Distance Vector Facts

Distance Vector Facts

Keep in mind the following principles about the distance vector method.

• Routers send updates only to their neighbor routers
• Routers send their entire routing table
• Tables are sent at regular intervals (each router is configured to specify its own update interval)
• Routers modify their tables based on information received from their neighbors

Because routers using the distance vector method send their entire routing table at specified intervals, they are susceptible to a condition known as a routing loop (also called a count-to-infinity condition). Like a bridging loop, a routing loop occurs when two routers share different information. The following methods can be used to minimize the effects of a routing loop.

Method	Characteristics
Split horizon	Using the split horizon method (also called best information), routers keep track of where the information about a route came from. Routers do not report route information to the routers on that path. In other words, routers do not report information back to the router from which their information originated.
Split horizon with poison reverse	Using the split horizon with poison reverse method (also called poison reverse or route poisoning), routers continue to send information about routes back to the next hop router, but advertise the path as unreachable. If the next hop router notices that the route is still reachable, it ignores the information. If, however, the path timeout has been reached, the route is immediately set to unreachable (16 hops for RIP). Convergence happens faster with poison reverse than with simple split horizon. However, it results in greater network traffic because the entire table is broadcast each time an update is sent.
Triggered updates	With the triggered update method (also known as a flash updates), routers that receive updated (changed) information broadcast those changes immediately rather than waiting for the next reporting interval. With this method, routers broadcast their routing tables periodically, punctuated by special broadcasts if conditions have changed. This method reduces the convergence time.
Hold-downs	With the hold-down method, routers will, for a period of time, "hold" an update that reinstates an expired link. The time period typically reflects the time required to attain convergence on the network. The hold-down timer is reset when the timer runs out or when a network change occurs.

The distance vector method has the following advantages:

• Stable and proven method (distance vector was the original routing algorithm)
• Easy to implement and administer
• Bandwidth requirements negligible for a typical LAN environment
• Requires less hardware and processing power than other routing methods

Distance vector has the following disadvantages:

• Relatively long time to reach convergence (updates sent at specified intervals)
• Routers must recalculate their routing tables before forwarding changes
• Susceptible to routing loops (count-to-infinity)
• Bandwidth requirements can be too great for WAN or complex LAN environments