Refer to the exhibit. The Los Angeles and New York routers are receiving routers from Chicago but not from each other. Which configuration fixes the issue?
A.
interface Tunnel1 no ip split-horizon eigrp 111
B.
interface Tunnel1 ip next-hop-self eigrp 111
C.
interface Tunnel1 tunnel mode ipsec ipv4
D.
interface Tunnel1 tunnel protection ipsec profile IPSec-PROFILE
The given answer is correct - A
Its important here to work out that Chicago is the Hub router in a DMVWN network.
If Chicago was a spoke it would need a mapping to the hub - and that is not in the output - ie "ip nhrp map" command with relevant tunnel and WAN ip addresses ip addresses for the hub.
The hub over its tunnel1 interface learns the routes from LA - it wants to advertise the LA routes to New York - but those advertisements would be back out Tunnel1 - split horizon will not allow this. Split horizon does not allow advertising routes back out the interface a router received them on.
So the fix is to disable split horizon - a valid fix in certain scenarios.
Note this is a phase 2 DMVPN solution
In Phase 2, the Hub is still the “hub” for the control plane. All routes are learned through the hub. Spokes cannot exchange routes with each other directly.
A phase 3 DMVPN solution does not have this split horizon issue.
We need to disable split-horizon on DMVPN hubs.
Split-horizon is a loop-prevention mechanism in EIGRP that works by not allowing a route to be advertised out from the same interface it was learned on. The problem is that in a DMVPN setup, the hub will need to learn and send out routes from the same tunnel interface. So we have to disable it.
As Surfside92 is saying Chicago is the HUB spoke, cause of the split horizon rule of eigrp you cannot advertise a prefix that you received in the same interface, so for disabling that we need to disable the split horizon rule
Suggested Answer A is correct.
@Surfside92: Hi I checked DMVPN-Ph3 topology in gns3, even here "no ip split-horizon eigrp <as>" config is required on the HUB site tunnel interface.
n this topology, Chicago router (Hub) will receive advertisements from Los Angeles (Spoke1) router on
its tunnel interface. The problem here is that it also has a connection with New York (Spoke2) on that
same tunnel interface. If we don’t disable EIGRP split-horizon, then the Hub will not relay routes from
Spoke1 to Spoke2 and the other way around. That is because it received those routes on interface
Tunnel1 and therefore it cannot advertise back out that same interface (splithorizon rule). Therefore we
must disable split-horizon on the Hub router to make sure the Spokes know about each other.
upvoted 7 times
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