The third category of routing protocols is advanced distance vector.
In theory, EIGRPP or enhanced interior gateway routing protocol takes the best of both distance vector
and link states routing protocols and combines that into a single routing protocol.
You may see that EIGRP is described as a hybrid routing protocol where it combines elements of both
distance-vector and link-state into a single routing protocol that isn't entirely true
and in the EIGRP videos, we'll discuss that in a lot more detail.
In theory, EIGRP takes the best of distance vector in that it's very easy to configure.
You could configure EIGRP on a router as an example by using just two commands and thus it's very
easy to configure in the same way that RIP in the past was easier to configure.
EIGRP however forms neighbor relationships in a similar way to link-state routing protocols such as
OSPF.
So unlike distance-vector routing protocols like RIP that do not form neighbor relationships, EIGRP does
form neighbor relationships.
So in theory it's taking the best of both distance factor and link-state and combining them into a single
routing protocol that gives you the power of a link-state routing protocol with the ease of configuration
of a distance vector routing protocol.
The disadvantage of EIGRP is that it's Cisco proprietary.
So unless you are running a pure Cisco network you may have to choose a different routing protocol such
as OSPF.
If however, all your devices are Cisco devices EIGRP is a very good choice.
It's very easy to configure.
It's very quiet.
It updates very quickly and is a good choice for most enterprise networks that only have Cisco devices.
Another very important concept to understand is administrative distance which is essentially the believability
factor of a route. When multiple routing protocols are configured on a router and are trying to put
routes into what's called the routing information base or RIB
also called the routing table a router will need to decide who to believe.
If OSPF says go left and EIGRP says go right to get to a network which writing protocol
does the router believe
and which route does it insert into the routing table?
In other words, what is the trustworthiness or believability of the routing information that the router is
receiving? Administrative distance is then used as a tiebreaker to determine which way to go when multiple
routing protocols tell the router to go different ways.
As an analogy, administrative distance is used as a tiebreaker to determine which way to forward traffic.
If OSPF says go left and EIGRP says go right the router uses administrative distance to determine
which routing protocol it believes most. Technically what happens is that the routing protocol with the
lowest administrative distance is more believable
and its routing information is inserted into the RIB or routing information base or routing table
in preference of other routing protocols that have a higher administrative distance.
So EIGRP has a lower administrative distance than OSPF, EIGRP has an administered distance of 90 OSPF
has an administrative distance of 110.
So internal EIGRP would be preferred over OSPF so the lower the administrative distance the more
believable a routing protocol is.
You cannot believe anyone more than yourself.
So a router will believe routes that it originates above routes received from neighboring devices a directly
connected interface has a believability factor or administrative distance of 0 a static route configured
to a next-hop IP address has a administrative distance of 1 when a static route is configured to a directly
connected interface it is deemed to be directly connected and therefore has a believability factor or
administrative distance of 0.
The reason why static routes have such a low administrative distance is that the router believes that
you know more than other routers do you as the administrator are supposed to know what you are doing
and thus the router believes you more than it believes routes received via routing protocols such as
OSPF and EIGRP.
EIGRP has an administrative distance of 90 for internal routes.
Those are routes that originated in EIGRP and were not redistributed or taken from a different routing
protocol and then put into EIGRP. OSPF has an administrative distance of 110, RIP has an administrative distance of 120
and unknown routes have an administrative distance of 255.
As you can see EIGRP has a lower administrative distance than RIP does and that's because EIGRP
is a better or more powerful routing protocol.
EIGRP will take bandwidth and delay into account when determining the best route, RIP only uses hop
count.
So EIGRP seemed to be better than OSPF which is better than RIP.