In this vlog, we're going to continue discussing administrative distances and the believability of
routing protocols. So on router 1 show IP route, why in this topology is this loopback available in
the routing table through BGP and why not for this route?
So this network 2.2.2.2 is available in the routing table and the route was
learnt via BGP.
This network was learnt via EIGRP, so why is BGP used for the one network and not the other?
The first thing you need to look at is look at this
admin distance here.
It's 20, so that BGP route has an administrative distance of 20.
What are the administrative distances of BGP?
And what you must remember is that you have two versions of BGP, EBGP and IBGP.
EBGP has an admin distance of 20.
IBGP has an admin distance of 200 so show IP BGP summary.
We have a neighbor relationship to router 2 as well as to router 3. Show IP BGP neighbor
the relationship to router 2 uses EBGP.
Notice the autonomous system number four router 2 is 2 and scrolling down.
This neighbor 10.1.2.2 which is router 3 is using IBGP, router 3 is an autonomous
system one.
The relationship is established.
Are we learning the routes through BGP? So show IP BGP
Yes, we are learning both the loopback of router 2 as well as the loopback of router 3 through BGP.
So BGP has both routes in the BGP routing table but this route is not being put into the IP routing
table and we can see that because of this
R it says rip failure.
The rip is the routing information base which from a CCNA point of view is the routing table.
So this route is not put into the IP routing table by BGP and that's because a better route is available
through EIGRP.
So 3.3.3.3 and 4.4.4.4 are both not put into the IP routing
table.
Show IP BGP, we're not learning quadruple 4 through BGP.
We are learning about this route, but because of the rip failure it's not put into the routing table.
Notice here I the route was learned through IBGP
and the reason why this route is not put into the routing table is because EIGRP has a lower administrative
distance than IBGP.
So to prove that, let's remove EIGRP from router 1.
So rather no router EIGRP 1 show IP route notice the route is now available through BGP.
Admin distance is 200.
So remember, IBGP has an administrative distance of 200.
EBGP has an admin distance of 20. If we shut the neighbor relationship down.
So neighbor 10.1.1.2 shut down, what we should see
is that the route is replaced by another routing protocol in this case it's OSPF. So I'm running OSPF
from router 1 and router 2 and router 1 has learned about that loopback through OSPF previously
because the route was learned through EBGP the EBGP route took precedence
but now because we've shut down the neighbor relationship OSPF
is able to add the route to the routing table. So no neighbor 10.1.1.2 shut down when that
relationship comes up.
Which it has, we should see the routes replaced by BGP route and we can. Now in this example rip took a long
time to converge
but notice RIP has been used as the routing protocol to get to this loopback rather than BGP. Rip has
an admin distance of 120, IBGP has an admin distance of 200, rip is more believable.
So if we removed rip
from the router what we should see is the route is now available through BGP.
So in summary, IBGP has an admin distance of 200, rip has an admin distance of 120,
ISIS has an admin distance of 115,
OSPF has an admin distance of 110,
EIGRP internal routes have an admin distance of 90,
static routes to next-hop addresses have a admin distance of 1, static route to directly connected interface
as an admin distance of 0.
The lower the number, the more believable the routing protocol.
Remember with BGP, we have both EBGP and IBGP,
EBGP admin distance is 20, IBGP admin distance is 200.
Make sure you know you administrative distances.
That concludes this vlog.
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