* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Chap 11 Routing
Survey
Document related concepts
Piggybacking (Internet access) wikipedia , lookup
Distributed firewall wikipedia , lookup
Zero-configuration networking wikipedia , lookup
Network tap wikipedia , lookup
Backpressure routing wikipedia , lookup
Internet protocol suite wikipedia , lookup
List of wireless community networks by region wikipedia , lookup
Deep packet inspection wikipedia , lookup
IEEE 802.1aq wikipedia , lookup
Wake-on-LAN wikipedia , lookup
Cracking of wireless networks wikipedia , lookup
Computer network wikipedia , lookup
Multiprotocol Label Switching wikipedia , lookup
Airborne Networking wikipedia , lookup
Recursive InterNetwork Architecture (RINA) wikipedia , lookup
Transcript
Chap 11 Routing Andres, Wen-Yuan Liao Department of Computer Science and Engineering De Lin Institute of Technology [email protected] http://www.cse.dlit.edu.tw/~andres Overview Internetworking function Routing and routed protocols Track distance between locations Distance-vector, link-state, and hybrid routing approaches Routing Basics Path determination Route packets from source to destination Network and host addressing Path selection and packet switching Routed versus routing protocol Network-layer protocol operations Multiprotocol routing Path determination Evaluate the available paths to a destination and to establish the preferred handling of a packet Network part of IP Layer 3 Route packets The consistency of Layer 3 addr. improves the use of bandwidth by preventing unnecessary broadcasts Broadcasts invoke unnecessary process overhead and waste capacity Network and host addressing The router uses the network address to identify the destination network of a packet Assigning host addresses Network administrator Be partially or completely dynamic Path selection and packet switching The router uses the network portion of the address to make path selections The switching function: accept a packet on one interface and forward it through a second interface Routed versus routing protocol Routed protocols define the field formats within a packet (IP): Carry data Routing protocols provide mechanisms for sharing routing information: Maintain table Routing protocol RIP (Routing Information Protocol) IGRP (Interior Gateway Routing Protocol) EIGRP (Enhanced Interior Gateway Routing Protocol) OSPF (Open Shortest Path First) Multiprotocol routing Routers are capable of supporting multiple independent routing protocols and maintaining routing tables for several routed protocols Why Routing Protocols are Necessary Static versus dynamic routes Default route Dynamic routing Various metrics Three classes of routing protocols Time to convergence Static versus dynamic routes Static route Be administered manually by a network administrator who enters it into a router's configuration Dynamic The route route knowledge: updated by a routing process Static route Dynamic routing: reveal everything known about an internetwork Security reasons Hide A stub parts of an internetwork network:A network is accessible by one path Default route A routing table entry that directs packets to the next hop when that hop is not explicitly listed in the routing table Dynamic routing Offers more flexibility Loadsharing Direct traffic from the same session over different paths in a network for better performance Dynamic routing operations 1. 2. Maintenance of a routing table Timely distribution of knowledge, in the form of routing updates, to other routers A routing protocol How to send updates What knowledge is contained in these updates When to send this knowledge How to locate recipients of the updates Three classes of routing protocols Distance-vector routing Determines the direction (vector) and distance to any link Link-state (shortest path first) Re-creates Balanced the exact topology hybrid approach Time to convergence Convergence The knowledge: an accurate, consistent view of new topology Converged All routers in an internetwork are operating with the same knowledge Fast convergence Distance-Vector Routing Distance-vector routing basics Exchange routing tables Topology changes propagate Routing loops Counting to infinity Defining a maximum Split horizon Hold-down timers Distance-vector routing basics Pass periodic copies of a routing table from router to router Do not allow a router to know the exact topology of an internetwork Exchange routing tables Directly-connected A distance of 0 network Topology changes propagate Topology change updates proceed step-by-step from router to router Send its entire routing table to each of its adjacent neighbor Routing loops A network's slow convergence on a new configuration causes inconsistent routing entries Routing loops Packets never reach their destination but instead cycle repeatedly through the same group of network nodes Counting to infinity The invalid updates will continue to loop until some other process stops the looping Sol: Defining a maximum Routing metric (hop count) Distance-vector protocols define infinity as a specific maximum no. The distance-vector default maximum of 15 hops Sol: Split horizon Split-horizon attempts: 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 Hold-down timers When a previously accessible network is now inaccessible, the router marks the route as inaccessible and starts a hold-down timer Hold-down timers They help prevent a router from immediately using an alternate route that includes the failed route Hold-down timers If at any time before the holddown timer expires an update indicates that the network is again accessible Marks the network as accessible and removes the hold-down timer Hold-down timers If an update arrives from a different neighboring router with a better metric, the router marks the network as accessible and removes the hold-down timer Hold-down timers If before the hold-down timer expires an update is received from a different neighboring router with a poorer metric, the update is ignored Hold-down timers Allows more time for the knowledge of a disruptive change to propagate through the entire network Link-State Routing Link-state routing basics Exchange routing tables Topology changes propagate Two link-state concerns Unsynchronized LSAs Link-state routing basics SPF (shortest path first): OSPF Maintain a complex database of topology information Maintains full knowledge of distant routers and how they interconnect Link-state routing Link-state advertisements (LSAs) A topological database The SPF algorithm; the SPF tree A routing table of paths and ports to each network Exchange routing tables Routers exchange LSAs with each other Each router in parallel with the others constructs a topological database Exchange routing tables The SPF algorithm computes network reachability. It then sorts these paths shortest path first (SPF) The router lists its best paths, and the ports to these destination networks Topology elements and status details Topology changes propagate Whenever a link-state topology changes, the routers send information to other routers or to a designated router Each Router Keeps track of its neighbors: ( name, up or down, cost of the link ) Constructs an LSA packet Sends out this LSA packet Each Router When it receives an LSA packet, updates the most recently generated LSA packet Completes a map of the internetwork by using the SPF algorithm Two link-state concerns Processing and memory requirements Hold information from various databases, the topology tree, and the routing table Dijkstra's algorithm Bandwidth Initial requirements link-state packet flooding Unsynchronized LSAs Networks become unreachable as a result of a disagreement among routers about a link Scaling up with link-state protocols on very large internetworks can expand the problem of faulty LSA packet distribution Different Routing Protocols Distance-vector vs. link-state routing Hybrid routing protocols LAN-to-LAN routing LAN-to-WAN routing Path selection and switching of multiple protocols and media Hybrid routing protocols Use distance vectors with more accurate metrics Using topology changes to trigger routing database updates Fewer resources OSI's IS-IS, and Cisco's EIGRP LAN-to-LAN routing Routers must be capable of seamlessly handling packets encapsulated into various lower-level frames without changing the packets' Layer 3 addressing Summary Network addressing and best path selection for traffic. Routed and routing protocols Distance-vector routing Link-state routing Balanced hybrid routing