A FEC for each of the IPv4 and IPv6 system interface addresses is advertised and resolved automatically by the LDP peers when the LDP session comes up, regardless of whether the session is IPv4 or IPv6.
To avoid the automatic advertisement and resolution of IPv6 system FEC when the LDP session is IPv4, the following procedure must be followed before and after the upgrade to the SR OS version which introduces support of LDP IPv6.
Before the upgrade, implement a global prefix policy which rejects prefix [::0/0 longer] to prevent IPv6 FECs from being installed after the upgrade.
In MISSU case:
If new IPv4 sessions are created on the node, the per-peer FEC-capabilities must be configured to filter out IPv6 FECs.
Until an existing IPv4 session is flapped, FEC-capabilities have no effect on filtering out IPv6 FECs. The import global policy must remain configured in place until the session flaps. Alternatively, a per-peer-import-policy [::0/0 longer] can be associated with this peer.
In cold upgrade case:
If new IPv4 sessions are created on the node, the per-peer FEC-capabilities must be configured to filter out IPv6 FECs.
On older, pre-existing IPv4 sessions, the per-peer FEC-capabilities must be configured to filter out IPv6 FECs.
When all LDP IPv4 sessions have dynamic capabilities enabled, with per-peer FEC-capabilities for IPv6 FECs disabled, then the GLOBAL IMPORT policy can be removed.