zebra: When processing route_entries ignore unusable routes

When zebra is processing routes to determine what to send
to the rib, suppose we have two routes (a) a route processed
earlier that none of it's nexthops were active and (b)
a route that has good nexthops but has a worse admin distance.

rib_process, would not relook at (a)'s nexthops because
the ROUTE_ENTRY_CHANGED flag was not true and it would
win when compared to (b) because it's admin distance
was better, leaving us with a state where we would
attempt and fail to install route (a) because it
was not valid.

Modify the code to consider the number of nexthops
we have as a determiner if we can use the route.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>

zebra/zebra_rib.c

@@ -1098,38 +1098,38 @@ static void rib_process(struct route_node *rn)
 		if (CHECK_FLAG(re->status, ROUTE_ENTRY_REMOVED))
 			continue;
 
-		/* Skip unreachable nexthop. */
+		/*
-		/* This first call to nexthop_active_update is merely to
+		 * If the route entry has changed, verify/resolve
-		 * determine if there's any change to nexthops associated
+		 * the nexthops associated with the entry.
-		 * with this RIB entry. Now, rib_process() can be invoked due
+		 *
-		 * to an external event such as link down or due to
+		 * In any event if we have nexthops that are not active
-		 * next-hop-tracking evaluation. In the latter case,
+		 * then we cannot use this particular route entry so
-		 * a decision has already been made that the NHs have changed.
+		 * skip it.
-		 * So, no need to invoke a potentially expensive call again.
-		 * Further, since the change might be in a recursive NH which
-		 * is not caught in the nexthop_active_update() code. Thus, we
-		 * might miss changes to recursive NHs.
 		 */
-		if (CHECK_FLAG(re->status, ROUTE_ENTRY_CHANGED)
+		if (CHECK_FLAG(re->status, ROUTE_ENTRY_CHANGED)) {
-		    && !nexthop_active_update(rn, re)) {
+			if (!nexthop_active_update(rn, re)) {
 				if (re->type == ZEBRA_ROUTE_TABLE) {
 					/* XXX: HERE BE DRAGONS!!!!!
-				 * In all honesty, I have not yet figured out
+					 * In all honesty, I have not yet
-				 * what this part does or why the
+					 * figured out what this part does or
-				 * ROUTE_ENTRY_CHANGED test above is correct
+					 * why the ROUTE_ENTRY_CHANGED test
-				 * or why we need to delete a route here, and
+					 * above is correct or why we need to
-				 * also not whether this concerns both selected
+					 * delete a route here, and also not
+					 * whether this concerns both selected
 					 * and fib route, or only selected
 					 * or only fib
 					 *
-				 * This entry was denied by the 'ip protocol
+					 * This entry was denied by the 'ip
-				 * table' route-map, we need to delete it */
+					 * protocol
+					 * table' route-map, we need to delete
+					 * it */
 					if (re != old_selected) {
 						if (IS_ZEBRA_DEBUG_RIB)
 							zlog_debug(
 								"%s: %s(%u):%s: imported via import-table but denied by the ip protocol table route-map",
 								__func__,
-							VRF_LOGNAME(vrf),
+								VRF_LOGNAME(
+									vrf),
 								vrf_id, buf);
 						rib_unlink(rn, re);
 					} else
@@ -1139,6 +1139,16 @@ static void rib_process(struct route_node *rn)
 
 				continue;
 			}
+		} else {
+			/*
+			 * If the re has not changed and the nhg we have is
+			 * not usable, then we cannot use this route entry
+			 * for consideration, as that the route will just
+			 * not install if it is selected.
+			 */
+			if (!nexthop_group_active_nexthop_num(&re->nhe->nhg))
+				continue;
+		}
 
 		/* Infinite distance. */
 		if (re->distance == DISTANCE_INFINITY &&