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I'm trying to become more familiar with DMVPN, NHRP, and mGRE. I just have one quick question regarding solutions that offer spoke-to-spoke connections:

1.) Why is it necessary to disable split-horizon in order to enable spoke-to-spoke connections?

Thank you for your time!

1 Answer 1

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The split horizon rule is essentially:

Don't advertise a route out of the same interface you learned it from.

With hub and spoke networks and mGRE, the hub is connected to the spokes by a single (logical) interface. So when the hub learns a route from Spoke1, split horizon prevents advertising the route to Spoke2, because the route was learned via the same interface.

By disabling split horizon, the hub can advertise routes from spoke1 to spoke2.

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  • Thank you! I was focusing too much on NHRP/mGRE which knows of the multiple tunnels and mappings, but they're still connected via a single interface on the hub.
    – Rumblebutt
    Aug 28, 2014 at 17:17

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