Overview

[[Networking]]

Used to overcome the shortcomings of static routing where static routing is simply impractical.

A dynamic routing table is made, maintained, and updated by routing protocol running within the router

The whole point of dynamic routing is to share routing info with other routers, discover remote networks and to chose the optimal path for your packets to travel.

Can automatically adapt to link failures and new nodes.

Classifying Dynamic Routing

• Interior Gateway Protocols (IGP)
  • Used fir routing within an Autonomous system
  • eg. RIP, EIGRP, OSPF, IS-IS
• Exterior Gateway protocols (EGP)
  • Used for routing between Autonomous systems
  • eg. BGP There are two distinct categories of dynamic routing protocols.
• Distance vector protocols
• Link state protocols

Distance-Vector

• Slow but simple
• All protocols share key characteristics:
  • Periodic updates of full routing table are sent to neighboring routers
  • Distance vectors suffer from convergence and are highly susceptible to loops
  • Some form of distance is used to calculate the route
  • The Bellman-Ford Algorithm is used to determine the shortest path
• Each node maintains 2 tables:
  • Distance table: Cost to each node via outgoing link
  • Routing table: Minimum cost to each node and next hop node
• Nodes exchange messages that contain routing info at the cost of a route
• Reception of said message triggers a re-calculation of routing table
• Major issue: The Bellman-Ford Algorithm cannot prevent loops. The count to infinity issue arises.

Link-State Routing Protocols

• Developed to prevent looping
• Takes into consideration all aspects such as link speed
• Tables:
  • Neighbor table
  • Topology table
  • Shortest route table OSPF (Open Shortest Path First):
• Open standard routing protocol
• Uses bandwidth as metric to determine route
• Features:
  • Wide scope
  • Fast
  • Loop free
  • Secure EIGRP (Enhanced Interior Gateway Routing Protocol):
• Cisco proprietary protocol
• A hybrid of Link-State and Distance Vector
• Feels like a mix of OSPF and RIP
• Uses metrics like:
  • Bandwidth
  • Load
  • Delay
  • Reliability
• Calculations are much more complex than OSPF and RIPv2

RIP (Routing Information Protocol)

• Classfull
• Easy to config
• Routing broadcast updated every 30 secs
• Metric = hop count
• 15 hop limit
• Does not work once subnets are introduces RIPv2 vs RIPv1:
• V2 is classless
• Increased efficiency
• Reduced routing entries
• Secure
• When routers advertise networks, they also show the subnetmask

Count to infinity problem: We have 3 routers with their own tables. Each table holds the distance to the nodes. Eventually each routing table with reach convergence; which is good. When one of those nodes goes offline the neighboring table will continue to try and count until it reaches the offline router. This makes the router hop count become infinity and then, as normal, the infinite hop router will then send that information around to the other nodes subsequently spreading broken information around. Solutions: - Route poisoning: - Prevents routers from sending packets through a route that has been deemed invalid within a network. A metric called “infinity” is used to flag tables with invalid info - Split Horizon: - Used to prevent advertising the cost to a neighbor is this neighbor is the next hop on the current path