Cisco Systems Networking Academy S2 C 11

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Cisco Systems Networking AcademyS2 C 11

• Routing Basics

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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

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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

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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

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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.

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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

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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

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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
  upper-layer functions (CCNA says 3 levels can be
  supported)
• End system addresses frame using MAC address of
  intermediate system

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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

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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

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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

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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

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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

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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

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Routing Metric ComponentsThe smaller the metric
the better

• Hop Count
• Ticks
• Cost
• Bandwidth
• Delay
• Load
• Reliability

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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

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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

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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

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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
  hold-down timer started) no conflicting poorer
  information accepted from other routers until
  time expires

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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

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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

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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.

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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

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Link State Concerns

• Processing Requirements
• Use Dijkstras 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

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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

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Comparison

• Distance Vector
• Views topology from neighbors 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

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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 OSIs IS-IS and Cisco EIGRP

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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.

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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