IP Routing

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IP Routing

• Redes de Datos 2002
• etapia_at_eie.fceia.unr.edu.ar

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IP Routing

• IP router
• Connects different physical networks
• Datagrams forwarding
• Until they reach a router connected to the
  destinations LAN segment
• Selects the next hop device used to reach the
  destination
• The next hop is the next device along the path to
  reach the destination

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IP Routing view
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IP Routing Tables

• Routing Protocol
• Dynamically build and manage IP routing tables
• Information about a routers locally connected
  networks
• Network topology changes
• Routing tables are updated with minimal or no
  manual intervention

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IP addresses
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Net and Host Numbers
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Recall that

• Any address starting with 127 is a loop back
  address
• It should never be used for addressing outside
  the host
• A host number of all binary 1's indicates a
  directed broadcast over the specific network
• For example, 200.1.2.255 would indicate a
  broadcast over the 200.1.2 network
• If the host number is 0, it indicates "this host
• If the network number is 0, it indicates "this
  network"

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Network and Hosts
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Two LAN Segments
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Netmask

• Used to specify which part of the address is the
  network number part, and which is the host part
• This is accomplished by a logical bitwise-AND
  between the netmask and the IP address.The result
  specifies the network number
• For Class C, the netmask will always be
  255.255.255.0
• For Class B, the netmask will always be
  255.255.0.0 and so on.

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VLMS

• XYZ Corporation
• Network number 160.6.0.0 from the InterNIC
• The world sees this company as 160.6.0.0
• Variable Length Subnet Masks (VLSM)
• Divide their world into a multi-level hierarchy
• Sub-subnets in a tree-structured network

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Hierarchical Subnetting Example

• XYZ Corp has 23 field offices in all
• 32 subnets using a mask of 255.255.252.0. In
  binary. The mask bits are 11111111 11111111
  11111100 00000000 (64 different subnetworks)
• The designers want to further subdivide each site
  with a router
• Central routers are subnetted 255.255.252.0 and
  they differentiate between field offices
• The field office routers, are subnetted with
  255.255.255.128

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VLMS view
Mask
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Autonomous System (AS)

• Logical portion of a larger IP network
• Internetwork within an organization
• Administered by a single management authority
• Interior Gateway Protocols (IGPs)
• Allow routers to exchange information within an
  AS
• Open Short Path First (OSPF)
• Routing Information Protocol (RIP)

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ASs
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AS Example
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Routing Protocols

• Exterior Gateway Protocols (EGPs)
• Allow exchange of summary information between ASs
• Border Gateway Protocol (BGP)

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A Routing Taxonomy

• Static Routing
• Preprogrammed definitions on paths through the
  network
• Dynamic Routing Protocols
• Routers automatically discover and maintain
  awareness of the paths through the Network
• Differ on the way they discover and calculate new
  routes to destination networks

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Dynamic Routing Protocols

• Distance vector protocols
• Link state protocols
• Hybrid protocols

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Routing Protocols Diversity

• Different requirements for security, stability,
  and scalability
• Routing within a network
• Routing between networks
• New protocols
• Address deficiencies in established protocols.
• Different-sized networks
• Small to medium-sized networks
• Large networks

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Static Routing

• Coordination Maintenance
• Static routes are not adaptative
• Network Topology changes
• Requires no network bandwidth
• To advertise routes between neighboring devices
• Uses less processor memory and CPU cycles
• To calculate network path

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Static Routing contd

• Utilization or line tariffs
• Undesirable to send routing advertisement traffic
  through lower-capacity WAN connections.
• Traffic for a host traverses certain network path
• Provide a more secure network environment
• The administrator is aware of all subnetworks
  defined in the environment
• The administrator specifically authorizes all
  communication permitted between these
• To provide more efficient resource utilization

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Distance Vector Routing

• Each router maintains the distance or cost from
  itself to every known destination
• Represents the overall desirability of the path
• The path represented by the smallest cost becomes
  the preferred path to reach the destination
• Distance vector table
• Periodically advertised to each neighboring
  router
• Each router processes
• Advertisements to determine the best paths
  through the network

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Dynamic Routing contd

• Advantages
• Easy to implement and debug
• Useful in small networks with limited redundancy
• Disadvantages
• Excessive convergence time in complex
  internetworks
• Routing loops -Unstable packet forwarding
• Solution Maximum number of hops contained in a
  single route.

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Distance Vector Routing Table

• Periodically transmitted
• To neighboring devices
• Even if no changes have been made
• Periods of increased utilization in reduced
  capacity environments
• Enhancements
• Convergence and counting to infinity
• RIP and BGP are two popular examples

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DV Routing Table
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Distance Vector Routing Messages
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Distance Vector Routing Updates
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Initiating Tables
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Initiating Tables contd
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Initiating Tables contd
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Updating Routing Tables
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Updating Routing Tables contd
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Example
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Example contd
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Example contd
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DVR Features
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Count to Infinity Problem
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Count to Infinity causes
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Count to Infinity Solution 1
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Count to Infinity Solution 2
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Link State Routing

• Causes
• The growth in the size and complexity of networks
  
• Link state
• A link state is the description of an interface
  on a router
• IP address, subnet mask, type of network and its
  relationship to neighboring routers
• Link state database.

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Link State Routing

• Routers determine network topology
• Each router identifies all other routing devices
  on the directly connected networks.
• Advertise a list of all directly connected
  network links and the associated cost of each
  link
• Link state advertisements (LSAs) with other
  routers in the network

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Link State Routing

• Each router
• Creates a database detailing the current network
  topology
• The topology database in each router is identical
• Uses the information in the topology database to
  compute the most desirable routes to each
  destination network
• Updates the IP routing table

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Shortest Path First (SPF)

• The SPF algorithm is used to process the
  information in the topology database
• Tree-representation of the network
• The device running the SPF algorithm is the root
  of the tree
• The output of the SPF algorithm
• List of shortest-paths to each destination
  network
• OSPF

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SPF Example
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Hybrid Protocols

• Combine the positive attributes of both distance
  vector and link state protocols
• Like distance vector, hybrid protocols use
  metrics to assign a preference to a route
• Accurate metrics. Like link state algorithms,
  routing updates in hybrid protocols are event
  driven rather than periodic
• Converge more quickly than distance vector
  protocols
• Reduce the overhead of link state updates and
  distance vector advertisements

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Hybrid Routing Protocols

• Open hybrid protocols exist
• Proprietary EIGRP algorithm
• EIGRP was developed by Cisco Systems, Inc.

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RIP History
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RIP Features
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RIP Packet Format
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RIP Packets

• Request packets
• Query to neighboring RIP devices to obtain their
  distance vector table
• Indicate if the neighbor should return either a
  specific subset or the entire contents of the
  table.
• Response packets
• To advertise the information maintained in its
  local distance vector table
• The table is automatically sent every 30 seconds.
• The table is sent as a response to a request
  packet generated by another RIP node.

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RIP Packets contd

• If triggered updates are supported, the table is
  sent when there is a
• change to the local distance vector table
• When a response packet is received
• Information contained in the update is compared
  against the local distance vector table
• If the update contains a lower cost route to a
  destination, the table is updated to reflect the
  new path

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RIPv2 Subnet Mask
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RIP Operation
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RIP on UDP

• RIP packets are transmitted using UDP Datagrams
• RIP sends and receives datagrams using UDP port
  520
• RIP datagrams have a maximum size of 512 octets
• Larger updates are advertised in multiple
  datagrams.
• LAN environments
• RIP datagrams are sent using the MAC all-stations
  broadcast address and an IP network broadcast
  address
• In point-to-point (non-broadcast)
• RIP datagrams are addressed to the destination
  device

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RIP modes of operation

• Active mode
• Advertise their distance vector table
• Receive routing updates from neighboring RIP
  hosts
• Routing devices
• Passive (or silent) mode
• Receive routing updates from neighboring RIP
  devices
• Do not advertise their distance vector table
• End stations

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RIP Limitations

• Path cost limits
• The counting to infinity problem enforces a
  maximum cost for a network path
• Upper limit on the maximum network diameter (15
  hops)
• Network-intensive table updates
• Periodic broadcasting of the distance vector
  table can result in increased utilization of
  network resources

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RIP Limitations

• Relatively slow convergence
• The algorithms rely on timers to initiate routing
  table advertisements.
• No support for variable length subnet masking
• Route advertisements in a RIP environment do not
  include subnet masking information

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RIPv2
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RIPv2 Packet Format
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RIP Security
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Routing inside UNIX
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Routing in UNIX contd
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References

• TCP/IP Tutorial and Technical Overview. IBM 2001
• http//ece.iisc.ernet.in/innic/docs/vlsm.htm