202012 version of #10119
Why I did it
If the VLAN interface is not up when ndppd starts, it will fail to enable allmulti mode on the interface and be unable to process received NDP packets
The following logs are seen:
/var/log/syslog.33.gz:Feb 18 10:33:12.825406 sonic INFO swss#/supervisord: ndppd (error) Failed to set allmulti: No such device
How I did it
Use the wait_for_link script currently used by radv to delay ndppd startup until the vlan interface is ready
How to verify it
Apply the changes to a device. config reload the device and confirm that the above error logs are not observed when ndppd starts. Run the arp/test_arp_dualtor.py::test_proxy_arp test case and verify it passes.
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