The article explains a commit error that was reported during a new BGP IPV6 neighbour provisioning.

root@PE20_RE# show| compare 

[edit]

+ protocols {

+   bgp {

+     group ebgp_v6 {

+       type external;

+       family inet {

+         unicast;

+       }

+       peer-as 65002;

+       local-as 65001;

+       neighbor 2001:1:1:1::2;

+     }

+   }

+ }

root@PE20_RE# commit check 

[edit protocols]

 'bgp'

  Error in neighbor 2001:1:1:1::2 of group ebgp_v6:

Local IPv4 address must be configured under non-IPv6 NLRI with an IPv6 neighb

error: configuration check-out failed

The issue was due to an incorrect configuration. The BGP IPv6 group configuration was inheriting BGP NLRI "family inet unicast" instead of "family inet6 unicast". Correcting the configuration to below resolved the issue

root@PE20_RE# show | compare 

[edit]

+ protocols {

+   bgp {

+     group ebgp_v6 {

+       type external;

+       family inet6 {

+         unicast;

+       }

+       peer-as 65002;

+       local-as 65001;

+       neighbor 2001:1:1:1::2;

+     }

+   }

+ }

[edit]

root@PE20_RE# commit check 

configuration check succeeds

Or else, if the configuration is intended but the BGP neighbor is flapping due to NLRI mismatch in following scenario.

1. On IPv4 session, IPV6 prefixes are received from peer where the neighbor was only negotiated for IPv4 unicast address family.
2. On IPv6 session, IPV4 prefixes are received from peer where the neighbor was only negotiated for IPv6 unicast address family.

In such case you may want the router to negotiate both address families(IPv4 & IPv6). IPv4 session becomes stable after configuring “family inet unicast”; however, for 'ipv6' session with 'family inet6 unicast' the router throws error

Error in neighbor 2001:1:1:1::2 of group ebgp_v6:
Local IPv4 address must be configured under non-IPv6 NLRI with an IPv6 
error: configuration check-out failed

To make the IPv6 session stable configure “family inet6 unicast local-ipv4-address <local-ip-address>”

Example: 'set protocols bgp group ebgp_v6 neighbor 2001:1:1:1::2 family inet6 unicast local-ipv4-address 10.0.0.1'

Reference - https://www.juniper.net/documentation/us/en/software/junos/bgp/topics/topic-map/multiprotocol-bgp.html#id-advertising-ipv4-routes-over-bgp-ipv6-sessions-overview