Routing and RoutersRoutingRouting – Street analogyRouting- What is it?Slide 5Slide 6Routing algorithmsSlide 8Slide 9Slide 10Slide 11Slide 12Slide 13Routed vs. Routing ProtocolsRouted versus Routing ProtocolsSlide 16Slide 17Slide 18Routing MetricsRouting metricsRouting metrics (cont.)Classes of Routing ProtocolsSlide 23Slide 24RoutersSlide 26Flashback – OSI ModelBasicsSwitch recapRouter ExamplesExamplesSlide 32Slide 33Slide 34Slide 35Slide 36Configuration TablesRouteRoute - GeneralSlide 40Slide 41Slide 42Slide 43Route - WindowsSlide 45Slide 46Contents of an IP Routing Table From: Microsoft TechNet: Understanding the IP Routing TableSlide 48Route - LinuxSlide 50Route - CommercialSlide 52Cisco IOS CommandsRouting and RoutersRoutingWhat is it?Routing – Street analogyRouting- What is it?Selecting of paths in a computer network along which to send dataDirects the forwarding and passing of logically addressed packetsFrom the source networkToward the ultimate destination through intermediary nodesTypically through hardware devices called routers. Usually directed by routing tablesa record of the best routes to various network destinationsConstructing good routing tables is very important for efficient routing.Routing- What is it?Small networks typically use manually configured routing tablesLarger networks involve complex topologies that can change constantlyThis makes manual construction of routing tables very problematicNote: Most of the Public Switched Telephone Network (PSTN) uses pre-computed routing tablesFallback routes if the most direct route becomes blockedDynamic routing attempts to solve this problemConstructs routing tables automatically, based on information carried by routing protocolsAllows the network to act nearly autonomously in avoiding network failures and blockages.Routing- What is it?Dynamic routing dominates the Internet. However, the configuration of the routing protocols often requires a skilled touchNetworking technology has not developed to the point of the complete automation of routingRouting algorithmsDistance vector algorithmsLink-state algorithmsRouting algorithmsDistance vector algorithmsUse the Bellman-Ford algorithm.Assigns a number, the cost, to each of the links between each node in the network. Nodes will send information from point A to point B via the path that results in the lowest total cost (i.e. the sum of the costs of the links between the nodes used).Routing algorithmsDistance vector algorithms (cont.)Operates in a very simple manner. When a node first starts, it only knows of its immediate neighbors, and the direct cost involved in reaching them. This information, the list of destinations, the total cost to each, and the next hop to send data to get there, makes up the routing table, or distance table.Each node, on a regular basis, sends to each neighbor its own current idea of the total cost to get to all the destinations it knows of. The neighboring node(s) examine this information, and compare it to what they already 'know'; anything which represents an improvement on what they already have, they insert in their own routing table(s). Over time, all the nodes in the network will discover the best next hop for all destinations, and the best total cost.Routing algorithmsDistance vector algorithms (cont.)When one of the nodes involved goes down, those nodes which used it as their next hop for certain destinations discard those entries, and create new routing-table information. They then pass this information to all adjacent nodes, which then repeat the process. Eventually all the nodes in the network receive the updated information, and will then discover new paths to all the destinations which they can still "reach".Routing algorithmsLink-state algorithmsEach node uses as its fundamental data a map of the network in the form of a graph. Each node floods the entire network with information about what other nodes it can connect toEach node then independently assembles this information into a map Using this map, each router then independently determines the best route from itself to every other nodeRouting algorithmsLink-state algorithms (cont.)Uses Dijkstra's algorithmBuilds another data structure (a tree)Current node itself as the rootContaining every other node in the networkStarting with a tree containing only itselfAdds, one at a time, nodes which have not added to the treeAdds the node which has the lowest cost to reach an adjacent node which already appears in the tree.Continues until every node appears in the tree.Routing algorithmsLink-state algorithms (cont.)This tree then serves to construct the routing tableGives the best next hop, etc, to get from the node itself to any other network.Routed vs. Routing Protocols Confusion often arises between:"routed protocols“- and -"routing protocols"Routed versus Routing ProtocolsRouted protocolsAny network protocol that provides enough information in its Network Layer addressTo allow a packet to be forwarded from one host to another host based on the addressing schemeWithout knowing the entire path from source to destinationRouted versus Routing ProtocolsRouted protocols (cont.)Define the format and use of the fields within a packetPackets generally are conveyed from end system to end system.Almost all layer 3 protocols and those that are layered over them are routableIP is an exampleRouted versus Routing ProtocolsRouted protocols (cont.)Layer 2 protocols such as Ethernet are necessarily non-routable protocolsThey contain only a link-layer address, which is insufficient for routing: Some higher-level protocols based directly on these without the addition of a network layer address are also non-routableFor example: NetBIOS,Routed versus Routing ProtocolsRouting protocolsUsed in the implementation of routing algorithms facilitate the exchange of routing information between networksallowing routers to build routing tables dynamicallyIn some cases, routing protocols can themselves run over routed protocols: for example, BGP runs over TCP: care is taken in the implementation of such systems not to create a circular dependency between the routing and routed protocolsRouting MetricsRouting metrics Any value used by routing algorithms to determine whether one route