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

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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