DMVPN

In phase 1 of DMVPN, why do we need to disable split horizon if we are sending a defualt route from the hub? I believed either we send a defualt from the hub and specifics from the spokes OR disable split horizon on the hub.

 

Also if we are using RIP, why do we need to do a passive on the spoke tunnel interfaces? I was going through an article and had these doubts. Would appreciate if anyone can clear it

Comments

  • Hi,

    First, I must define what split-horizon does. It ensures that routes learnt from an interface are not advertized out the same interface. Now, we need to disable split horizon when we are advertizing routes learnt from one spoke to another spoke, which is a common scenario. However, another common scenario is when each of the spokes has a small routing table, and for anything else, it has to go through the hub. Now, in this case, all you have to do is advertise a default route pointing towards the hub and since you are advertising a default route, you don't need to advertise routes learnt from one spoke to the other, hence eliminating the need of advertising routes out the same interface they were learnt from, and ultimately eliminating the need of split-horizon.

    I hope that is clear.

     

    Why should we make the tunnel interface passive? I can't think of a reason why since that would stop routing tables from being exchanged over the tunnel. Can you quote the exact lines from the article that said this? The only thing that makes sense is to make all interfaces except the tunnel interface passive.

     

    Thank you.

  • Hi all,

        Split horizon does not really stop advertising a prefix back on the interface it was received, as most of the books say. Split horizon says the following: if i receive a prefix on interface Fa0/0 and this is my best route for this prefix, i will not advertise back on the same interface; if i receive a prefix on interface Fa0/0 BUT my best route for this prefix is over Fa0/1 i WILL advertise back on Fa0/0 that prefix saying about my best route to it.

       Now about "passive-interface" on RIP, it disables sending of broadcast and multicast RIP updates (unicast updates by neighbor command can still be send), but it does NOT stop receiving RIP updates. So basically in this DMCPN case, the hub will learn RIP prefixes from spokes, but not advertise anything to spokes, which probably have a static defaut route through the hub configured. Why is this used? In certain scenarios for scalling DMVPN deployments with routing protocol scalability issued on hub.

    Hope this has been informative!

  • Thank you for pointing this out!!

  • Thanks a lot Cristian. I got the RIP part . But for the split horizon I still have that doubt. If routes learnt from one spoke need not be sent to the other spoke , then split horizon should be kept enabled not disabled. Right?

  • Definition of Split Horizon: When router receives the routing updates on one inteface, it doesn't send back through the same interface.

    Purpose: Controlling Routing loops.

    We should disable if we have the HUB and SPOKE topology.

    When you have the HUB and SPOKE topology, every SPOKE sends the routing updates to HUB. When HUB receives the routes from one SPOKE HUB should forward to another SPOKE for end to end reachability. 

    Like R1 HUB, it's serial 0/0 connected to Frame-Relay switch and R2, R3, R4 Spokes are also connected to Frame-Relay switch. When R2 sends routing updates to R1, R1 should install into it's routing table as well as should sends updates to R3, R4 . If Split-Horizon is enabled on interface, R1 couldn't send the routes coming from R2 to R3 and R4 similarly R3 to R2 and R4, R4 to R2 and R3.

     

    For RIP

    use no ip split-horizon

    For EIGRP

    use no ip split-horizon eigrp <AS> number

    Check using following command

    R1#show ip interface serial 0/0.2 | in Split
      Split horizon is enabled

  • Thanks a lot Cristian. I got the RIP part . But for the split horizon I still have that doubt. If routes learnt from one spoke need not be sent to the other spoke , then split horizon should be kept enabled not disabled. Right?

     

    Hi,

     

    That's correct.. if routes from spokes need not be advertised to each other.. than split-horizon should be enabled.. which is the default setting.

     

    Thank You!

  • Hi nnn,

      This is not the correct definition of split-horizon. Did you check my previous post on this topic?

    Good luck with your studies!

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