Routing Protocol Theory

IGP ( Interior Gateway Protocol ) : a routing protocol that was designed and intended for use in a single autonomous system. IGP are assigned a Autonomous System Number ( ASN ).
Example: Home office and or single company

EGP ( Exterior Gateway Protocol ) : a routing protocol that was designed for use between autonomous systems. Exchanges data between routers, Border Gateway Protocol BGP is the only legitimate EGP at this time.
Example: Internet Service Provider


Types of IGP Routing Protocol Algorithms


  • Distance Vector 
  • Link-State
  • Balanced hybrid \ enhanced distance vector
Metrics - each routing protocol algorithm uses different metrics, metrics are defined by hop count, cost, and or composite bandwidth and delay.











Route distribution - method of different routing protocols sharing information.

Administrative Distance - value given to routing protocols to assist with picking the best path to any given destination. 


Distance Vector Protocols facts


  • Periodic updates - timers are triggered at intervals updating routes. RIP uses 30 seconds by default for example
  • Full updates - sends full routing table every time an update is triggered. 
  • Full updates limited by split-horizon rules - permits the use of split-horizion to omit some routes from the periodic full update.
Distance Vector Loop Prevention

Route Poisoning - is the method distance vector use to advertise failed links. It advertises a failed link with a special metric value called infinity.

Counting-to-infinity - is a term used when a distance vector protocol has created a routing loop. A routing loop can happen when a route poison update does not make it to all routers before the periodic update is next sent. Creating a count to infinity which is 16 in distance vector protocols.

Split Horizon - Is used to prevent the counting to infinity routing problem with distance vector protocols. It does this by preventing routers from advertising routers back to the source of that learned route. Example R1 learns a route from R2, R1 will not turn around and advertise that route to R2.

Triggered update - sends an immediate triggered update listing the poisoned router, rather than waiting for the period timer to go off.

Poison reverse - When learning about a failed route, poison reverse suspends split-horizon rule for the route and advertises the poisoned router back to the sender as well.

Hold down timer - process that tells routers to ignore all new information learned about a down router for a certain period of time, giving all the other routers a chance to update.

Hold down timer process summary - after hearing a poisoned route the holddown timer starts for that one route. Until that time expires the routers will ignore or not believe any new information learned about that downed route to prevent routing loops caused by counting to infinity. However, information learned from the originally advertised the working router can be trusted before the timer expires.