VMware VeloCloud SD-WAN supports RFC 3021

Products

• VMware VeloCloud SD-WAN

Issue/Introduction

RFC 3021: Using 31-Bit Prefixes on IPv4 Point-to-Point Links‌. RFC 3021, published in December 2000, defines a method to conserve IPv4 address space by enabling the use of ‌31-bit subnet masks‌ on point-to-point (P2P) links‌12. This standard addresses the inefficiency of traditional IPv4 addressing, where a /30 subnet (requiring 4 addresses) was historically used for P2P connections, leaving two addresses unused‌

Environment

VMware VeloCloud SD-WAN Edge and Gateway

Resolution

VMware VeloCloud SD-WAN Edge and Gateway officially support RFC 3021. Configuring a 31-bit prefixes should not bring routing issue by design.

Additional Information

Known bug#129522

Fixed Issue 129522: On the Global Settings > Customer Configuration > Configure BGP and BFD page, addresses with 31 bit prefixes cannot be used as a Local IP value for the Handoff Interface.
The issue is caused by the Orchestrator not supporting RFC 3021 networks (/31 networks).

 

OSPF related consideration for RFC 3021:

When using a ‌31-bit subnet mask‌ (RFC 3021) on a router interface for OSPF, ‌explicitly configuring the OSPF network type as point-to-point (P2P)‌ is strongly recommended, though not strictly mandatory. Here’s the rationale and configuration guidance:

Why P2P Mode Is Preferred‌
‌RFC 3021 Constraints‌:

• A 31-bit subnet supports only ‌two usable IP addresses‌ (no broadcast address)‌12, making it inherently suited for ‌strict two-node links‌ (e.g., router-to-router connections).
• Traditional OSPF ‌broadcast network types‌ require a Designated Router (DR)/Backup DR (BDR) election process, which is unnecessary and problematic on 31-bit subnets due to the absence of a broadcast address‌

Default OSPF Behavior‌:

• If the interface is not explicitly set to P2P, OSPF may default to ‌broadcast mode‌ (for Ethernet-like interfaces) or ‌non-broadcast multi-access (NBMA)‌ mode, leading to DR/BDR election failures‌.