- [Instructor] In this video, we want to consider a couple
of features that RIP, has Split Horizon and Poison Reverse.
Now, Split Horizon is a rule that's used not just by RIP
but also by EIGRP and that rule says that
if a router receives an advertisement
for a network in a specific interface,
that router will not advertise that same network out
of the interface on which it was learned
and that can prevent a routing loop
or what's called a count to infinity problem.
Consider this example,
let's imagine that Split Horizon has been disabled
and R3 is connected to the 10.1.1.0/24 network
and we see in R3s routing table,
it's zero hops away because it's directly connected
and it has advertised to previously over to R2
that it's directly connected and R2 knows
that the 10.1.1.0/24 network is available to it
if it just hops through or transits one router.
So it has a hop count of one from the perspective of R2
but let's say something happens to that network,
it goes down, when that happens,
R3s routing table says we're no longer directly connected.
We're infinitely far away from this, but let's say
that R2 then advertised just to R3,
I can reach 10.1.1.0/24 in one hop
and R3 says, oh that is great news
because I could not reach it previously.
If you're one hop away and I go through you
that means I'm two hops away.
Do you see the issue here?
R2 is not one hop away from that network any longer
because that network is down and R3 has a false sense
of assurance that it can reach that network.
When R3 does advertise that network back to R2,
saying it's two hops away,
R2, since it learned the network from R3 in the beginning,
it's going to update its hop count to say it's three hops away
and following that same logic R2 advertises to R3
that it's three hops away,
R3 says, I'm now four hops away
and this goes on and on until we reach a hop count of 16.
Remember when we get to a hop count of 16 in RIP,
that network is considered to be infinitely far away.
It's unreachable.
This is a count to infinity problem,
but by having Split Horizon enabled as it is by default,
router R2 is not going to advertise back to R3
any networks that it has learned from R3,
that's going to prevent this count to infinity problem.
Another way that RIP attempts to prevent a routing loop
like this or a count to infinity problem
is a feature called Poison Reverse.
This says when a network goes down,
a router is going to advertise a poisoned route,
a route with an infinite metric saying
that this route is unreachable.
Going back to our previous topology, let's once again say
that the 10.1.1.0/24 24 network off of R3 goes down,
R3s hop count to that network goes from zero to infinity,
but with Poison Reverse
it's going to send an advertisement over to R2
saying that the hop count to 10.1.1.0/24 is 16
which is infinitely far away.
That's going to make R2 look at that advertisement,
that poisoned route advertisement specifically
and say that network is infinitely far away
and that's a look at two features RIP uses
to prevent a routing loop or a count to infinity problem,
and those were Split Horizon and Poison Reverse.