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

Co-authored-by: Darren Loher <dloher@google.com>

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

@@ -47,34 +47,42 @@ 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. Setting `forwarding-viable = false` is not 
+        used with partial capacity. Setting `forwarding-viable = false` is not
         equivalent to administratively disabling the interface.
-        Some rules to follow when an interface or aggregate interface is set for 
+        Some rules to follow when an interface or aggregate interface is set for
         Forwarding-viable=False:
-          1. When forwarding-viable is FALSE `/interfaces/interface/state/oper-status` 
-          should reflect the L2 status, which when UP should be treated as such for 
-          aggregate interface `min-links` checks.
-
-          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. For L3 packets, control-plane as well as data-plane packets must be 
-          received but not transmitted on such ports/bundles. Since the port 
-          is receiving L3 packets, inflight data-plane traffic will continue to 
-          be delivered to its destination post FIB lookup. Also, received 
-          control-plane traffic must be handled normally. Therefore, when all 
-          ports in a bundle are set to FV=False, it is possible that the 
-          dead-interval or Hold-down timer of L3 protocols like IS-IS/BGP on the 
-          peer router may expire taking down the adjacency or peering on that 
-          connection. However, the peer may still continue to use the bundle for 
-          forwarding traffic towards the subject router if the peer's forwarding 
-          table is programmed by an external programming entity. In this scenario, 
-          the subject router will continue to recieve traffic indefinitely on 
-          such a bundle.
-
-          4. An implementation should follow rule #3 even when the subject
-          interface is the last resort of communication for the device.";
+          1. Aggregate interface '/interfaces/interface/aggregation/state/min-links'
+           checks should be evaluated based on
+          `/interfaces/interface/state/oper-status`.  'min-links' should not be
+          affected by the use of forwarding viable.
+
+          2. L2 protocols like LLDP and LACP must be processed normally on
+          transmit and receive on such ports/bundles.  IS-IS PDUs should be
+          handled as per the requirements for L3 packets below.
+
+          3. L3 packets must not be transmitted on the interface.  
+          
+          4. Received L3 packets must be processed normally.  Received data-plane
+          traffic will continue to forwarded to its destination post FIB lookup.
+          Received control-plane traffic must also be processed normally. 
+          
+          5. It is possible that the dead-interval or hold-down timer of L3
+          protocols like IS-IS/BGP on the peer router may expire taking down the
+          adjacency or peering on that connection. However, the peer may still
+          continue to transmit packets which are received by the local device.  
+          These received packet should continue to be processed normally as
+          per rule #4 above.
+
+          For example, if the peer's forwarding table is programmed using gRIBI
+          by an external controller, the local device will continue to receive
+          packets.
+
+          6. An implementation should follow rule #3 even when the subject
+          interface on the local device is the last resort of communication for a
+          given destination.  For example, the only nexthop for a destination is
+          an aggregate interface which has all member interfaces set to
+          forwarding-viable = false.  In this scenario all L3 packets for that
+          destination will be dropped.";
     }
   }