Update description for forwarding-viable

release/models/interfaces/openconfig-if-sdn-ext.yang

@@ -47,11 +47,26 @@ module openconfig-if-sdn-ext {
         This is used by an external programming entity to disable an interface
         (usually part of an aggregate) for the purposes of forwarding
         traffic. This allows a logical aggregate to continue to be
-        used with partial capacity, for example.  Note that setting
-        `forwarding-viable = false` is not equivalent to
-        administratively disabling the interface -- in particular, the
-        interface is expected to participate in L2 protocols such as
-        LLDP or LACP even if it blocked from forwarding traffic.";
+        used with partial capacity. Setting `forwarding-viable = false` is not 
+        equivalent to administratively disabling the interface.
+        Some rules to follow when a port/bundle is set for 
+        Forwarding-viable=False:
+          1. Given that L2 protocols like LACP should be allowed to be 
+          transmitted and received on such ports, a device mustn't consider 
+          such ports in the calculation for minimum number of links required 
+          for an aggregate bundle when such a configuration exists on the box.
+          Hence, the device mustn't disable the bundle because the minimum
+          number of member links requirement is not met for the bundle.
+          2. L2 protocols like LLDP, LACP must be allowed for transmit and 
+          receive on such ports/bundles, IS-IS PDUs should be handled as per the 
+          requirements for L3 packets below.
+          3. L3 packets, control-plane as well as data-plane must be received
+          but not transmitted on such ports/bundles. By doing so, expectation
+          is that the other end of the connection ceases to use its respective
+          ports for transmit and receive when either the dead-timer or the
+          hold-down timer of its L3 protocol expires. 
+          4. An implementation should follow rule#3 even when the subject
+          port/bundle is the last resort of communication for the device.";
     }
   }