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Chap 11 Routing

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Chap 11 Routing Andres, Wen-Yuan Liao Department of Computer Science and Engineering De Lin Institute of Technology andres_at_dlit.edu.tw http://www.cse.dlit.edu.tw/~andres – PowerPoint PPT presentation

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Title: Chap 11 Routing


1
Chap 11 Routing
  • Andres, Wen-Yuan Liao
  • Department of Computer Science and Engineering
  • De Lin Institute of Technology
  • andres_at_dlit.edu.tw
  • http//www.cse.dlit.edu.tw/andres

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Overview
  • Internetworking function
  • Routing and routed protocols
  • Track distance between locations
  • Distance-vector, link-state, and hybrid routing
    approaches

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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

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Path determination
  • Evaluate the available paths to a destination and
    to establish the preferred handling of a packet
  • Network part of IP
  • Layer 3

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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

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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

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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

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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

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Routing protocol
  • RIP (Routing Information Protocol)
  • IGRP (Interior Gateway Routing Protocol)
  • EIGRP (Enhanced Interior Gateway Routing
    Protocol)
  • OSPF (Open Shortest Path First)

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Multiprotocol routing
  • Routers are capable of supporting multiple
    independent routing protocols and maintaining
    routing tables for several routed protocols

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Why Routing Protocols are Necessary
  • Static versus dynamic routes
  • Default route
  • Dynamic routing
  • Various metrics
  • Three classes of routing protocols
  • Time to convergence

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Static versus dynamic routes
  • Static route
  • Be administered manually by a network
    administrator who enters it into a router's
    configuration
  • Dynamic route
  • The route knowledge updated by a routing process

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Static route
  • Dynamic routing reveal everything known about an
    inter-network
  • Security reasons
  • Hide parts of an internetwork
  • A stub networkA network is accessible by one path

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Default route
  • A routing table entry that directs packets to the
    next hop when that hop is not explicitly listed
    in the routing table

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Dynamic routing
  • Offers more flexibility
  • Loadsharing
  • Direct traffic from the same session over
    different paths in a network for better
    performance

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Dynamic routing operations
  1. Maintenance of a routing table
  2. Timely distribution of knowledge, in the form of
    routing updates, to other routers

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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

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Three classes of routing protocols
  • Distance-vector routing
  • Determines the direction (vector) and distance to
    any link
  • Link-state (shortest path first)
  • Re-creates the exact topology
  • Balanced hybrid approach

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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

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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

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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

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Exchange routing tables
  • Directly-connected network
  • A distance of 0

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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

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Routing loops
  • A network's slow convergence on a new
    configuration causes inconsistent routing entries

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Routing loops
  • Packets never reach their destination but instead
    cycle repeatedly through the same group of
    network nodes

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Counting to infinity
  • The invalid updates will continue to loop until
    some other process stops the looping

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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

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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

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Hold-down timers
  • When a previously accessible network is now
    inaccessible, the router marks the route as
    inaccessible and starts a hold-down timer

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Hold-down timers
  • They help prevent a router from immediately using
    an alternate route that includes the failed route

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Hold-down timers
  • If at any time before the hold-down timer expires
    an update indicates that the network is again
    accessible
  • Marks the network as accessible and removes the
    hold-down timer

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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

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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

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Hold-down timers
  • Allows more time for the knowledge of a
    disruptive change to propagate through the entire
    network

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Link-State Routing
  • Link-state routing basics
  • Exchange routing tables
  • Topology changes propagate
  • Two link-state concerns
  • Unsynchronized LSAs

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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

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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

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Exchange routing tables
  • Routers exchange LSAs with each other
  • Each router in parallel with the others
    constructs a topological database

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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

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Topology changes propagate
  • Whenever a link-state topology changes, the
    routers send information to other routers or to a
    designated router

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Each Router
  • Keeps track of its neighbors ( name, up or down,
    cost of the link )
  • Constructs an LSA packet
  • Sends out this LSA packet

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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

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Two link-state concerns
  • Processing and memory requirements
  • Hold information from various databases, the
    topology tree, and the routing table
  • Dijkstra's algorithm
  • Bandwidth requirements
  • Initial link-state packet flooding

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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

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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

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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

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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

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Summary
  • Network addressing and best path selection for
    traffic.
  • Routed and routing protocols
  • Distance-vector routing
  • Link-state routing
  • Balanced hybrid routing
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