Dynamic Routing

• Dynamic Routing


  Networking technique that enables routers to select paths according to the real-time logical network topology
• Dynamic Routing


  • Routers automatically learn about remote networks from other routers
  • Networks and best path to each are added to the routing table
• Dynamic Routing Protocols


  Used to facilitate the sharing of information about the reachability and status of remote networks between routers
• Dynamic Routing Protocol Operations

  1. Discovery of remote networks

  2. Maintaining up-to-date routing information

  3. Choosing the best path to destination networks

  4. Ability to find a new best path if the current path is no longer available
• Dynamic routing protocols have been used in networks since the late 1980s
• Routing Protocol Classification


  • Interior Gateway Protocols
  • Exterior Gateway Protocols
  • Distance Vector
  • Link-State
  • Path Vector
• Routing Protocols


  • IPv4: RIPv2, EIGRP, OSPFv2, IS-IS, BGP-4
  • IPv6: RIPng, EIGRP for IPv6, OSPFv3, IS-IS for IPv6, BGP-MP
• Autonomous System (AS)


  A network or set of networks under the administrative control of a single entity
• Interior Gateway Protocols (IGPs)


  Used to exchange routing information between routers within an AS
• Exterior Gateway Protocols (EGP)

  Used for the exchange of routing information between routers from different AS
• Distance Vector Routing Protocols

  • Aware of reachable networks only by their distance and vector and do not know the complete path that packets take to their destination
  • Examples: RIP, EIGRP
• Link-State Routing Protocols


  • Learn about reachable networks by having a full view of the entire topology
  • Routers use link-state information received from other routers to create a topology map and select the best path to all destination networks
  • Examples: OSPF, IS-IS
• Dynamic Routing Protocol Components

  • Data structures
  • Routing protocol messages
  • Algorithm
• Dynamic Routing Protocol Operation

  1. The router sends and receives routing messages on its interfaces

  2. The router shares routing messages and routing information with other routers using the same routing protocol

  3. Routers exchange routing information to learn about remote networks

  4. When a router detects a topology change the routing protocol can advertise this change to other routers
• Routing Protocol Metric


  Represents the distance to reach a network
• Routing Protocol Metrics


  • RIP: Hop count
  • OSPF: Cost based on cumulative bandwidth
  • EIGRP: Based on slowest bandwidth and delay, could also include load and reliability
• Convergence


  The network has converged when all routers have complete and accurate information about the network
• Convergence Speed


  • Affects how quickly routers adjust to topology changes
  • Influenced by the size of the network
  • A network is not completely operable until it has converged
  • Routing protocols with shorter convergence times are preferred
• Routing Information Protocol (RIP)

  A distance vector routing protocol that uses hop count as its metric
• RIP


  • Uses the Bellman-Ford algorithm
  • Limited to 15 hops maximum
  • Periodic routing updates - Entire routing table is broadcast every 30 seconds to neighbors
  • Assigned an AD of 120 on Cisco routers
  • Available versions: RIPv1, RIPv2, RIPng
• RIP Operation - Starting Up

  Router initially discovers its own directly connected networks and adds these to its routing table
• RIP Operation - Initial Exchange

  1. Routers begin sending routing updates containing their known networks to their neighbors

  2. Receiving neighbor increments the hop count of the received network and places the entry in the routing table
• RIP Operation - Periodic Exchange

  The routers repeat the exchange at regular intervals (30 seconds), passing their respective route information to neighbors, including those that they learned from other routers
• RIP Operation - Convergence and Maintenance


  Once all routers have the complete set of networks in their respective tables, the network has then converged, but routers continue to send periodic updates to keep tables up-to-date
• Routing Loops


  • A condition in which a packet is continuously transmitted within a series of routers without ever reaching its destination
  • Can result in excess use of bandwidth and degraded network convergence
  • May be caused by slow convergence and incorrect management of routing updates
• Routing Loop Prevention


  RIP uses the following mechanisms: Split horizon rule, Route poisoning, Hold-down timer
• Split Horizon Rule


  Prevents routers from advertising a route out the same interface where it was received
• Route Poisoning


  A router that detects an unreachable network must deliberately advertise it as 'down' to its neighbor by tagging it with an unreachable metric (16)
• Routing Loop Prevention

  1. Split Horizon

  2. Route Poisoning

  3. Hold-down Timer
• In route poisoning, a router that detects an unreachable network must deliberately advertise it as 'down' to its neighbor by tagging it with an unreachable metric (16)
• Hold-down timers


  Prevent a router from accepting further updates to a route for a specified period of time (180 seconds default) after it is advertised as unreachable (down)
• When the timer expires and a route is still being advertised as unreachable, it is removed from the routing table
• RIP Configuration Procedure


  1. Enter RIP configuration mode

  2. Set RIP version (case-to-case basis)

  3. Identify directly connected networks to advertise and participate in routing updates

  4. Select passive interfaces (optional but recommended)

  5. Advertise a static default route (case-to-case basis)

  6. Disable automatic route summarization (case-to-case basis)
• Router RIP Configuration Mode

  Use the 'router' command on global config mode to enable a routing protocol
• Advertising Networks


  The 'network' command enables RIP on all interfaces of a router that belong to the specified network and also directs the router to advertise known networks to neighbors connected on these interfaces
• Setting Passive Interfaces


  Disable sending updates out an interface to save bandwidth, resources, and security
• Propagating a Default Route

  Edge routers connected to an external network can use RIP to pass default route information to other routers within the routing domain
• Automatic Summarization


  Route summarization is commonly supported by routing protocols to reduce routing table size, update bandwidth, and impact of topology changes
• By default, RIP automatically summarizes networks into their classful boundaries according to their starting octet value when advertising routes to neighbors
• Automatic summarization should be disabled when the network topology uses discontiguous subnet allocation that can lead to incorrect routes advertised by routers