YANG constraints are used to automate the validation of complicated system configuration. All YANG constraints must be validated before a configuration change can take effect.

YANG constraints include:

  • Must statements (referential integrity)

  • When statements (conditional data nodes)

  • Leafref statements (cross references)


Below are some examples of YANG constraints using the ietf-interface YANG module from RFC 7223.



This module can be augmented by the module ex-vlan.yang found in RFC 7223 Appendix C.




The vlan-tagging leaf is only added to the interface entry if the “when” statement conditions are met.



This augment statement only adds the base-interface leaf when the interface-type is “l2vlan”.



The leaf base-interface has a leafref to interface eth0 which MUST have vlan-tagging = 'true'.




The ietf-interface YANG module after all of these YANG constraints have been validated. Below is the YANG structure:


       +--rw interfaces

         |  +--rw interface* [name]

         |     +--rw name                        string

         |     +--rw description?                string

         |     +--rw type                        identityref

         |     +--rw enabled?                    boolean

         |     +--rw link-up-down-trap-enable?   enumeration


Via augments:


         |     +--rw vlan-tagging?               boolean

         |     +--rw base-interface?             leafref

         |     +--rw vlan-id?                    uint16



To see how YumaPro performs processing YANG Constraints compared to Open Source, check out our Transaction Performance page.