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Dynamic Routing Protocols

Dynamic Routing Protocols. Static Routing. Easy to configure Easy to troubleshoot Doesn’t scale well Limited redundancy options. Routing Protocols.

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Dynamic Routing Protocols

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  1. Dynamic Routing Protocols RCC College

  2. Static Routing • Easy to configure • Easy to troubleshoot • Doesn’t scale well • Limited redundancy options RCC College

  3. Routing Protocols • A routing protocol is the language a router speaks with other routers in order to share information about the reachability and status of networks. • Routing protocols also determine the next best path if the current best path to a destination becomes unusable. RCC College

  4. Routing Algorithm specifies: • A procedure for passing reachability information about networks to other routers • A procedure for receiving reachability information from other routers • A procedure for determining optimal routes based on the reachability information it has and for recording this information in a routing table • A procedure for reacting to, compensation for, and advertising topology changes in an internetwork RCC College

  5. Example: 3-router internetwork RCC College

  6. Metrics • When there are multiple routes to the same destination, a router must have a mechanism for calculating the best path • A metric is a variable assigned to routes to rank them from best to worst • Different protocols use different, and sometimes multiple, metrics RCC College

  7. Hop Count • A hop count metrics simply counts router hops. • Is the A-B link really the best path? What if the A-B link is a DS-0 and the A-C and C-B links are T-1s? RCC College

  8. Bandwidth • A bandwidth metric would choose a higher-bandwidth path over a lower-bandwidth link. • What if the higher-bandwidth link has a higher delay and we transfer voice traffic? • What if the T1 links are highly loaded and a 56Kb link is lightly loaded? RCC College

  9. Load • This metric reflects the amount of traffic utilizing the links along the path. The best path is the one with the lowest load. • Unlike hop count and bandwidth, the load on a router changes, and therefore the metric will change. If the metric changes too frequently, route flapping - the frequent change of preferred routes - may occur. RCC College

  10. Delay • Delay is a measure of the time it takes a packet to travel a route. • A routing protocol using delay as a metric would choose the path with the least delay as the best path. • Delay changes with traffic load. RCC College

  11. Reliability • Reliability measures the likelihood that the link will fail in some way and can be either variable or fixed. • Examples: the number of times a link has failed or the number of errors it has received within a certain period of time. RCC College

  12. Cost • Cost is configured by the network administrator to reflect more- or less- preferred routes. • The term cost is often used as a generic term when speaking of route choices. RCC College

  13. Convergence • The process of bringing all route tables to the state of consistency is called convergence. • The time it takes to share information across an internetwork and for all routers to calculate best paths is called the convergence time. RCC College

  14. Distance Vector Routing Protocols • Most routing protocols fall into one of two classes: distance vector or link state. • Distance vector algorithms are based on the work done of Bellman, Ford, Fulkerson and occasionally referred to as Bellamn-Ford or Ford-Fulkerson. RCC College

  15. Distance Vector Routing Protocols (cont’d) • The name distance vector is delivered from the fact that routes are advertised as vectors of distance, where distance is defined in terms of metric, and direction is defined in terms of the next-hop router. • Example: Destination A is at a distance of 5 hops away, in the direction of next-hop router X. RCC College

  16. Distance vector routing protocols: • Routing Information Protocol (RIP) for IP • Cisco’s Internet Gateway Routing Protocol (IGRP) • Novell’s IPX RIP • AppleTalk’s Routing Table Maintenance Protocol (RTMP) RCC College

  17. Common Characteristics of DVPs • Periodic updates • Neighbors • Broadcast Updates • Full Routing Table Updates RCC College

  18. Periodic Updates • Periodic Updates mean that at the end of a certain time period, updates will be retransmitted. • These periods typically range from 10 to 90 seconds. • If updates are sent too infrequently, convergence time may be unacceptably high; if updates are sent too frequently, network traffic may be unacceptably high. RCC College

  19. Neighbors • In the context of routers, neighbors always mean routers sharing a common data link. • A distance vector routing protocol sends its updates to neighboring routers and depends on them to pass the update information along to their neighbors. • For this reason, distance vector routing is said to use hop-by-hop updates. RCC College

  20. Broadcast Updates • When a router first becomes active on a network, how does it find other routers and how does it announce its own presence? • The simplest method - to send the updates to the broadcast address 255.255.255.255. • Hosts and other devices uninterested in the routing updates will simply drop the packets. RCC College

  21. Full Routing Table Updates • Most distance vector protocols take the very simple approach of telling their neighbors everything they know by broadcasting the entire routing table • Neighbors receiving these updates glean the information they need and discard everything else. RCC College

  22. Routing by Rumor RCC College

  23. Route Invalidation Timers • What if a router goes down? Information about a path is no longer valid, but there is no router to inform about this fact. • The problem is handled by setting a route invalidation timer for each entry in the routing table. • Typical periods for route timeouts range from 3 to 6 update periods. RCC College

  24. Split Horizon • Each router broadcasts its entire route table to every neighbor every update period. • Is this really necessary? • A router pointing back to the router from which packets were received is called a reverse route. • Split horizon is a technique for preventing reverse routes between two routers. RCC College

  25. Split Horizon (cont’d) • Split Horizon prevents routing loops problems. • 2 categories of split horizon: simple split horizon and split horizon with poisoned reverse. RCC College

  26. Poisoned Reverse • When sending updates out of a particular interface, designate any network that was learned from updates received on that interface as unreachable. • Split horizon with poisoned reverse is considered safer and stronger than simple split horizon - it helps to remove corrupted routing information. RCC College

  27. Counting to Infinity • Sometimes split horizon doesn’t resolve the loop problems. • The way to alleviate the effect of counting to infinity is to define infinity. • For example, RIP defines infinity as 16 hops. As soon as hop count increases to 16, the network is considered unreachable. • Setting a maximum hop count to 15 helps to solve the counting-to-infinity problem, but convergence will be very slow. Given an update period of 30 seconds, a network could take up to 7.5 minutes to reconverge. RCC College

  28. Triggered updates • Triggered updates, also known as flash updates, are very simple: • If a metric changes for better or for worse, a router will immediately send out an update without waiting for its update timer to expire. • Reconvergence will occur quicker. • Regular updates may still occur along with triggered updates. RCC College

  29. Holddown Timers • Holddown timers introduce a certain amount of skepticism to reduce the acceptance of bad routing information. • If the distance to a destination increases (for example, the hop count increases from 2 to 4), the router sets a holddown timer for that route. • Until the timer expires, the router will not accept any new updates for the route. RCC College

  30. Asynchronous Updates • As a few routers become synchronized, collisions will begin to occur, further contributing to system delays, and eventually all routers sharing the broadcast network may become synchronized. • Asynchronous updates may be maintained by a small random time, or timing jitter, which is added to each update period as an offset. RCC College

  31. Link State Routing Protocols • Unlike the routing-by-rumor approach of distance-vector, link state routers have firsthand information from all their peer routers. • Each router originates information about itself, its directly connected links, and the state of those links. • This information is passed around from router to router, each router makes a copy of it, but never changes it. RCC College

  32. Link State Routing Protocols • Every router has identical information about the internetwork, and each router will independently calculate its own best path. • Link state protocols, sometimes called shortest path first or distributed database protocols, are built around a well-known algorithm from graph theory, E.W. Dijkstra’s shortest path algorithm. RCC College

  33. Link-state routing protocols: • Open Shortest Path First (OSPF) for IP • The ISO’s Intermediate System to Intermediate System (IS-IS) for IP • Novell’s NetWare Link Service Protocol (NLSP) RCC College

  34. Basic functionality of LSRP • Each router establishes a relationship - an adjacency - with each of its neighbors. • Each router sends link state advertisements (LSAs), sometimes called link state packets (LSPs), to each neighbor. • Each router stores a copy of all the LSAs it has seen in a database. • The completed topology database, also called link state database, describes a graph of the internetwork. • Each router calculates the shortest path to each network and enters this information into the route table. RCC College

  35. Autonomous System (AS) • On the Internet, an autonomous system (AS) is the unit of router policy, either a single network or a group of networks that is controlled by a common network administrator. • An autonomous system is assigned a globally unique number, called an Autonomous System Number (ASN). RCC College

  36. Interior and Exterior Gateway Protocol • Networks within an autonomous system communicate routing information to each other using an Interior Gateway Protocol. • An autonomous system shares routing information with other autonomous systems using an Exterior Gateway Protocol. RCC College

  37. Want to know more? • Jeff Doyle ‘Routing TCP/IP, volume 1’ • Perlman, R. ‘Interconnections: Bridges and Routers.’ RCC College

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