Dynamic Routing Protocols

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A dynamic routing protocol uses an algorithm and metrics to build and maintain a routing information base.

• database stores information about the networks to which the router is connected
• if multiple paths, the algorithm and metrics prioritize one over others
• info can be shared with the router’s neighbor
• a learned route is one that was communicated to a router by another router
  • router can add learned routes from one or more routing protocols to its IP routing table

Topology and Metrics

• algorithms used for path selection can be categorized by the topology and metrics that they use to build and update a routing information base and prioritize optimal (or least-cost) paths
• most are classified as either distance vector or link state
  • some protocols use a hybrid of different methods to perform path selection more efficiently
• for each protocol that it runs, a router maintains a routing information base of routes discovered by that protocol
  • these databases are separate to the IP routing table used to determine the forwarding path
    • might contain more than one route to the same destination prefix
    • in this case, a metric is calculated to determine which path will be selected for use in the IP routing table
    • path with the lowest cost metric is preferred
    • the type of algorithm determines which factors are used to calculate the metrics
      • E.g., distance vector protocols used the number of hops to the destination

Convergence

Convergence is the process whereby routers running dynamic routing algorithms agree on the network topology.

• routers must be capable of adapting to changes such as:
  • newly added networks
  • router or router interface failures
  • link failures
  • etc.
• routers must be able to communicate changes to other routers quickly to avoid sinkholes and loops
  • sinkhole means that a packet is discarded without notification back to the source
  • loop causes a packet to be forwarded around the network until its TTL expires
• a network where all the routers share the same topology is described as steady state
  • time taken to reach steady state is a measure of a routing protocols convergence performance

Info

• flapping interface is one that frequently changes from online to offline
• similarly, route flapping refers to a router changing the properties of a route it is advertising quickly and often
  • can cause serious convergence problems

Autonomous Systems

• routing protocols can be classified according to the way they deal with administrative boundaries
• a network under the administrative control of a single owner is referred to as an autonomous system (AS)
• an Interior Gateway Protocol (IGP) is one that identifies routes within an AS
• an Exterior Gateway Protocol (EGP) is one that can advertise routes between autonomous systems
  • includes a field to communicate the network’s autonomous system ID
  • allows network owners to determine whether they can use paths through another organization’s network