In an Aruba CX EVPN-VXLAN fabric, an underlay routing protocol is required to provide IP reachability between the Virtual Tunnel Endpoints (VTEPs), which are typically the leaf switches. OSPF is the recommended and most commonly deployed Interior Gateway Protocol (IGP) for this purpose within a single data center fabric (referred to as a "room" in the scenario). It is a robust, standards-based, and well-understood link-state protocol that efficiently establishes the necessary underlay connectivity for the VXLAN overlay to function. The design ensures that all VTEP loopback addresses are reachable across the fabric.

A. iBGP: iBGP is used for the EVPN overlay control plane to exchange MAC/IP address information between VTEPs within the same Autonomous System, not for underlay reachability.

B. eBGP: While eBGP can be used as an underlay protocol (a "BGP-as-underlay" design), OSPF is the more common and standard IGP choice for a single Aruba CX fabric.

D. IS-IS: IS-IS is a valid link-state protocol for an underlay, but it is less commonly used and documented in Aruba enterprise data center designs compared to OSPF.

1. ArubaOS-CX 10.11 EVPN-VXLAN Guide

Hewlett Packard Enterprise Development LP

(2023). In the "Configuring the underlay" section

the guide states

"The underlay network uses a routing protocol

such as OSPF

to provide unicast routing of traffic between VTEPs." The subsequent configuration examples exclusively use OSPF for the underlay. (Page 16).

2. HPE Aruba Networking Validated Solution Guide - Data Center Networking with EVPN-VXLAN

Hewlett Packard Enterprise Development LP

(2022). In the chapter on "Underlay Network Design

" the guide specifies

"An Interior Gateway Protocol (IGP) is required to provide reachability between VTEPs in the underlay network. OSPF is a common choice for the IGP in a spine-and-leaf fabric." (Section: Underlay Routing).