LANs. Very small subnets are created for WAN links and other special cases. VLSM helps to manage IP addresses. VLSM allows for the setting of a subnet mask that suits the link or the segment requirements. A subnet mask should satisfy the requirements of a LAN with one subnet mask and the requirements of a point-to-point WAN with another. Addresses are assigned in a...

Default routes 1.2.7  This page will describe default routes and explain how they are configured. By default, routers learn paths to destinations three different ways: Static routes – The system administrator manually defines the static routes as the next hop to a destination. Static routes are useful for security and traffic reduction, as no other route is known. Default routes – The system administrator also manually defines default routes as the path to take when there is no known route to the destination. Default routes keep routing tables shorter. When an entry for a destination network does not exist in a routing table, the packet is sent to the default network. Dynamic routes – Dynamic routing means that the router learns of paths to destinations by receiving periodic updates from other routers. In Figure , the static route is indicated by the following command: Router(config)# ip route 172.16.1.0...

Troubleshooting RIP v2 1.2.6  This page explains the use of the debug ip rip command. Use the debug ip rip command to display RIP routing updates as they are sent and received. The no debug all or undebug all commands will turn off all debugging. The example shows that the router being debugged has received updates from one router at source address 10.1.1.2. The router at source address 10.1.1.2 sent information about two destinations in the routing table update. The router being debugged also sent updates, in both cases to the multicast address 224.0.0.9 as the destination. The number in parentheses is the source address encapsulated into the IP header. Other outputs sometimes seen from the debug ip rip command includes entries such as the following: RIP: broadcasting general request on Ethernet0 RIP: broadcasting general request on Ethernet1 These outputs appear at startup or when an event occurs such as an interface transition or a user manually clears...