For OSPF to function correctly over a DMVPN network, the OSPF network type should be set to point-to-multipoint on all participating routers (hub and spokes). This network type is designed for Non-Broadcast Multi-Access (NBMA) environments like DMVPN. It prevents the election of a Designated Router (DR) and Backup Designated Router (BDR), which would fail in a standard hub-and-spoke topology where spokes cannot directly communicate with each other initially. By treating the network as a collection of point-to-point links, it ensures proper next-hop resolution, which is critical for enabling direct spoke-to-spoke communication in DMVPN Phase 2 and 3.

B. ip ospf network point-to-point on the hub router: The hub's multipoint GRE (mGRE) interface connects to multiple spokes, not a single peer, making the point-to-point network type unsuitable.

C. ip ospf network point-to-multipoint on One spoke router: OSPF parameters, including the network type, must be consistent across all routers on a network segment to form adjacencies.

D. ip ospf network point-to-point on both spoke routers: The spokes connect to a multipoint network via the hub, not a simple point-to-point link, making this network type incorrect.

1. Cisco Systems

"OSPF over DMVPN Design Guide". In the "OSPF Network Type" section

the document states

"The recommended OSPF network type to run on a DMVPN network is point-to-multipoint. This network type does not have a DR/BDR

so it does not matter which routers can speak to each other." This directly supports using point-to-multipoint to avoid DR/BDR issues inherent in the DMVPN topology.

2. Cisco Systems

"IP Routing: OSPF Configuration Guide

Cisco IOS XE Gibraltar 16.12.x". The "OSPF Network Types" section describes the point-to-multipoint network type as a "numbered point-to-point link that has one or more neighbors

" which accurately models the hub-to-spoke relationship in DMVPN.

3. Lacoste

R.

& Edgeworth

B. (2020). CCNP Enterprise Advanced Routing ENARSI 300-410 Official Cert Guide. Cisco Press. Chapter 11

"Dynamic Multipoint VPN (DMVPN)

" in the section "DMVPN and Routing Protocols

" explicitly recommends

"The recommended OSPF network type for DMVPN is point-to-multipoint. This network type does not elect a DR/BDR

which is ideal for a partial-mesh topology." The configuration examples in this chapter show the ip ospf network point-to-multipoint command applied to both hub and spoke tunnel interfaces.