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Cisco Systems Networking Academy S2 C 11 Routing Basics Routing Tables • An IP routing table consists of destination network addresses and next hop pairs • At each stop, the next destination is calculated • The network layer provides best-effort end-to-end packet delivery across interconnected networks • After the path is selected, the router forwards the packet How Routers Route • As frames are received, the data link layer header is removed and discarded and the network layer frame is sent to the appropriate network layer process • Network protocol header is examined to determine destination of packet • Packet is then passed back to data link layer where it is encapsulated in a new frame and queued for delivery to appropriate interface How Routers Route - 2 • Each line between the routers has a number that routers use as a network address • Consistency of Layer 3 addresses across internetwork improves use of bandwidth by preventing unnecessary broadcasts • Consistent end-to-end addressing enables network layer to find a path to destination without burdening devices or links with broadcasts Network and Host Addressing • Network Address – path part used by router • Host Address – specific port or device • Destination router Ands subnet mask to network part of address to determine subnet that contains the host address • Most network protocol addressing schemes use some form of a host or node address. Path Selection and Packet Switching • A router generally relays a packet from one data link to another, using two basic functions: – a path determination function – a switching function • The switching function allows a router to accept a packet on one interface and forward it through a second interface. • The path determination functions selects best interface to use to send out the packet Routed and Routing Protocol • Routed protocol used between routers to direct user traffic – examples IP, IPX • Routing protocol used between routers to maintain routing tables – examples RIP, IGRP, EIGRP, OSPF Network Layer Protocol Operations • Layer 2 addresses may be changing constantly as packets work their way through network but layer 3 addresses are constant • Each router provides its services to support upperlayer functions (CCNA says 3 levels can be supported) • End system addresses frame using MAC address of intermediate system Multiple Routing Protocols • Routers can support multiple independent routing protocols – This capability allows router to deliver packets from several routed protocols over the same data links Static Dynamic Routes • Static Route – Uses programmed route the network administrator physically enters into router • Dynamic Route – Uses route that routing protocol adjusts automatically for topology or traffic changes Static Routes • Allow you to hide parts of network – Dynamic routing reveals everything known about a network – Static routing allows you to specify information to reveal • Stub network (only one possible path) – conserves resources Default Route • Default route used when next hop is not specified in routing table – Assumes and trusts next router will have a best path to destination or contain another default route Why Dynamic Routing? • An alternate route can substitute for a failed route • Dynamic routing protocols can also direct traffic from the same session over different paths in a network for better performance – Known as load sharing Dynamic Routing Operations • Success depends on: – Maintenance of routing tables – Timely distribution of knowledge in form of routing updates • Routing Protocol describes – – – – How to send updates What knowledge is contained in updates When to send updates How to locate recipients of the updates Routing Metric Components The smaller the metric the better • • • • • • • Hop Count Ticks Cost Bandwidth Delay Load Reliability Three Classes of Routing Protocols • Distance Vector – Determines direction and distance (hop count) • Link State a.k.a. Shortest Path First – Recreates exact topology of entire network • Hybrid – combination of distance vector and link state – Combines aspects of distance vector & link state Time to Convergence • The time it takes all routers to share the same information about the network • When topology changes routers must recompute routes (disrupts routing) • Time to reconvergence varies with routing protocols Distance Vector • Routers pass period copies of routing tables communicating topology changes • Each routeer receives routing tables from directly connected routers • Accumulates network distances • Does not allow router to know exact topology of entire network • Each router sends entire routing table – Contains total path cost and logical address of first router on path Routing Loops • Occur when slow convergence causes inconsistent routing entries • New updates contain paths to failed routes – Information is propagated to other routers – Invalid updates will continue to loop until some process stops the looping • Condition called “COUNT TO INFINITY” – Avoid by defining infinity (number of loops) – Hold-down timers (route marked inaccessible and holddown timer started) – no conflicting poorer information accepted from other routers until time expires Split Horizon • Incorrect information sent to a router contradicts correct information it just sent • Split Horizon solves problem – if a routing update about Network 1 arrives from Router A, Router B or Router D cannot send information about Network 1 back to Router A. – Thus is reduces incorrect routing information and routing overhead Link State Basics • Shortest Path First – Complex database of topology information – Table maintains full knowledge of distant routers • Uses – – – – link-state advertisements (LSAs) a topological database the SPF algorithm, and the resulting SPF tree a routing table of paths and ports to each network Link State Routing 2 • Algorithms rely on using same link-state updates – Whenever topology changes, routers share information • Convergence achieved because each router – keeps track of its neighbors: each neighbor's name, whether the neighbor is up or down, and the cost of the link to the neighbor. – constructs an LSA packet that lists its neighbor router names and link costs, including new neighbors, changes in link costs, and links to neighbors that have gone down. – Achieving Convergence Continued – sends out this LSA packet so that all other routers receive it. – when it receives an LSA packet, records the LSA packet in its database so that it updates the most recently generated LSA packet from each router. – completes a map of the internetwork by using accumulated LSA packet data and then computes routes to all other networks by using the SPF algorithm. • Each time LSA packet caused change in link-state database, SPF (link-state algorithm) recalculates best paths & updates routing table Link State Concerns • Processing Requirements – Use Dijkstra’s algorithm to compute the SPF (requires processing task proportional to number of links in network multiplied by number of routers) • Memory Requirements • Bandwidth requirements – During initial discovery process, all routers send LSA packets to each other – floods network and temporarily reduce bandwidth available for routed traffic Link State Continued • Most important aspect to to make certain all routers get necessary LSA packets • Need to synchronize large networks to keep updates correct • Order of router startup alters topologies learned • If LSA distribution is not done correctly, result is invalid routes • Scaling up on large networks can expand the problem Comparison • Distance Vector – Views topology from neighbor’s view – Adds distance vectors from router to router – Frequent, periodic updates; slow convergence – Copies of routing tables passed to neighbors • Link State – Common view of entire network topology – Shortest path calculated to routers – Event-triggered updates; faster convergence – Link-state routing updates passed Hybrid • Balanced-hybrid routing – Uses distance vectors with more accurate metrics – Use topology changes to trigger routing database updates – Converges rapidly – Uses fewer resources (bandwidth & memory) • Example is OSI’s IS-IS and Cisco EIGRP LAN to WAN Routing • Routers enable LAN-to-WAN packet flow by keeping the end-to-end source and destination addresses constant while encapsulating the packet in data link frames, as appropriate, for the next hop along the path. Routers • Devices that implement network services • Provide interfaces for wide range of links and subnetworks at wide range of speeds • Active and intelligent network nodes that help manage the network – Provide dynamic control over resources – Support tasks and goals for connectivity, reliable performance, mgm control, & flexibility – Route and switch but also sequence based on priority and filtering