Update description for forwarding-viable

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

@@ -47,11 +47,34 @@ 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 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.";
     }
   }