Why do you disable EIGRP split horizon on a DMVPN hub?
Show the answer
Say this first
Split horizon stops a router advertising a route out the interface it learned it on. A DMVPN hub reaches every spoke through one multipoint tunnel interface, so spoke A's LAN, learned on Tunnel0, would never be sent back out Tunnel0 to spoke B. Turning split horizon off on the hub tunnel lets the spokes learn each other's routes. Split horizon is on by default on every EIGRP interface.
Go deeper
The commands
Classic mode, on the hub tunnel:
interface Tunnel0
no ip split-horizon eigrp 100Named mode:
router eigrp WAN
address-family ipv4 unicast autonomous-system 100
af-interface Tunnel0
no split-horizonDMVPN phases
- Phase 1: all traffic goes through the hub. The hub can send just a summary or a default, and split horizon does not matter.
- Phase 2: spokes build direct tunnels, so each spoke needs the other spoke's real next hop. The
hub needs split horizon off and
no ip next-hop-self eigrp 100so it does not rewrite the next hop. - Phase 3: NHRP redirects and shortcuts handle spoke to spoke, so the hub can summarize to the spokes again. This is the design I would pick.
Is it safe?
Yes. For EIGRP, split horizon is an optimization. DUAL's feasibility condition still prevents loops when it is off.
They will ask next
Where else does split horizon bite?
Any multipoint interface where several neighbors sit behind one interface, such as old Frame Relay multipoint. Point-to-point subinterfaces avoid it because each neighbor has its own interface.
Traps
- Disabling split horizon on the spokes. It only matters on the hub's multipoint interface.