Finals CS 123

• Router


  The basic backbone for the Internet. The main function is to connect two or more networks and forward packets from one network to another

  
• Router


  • Has multiple interfaces that each belong to a different IP network
  • Determines which interface to use to forward a packet to its destination

  
• Router forwarding packets

  1. Receives IP packet on one interface

  2. Determines which interface to use to forward the packet

  3. Encapsulates the IP packet into the data link frame of the outgoing interface

  4. Forwards the packet toward its destination
• Routing


  The primary responsibility of a router is to direct packets destined for local and remote networks

  
• Routing


  • Determining the best path to send packets
  • Forwarding packets toward their destination
• Routing table


  Used by the router to determine the best path to forward a packet

  
• Static routes


  Configured manually by network administrators

  
• Static routes


  • Require a lot of administrative time to maintain in large networks
  • Often configured in conjunction with a dynamic routing protocol in large networks

  
• When to use static routing

  • Network consists of only a few routers
  • Network is connected to the Internet through a single ISP
  • Large network is configured in a hub-and-spoke topology

  
• Connected routes


  Networks that are directly connected to the router, automatically routed by the router

  
• Dynamic routing


  Routing protocol adjusts routes automatically for topology or traffic changes

  
• Non-adaptive routing algorithm


  Router consults a static table to determine where to send a packet

  
• Adaptive routing algorithm


  Router examines traffic conditions to determine an optimal route

  
• Routing protocol


  Communication used between routers to share information about networks and their proximity

  
• Autonomous System (AS)


  A collection of networks under a common administration that share a common routing strategy

  
• Dynamic Routing Protocols


  • Interior Gateway Protocol (IGP)
  • Exterior Gateway Protocol (EGP)

  
• Metric


  A value used by routing protocols to assign costs to reach remote networks and determine the best path

  
• Distance Vector Routing Algorithm


  Routes are advertised as vectors of distance and direction. Router knows the direction/interface to forward packets and the distance to the destination network

  
• Initial update in a distance vector protocol

  1. Router sends updates about its connected networks

  2. Router receives updates from neighbors about their connected networks

  3. Router stores the received network information in its routing table

  
• Next update in a distance vector protocol

  1. Router sends updates about all the networks it knows about

  2. Router receives updates from neighbors about new networks

  3. Router stores the new network information in its routing table

  
• 0.0

  Metric of 1
• Stores network 10.4.0.0 in the routing table

  Metric of 1

  
• Receives an update from R2 about network 10.2.0.0

  Metric of 1
• Stores network 10.2.0.0 in the routing table

  Metric of 1
• After this first round of update exchanges, each router knows about the connected networks of their directly connected neighbors. However, R1 does not yet know about 10.4.0.0 and R3 does not yet know about 10.1.0.0. Full knowledge and a converged network will not take place until there is another exchange of routing information.

  
• R1 receives an update from R2 about network 10.4.0.0

  Metric of 2
• R1 stores network 10.4.0.0 in the routing table

  Metric of 2

  
• The update from R2 contains information about network 10.3.0.0 with a metric of 1. There is no change; therefore, the routing information remains the same.

  
• R2 receives an update from R1 about network 10.1.0.0. There is no change; therefore, the routing information remains the same.

  
• R2 receives an update from R3 about network 10.4.0.0. There is no change; therefore, the routing information remains the same.

  
• R3 receives an update from R2 about network 10.1.0.0


  Metric of 2
• R3 stores network 10.1.0.0 in the routing table

  Metric of 2

  
• The update from R2 contains information about network 10.2.0.0 with a metric of 1. There is no change; therefore, the routing information remains the same.

  
• Split horizon prevents information from being sent out the same interface from which it was received.
• Link State Routing Algorithm


  Also known as Shortest path Routing algorithm

  
• Link states


  • Information about the state of (Router interfaces) links

  
• Link state information


  • The interface's IP address and subnet mask
  • The type of network, such as Ethernet (broadcast) or Serial point-to-point link
  • The cost of that link
  • Any neighbor routers on that link

  
• Link-state routing process to reach a state of convergence

  1. Each router learns about its own links, its own directly connected networks

  2. Each router meets its neighbors on directly connected networks by exchanging Hello packets

  3. Each router builds a Link-State Packet (LSP) containing the state of each directly connected link

  4. Each router floods the LSP to all neighbors, who then store all LSPs received in a database

  5. Each router uses the database to construct a complete map of the topology and computes the best path to each destination network

  
• Advantages of Link state Routing protocol

  • Build the topological map
  • Faster Convergence
  • Event Driven Updates

  
• Differences between Distance Vector and Link State

  • Uses hop count as Metric vs Uses shortest path
  • View the network from the perspective of neighbor vs Gets common view of entire network topology
  • Has frequent and periodic updates vs Has event triggered updates
  • Slow convergence vs Faster convergence
  • Susceptible to routing loops vs Not as susceptible to routing loops
  • Easy to configure and administer vs Difficult to configure and administer
  • Requires less memory and processing power of routers vs Requires more precessing power and memory than distance vector
  • Consumes a lot of Bandwidth vs Consumes less BW than distance vector
  • Passes copies of routing table to neighbor routers vs Passes link-state routing updates to other routers
  • Eg. RIP vs Eg. OSPF