[linksync] Netdev oper status determination using IFF_RUNNING (#1568)

The objective of this change is to make orchagent/linksync align with FRR/Zebra in handling interface states. Zebra uses IFF_RUNNING flag to determine interfaces' oper state while linksync uses IFF_LOWER_UP. Zebra rightly depends on IFF_RUNNING flag. As a routing daemon, zebra wants to know whether the interface is capable of passing packets or not. The flag IFF_RUNNING indicates exactly that (based on comment in if.h, this flag reflects the interface oper up state as defined in RFC2863). Since orchagent uses IFF_LOWER_UP, which comes earlier than IFF_RUNNING, there is interface state mismatch between zebra and orchagent for a window of time. Since with voq implementation we write static neigh/routes when the netdev (inband port) becomes oper up and bgp depends on these kernel entries, there is a need for this change to have the interface state timing same in both orchagent and zebra.

Refer issue sonic-net/sonic-buildimage#6295 for detailed information.

Signed-off-by: vedganes <vedavinayagam.ganesan@nokia.com>

portsyncd/linksync.cpp

@@ -174,7 +174,7 @@ void LinkSync::onMsg(int nlmsg_type, struct nl_object *obj)
 
     unsigned int flags = rtnl_link_get_flags(link);
     bool admin = flags & IFF_UP;
-    bool oper = flags & IFF_LOWER_UP;
+    bool oper = flags & IFF_RUNNING;
 
     char addrStr[MAX_ADDR_SIZE+1] = {0};
     nl_addr2str(rtnl_link_get_addr(link), addrStr, MAX_ADDR_SIZE);