Now that we have covered the EIGRP routing concepts in the previous modules,
the basics of forming a neighborship, timers,
and the structure of EIGRP packets,
let's go a little bit deeper into the concepts required for EIGRP
devices to communicate and exchange information and state.
Let's begin by reviewing EIGRP route states.
Every route that exists in the EIGRP topology table will also be
assigned one of two states depending on its status.
These include passive and active.
When a route is in the passive state,
at least one successor exists to the destination,
meaning that a loop-free path has been learned by the local device and is
able to be placed into the topology and routing tables.
Under stable network conditions, all EIGRP routes should be in the passive state.
When a route is in the active state,
its successor or successor's path has been lost,
and no feasible successors are available.
When this happens, the route is placed into the active state,
and it begins to actively search for an alternative route
using its other available neighbors.
But it's important as well to note that these messages that
will go out in search of a new route are subject to split
horizon and poison reverse roles,
just like messages advertising new or changing network knowledge.
While I noted this in the first module,
I didn't go into what these specifically were.
So let's cover them one at a time.
Split horizon is a concept that is often implemented in
distance factor routing protocols that states,
never advertise a route out of the interface through which it was learned.
Cisco's EIGRP implementation of split horizon is interpreted per the RFC as,
if you have a successor route to a destination,
never advertise the route out the interface on which it was learned.
This is used as an additional way to ensure that routing loops are not formed.
Poison reverse is often implemented along with split horizon.
What it does is it poisons a route back out the
interface that it was learned on.
With EIGRP,
this results in a route being advertised with an infinite delay
metric and is only applied on successor interfaces that are
currently being used to reach a destination.
Poison reverse can also be used in situations where an EIGRP device
wants to explicitly advertise that it doesn't, or no longer has a suitable path to a specific destination.