In a hub-and-spoke DMVPN topology using EIGRP, the hub router learns routes from all spokes via a single multipoint GRE (mGRE) tunnel interface. By default, EIGRP's split-horizon rule prevents the hub from advertising a route back out of the same interface on which it was learned. This means the hub will not advertise routes learned from Spoke 1 to Spoke 2, and vice-versa. To enable spoke-to-spoke communication, spokes must learn the routes to each other's networks. Disabling EIGRP split-horizon on the hub's tunnel interface allows the hub to reflect these routes, enabling spokes to establish direct tunnels.

A. no ip eigrp 1 mode multipoint: This is not a valid EIGRP configuration command. It appears to be a distractor combining EIGRP syntax with a generic interface mode concept.

C. no ip eigrp 1 tunnel-redirect: This is not a valid EIGRP command. It is likely a distractor referencing the NHRP redirect mechanism (ip nhrp redirect), which is separate from EIGRP routing advertisements.

D. no ip eigrp 1 mode mgre: This is not a valid EIGRP configuration command. It incorrectly merges EIGRP syntax with the mGRE interface type.

1. Cisco Systems

Inc. (2019). IP Routing: EIGRP Configuration Guide

Cisco IOS XE Gibraltar 16.12.x.

Section: "Configuring EIGRP

" Sub-section "Disabling or Enabling Split Horizon."

Content: This document details the no ip split-horizon eigrp command. It explains that split horizon is enabled by default on most interfaces and that disabling it may be necessary on a hub router in a hub-and-spoke network to allow the hub to advertise information learned from one spoke to other spokes.

2. Cisco Systems

Inc. (2021). Dynamic Multipoint VPN Configuration Guide

Cisco IOS XE 17.

Section: "Configuring DMVPN

" Sub-section "DMVPN Phase 2: Hub-and-Spoke with Spoke-to-Spoke Tunnels."

Content: The guide explicitly states: "For certain routing protocols like EIGRP

you must disable split horizon on the hub to allow the hub to forward routing updates from one spoke to another." This directly supports the need to disable split-horizon for the described scenario.

3. Martin

R.

& Tafa

S. (2020). CCNP Enterprise Advanced Routing ENARSI 300-410 Official Cert Guide. Cisco Press.

Chapter 6: "Dynamic Multipoint VPN

" Section "DMVPN Phase 2."

Content: The text explains that for EIGRP to support spoke-to-spoke communication in DMVPN Phase 2

split-horizon must be disabled on the hub's mGRE interface. It provides the specific command no ip split-horizon eigrp and explains its function in this context.