Chapter 27
description
Transcript of Chapter 27
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Chapter 27Chapter 27
Internetwork Routing(Static and automatic routing;
route propagation; BGP, RIP, OSPF; multicast routing)
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TerminologyTerminology
ForwardingRefers to datagram transferPerformed by host or routerUses routing table
RoutingRefers to propagation of routing informationPerformed by routersInserts / changes values in routing table
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Two Forms of Internet Routing
Two Forms of Internet Routing
Static routingTable initialized when system bootsNo further changes
Automatic routingTable initialized when system bootsRouting software learns routes and updates tableContinuous changes possible
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Static RoutingStatic Routing
Used by most Internet hostsTypical routing table has two entries:
Local network → direct deliveryDefault → nearest router
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Example of Static RoutingExample of Static Routing
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Automatic RoutingAutomatic Routing
Used by IP routersRequires special softwareEach router communicates with neighborsPass routing informationUse route propagation protocol
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Example of Route Propagation
Example of Route Propagation
Each router advertises destinations that lie beyond it
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The Point of Routing Exchange
The Point of Routing Exchange
Each router runs routing software that learns about destinations other routers can reach, and informs other routers about destinations that it can reach. The routing software uses incoming information to update the local routing table continuously.
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Autonomous System Concept
Autonomous System Concept
Set of networks and routers under one administrative authority
Flexible, soft definitionIntuition: a single corporationNeeded because no routing protocol can
scale to entire InternetEach AS chooses a routing protocol
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Classifications of Internet Routing Protocols
Classifications of Internet Routing Protocols
Two broad classesInterior Gateway Protocols (IGPs)
Used among routers within autonomous systemDestinations lie within IGP
Exterior Gateway Protocols (EGPs)Used among autonomous systemsDestinations lie throughout Internet
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Illustration of IGP / EGP Use
Illustration of IGP / EGP Use
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The Concept of Route and Data Flow
The Concept of Route and Data Flow
Each ISP is an autonomous system that uses an Exterior Gateway Protocol to advertise its customers’ networks to other ISPs. After an ISP advertises destination D, datagrams destined for D can begin to arrive
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Specific Internet Routing Protocols
Specific Internet Routing Protocols
Border Gateway Protocol (BGP)Routing Information Protocol (RIP)Open Shortest Path First Protocol (OSPF)
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Border Gateway Protocol (BGP)
Border Gateway Protocol (BGP)
Provides routing among autonomous systems (EGP)Policies to control routes advertisedUses reliable transport (TCP)Gives path of autonomous systems for each
destinationCurrently the EGP of choice in the InternetCurrent version is four (BGP-4)As of January, 2009, the backbone Internet routing
table has approximately 300,000 entries
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The Routing Information Protocol (RIP)
The Routing Information Protocol (RIP)
Routing within an autonomous system (IGP)Hop count metricUnreliable transport (uses UDP)Broadcast or multicast deliveryDistance vector algorithmCan propagate a default routeImplemented by Unix program routedPassive version for hosts
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Illustration of RIP Packet Format
Illustration of RIP Packet Format
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The Open Shortest Path First Protocol (OSPF)
The Open Shortest Path First Protocol (OSPF)
Routing within an autonomous system (IGP)Full CIDR and subnet supportAuthenticated message exchangeAllows routes to be imported from outside
the autonomous systemUses link-status (SPF) algorithmSupport for multi-access networks (e.g.,
Ethernet)
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OSPF Areas and EfficiencyOSPF Areas and Efficiency
Allows subdivision of AS into areasLink-status information propagated within
areaRoutes summarized before being propagated
to another areaReduces overhead (less broadcast traffic)
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Link-Status in the InternetLink-Status in the Internet
Router corresponds to node in graphNetwork corresponds to edgeAdjacent pair of routers periodically
Test connectivityBroadcast link-status information to area
Each router uses link-status messages to compute shortest paths
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Illustration of OSPF GraphIllustration of OSPF Graph
(a) an interconnect of routers and networks, and (b) an equivalent OSPF graphRouter corresponds to a node in the graph
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OSPF and ScaleOSPF and Scale
Because it allows a manager to partition the routers and networks in an autonomous system into multiple areas, OSPF can scale to handle a much larger number of routers than other IGPs
Some networks are to big even for OSPF. For these networks BGP can be used as IGP.
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Internet Multicast RoutingInternet Multicast Routing
Difficult because Internet multicast allowsArbitrary computer to join multicast group at any timeArbitrary member to leave multicast group at any timeArbitrary computer to send message to a group (even if
not a member)Internet Group Multicast Protocol (IGMP)
Used between computer and local routerSpecifies multicast group membership
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Multicast architectureMulticast architecture
IGMP is used both by the client computer and the adjacent network switches to connect the client to a local multicast router. Protocol Independent Multicast (PIM) is then used between the local and remote multicast routers, to direct multicast traffic from the video server to many multicast clients
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Multicast Routing Protocols
Multicast Routing Protocols
Several protocols existDistance Vector Multicast Routing Protocol (DVMRP)Core Based Trees (CBT)Protocol Independent Multicast – Sparse Mode (PIM-SM)Protocol Independent Multicast – Dense Mode (PIM-DM)Multicast extensions to the Open Shortest Path First
(MOSPF)None best in all circumstances
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SummarySummary
Static routing used by hostsRouters require automatic routingInternet divided into autonomous systemsTwo broad classes of routing protocols
Interior Gateway Protocols (IGPs) provide routing within an autonomous system
Exterior Gateway Protocols (EGPs) provide routing among autonomous systems
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Summary (continued)Summary (continued)
Border Gateway Protocol (BGP) is current EGP used in Internet
Interior Gateway Protocols include:Routing Information Protocol (RIP)Open Shortest Path First protocol (OSPF)
Internet multicast routing difficultProtocols proposed include: DVMRP, PIM-SM,
PIM-DM, MOSPF