在tag route下， dubbo2.6作为client调用dubbo2.7 server，路由规则已经起效果； dubbo2.7作为client调用dubbo2.6路由规则没有生效。（在dubbo2.7，ops没有配置路由规则的前提下，只通过应用url的tag打标）。
发现问题代码, org.apache.dubbo.rpc.cluster.router.tag.TagRouter：
```
if (CollectionUtils.isEmpty(invokers)) {
            return invokers;
        }

        if (tagRouterRule == null || !tagRouterRule.isValid() || !tagRouterRule.isEnabled()) {
            return invokers;
        }
```
没在ops里配置路由规则的时候，需要继续往下走。

----transplate here 

Under tag route, dubbo2.6 calls dubbo2.7 server as client, routing rules have been effective; dubbo2.7 as client calls dubbo2.6 routing rules have not taken effect. (In dubbo2.7, ops is not configured with routing rules, only by applying the url tag).
Found the problem code, org.apache.dubbo.rpc.cluster.router.tag.TagRouter:
```
if (CollectionUtils.isEmpty(invokers)) {
            return invokers;
        }

        if (tagRouterRule == null || !tagRouterRule.isValid() || !tagRouterRule.isEnabled()) {
            return invokers;
        }
```
When you do not configure routing rules in ops, you need to continue down.
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 * limitations under the License.
 */
package org.apache.dubbo.rpc.cluster.router.tag;
import org.apache.dubbo.common.Constants;
import org.apache.dubbo.common.URL;
import org.apache.dubbo.common.logger.Logger;
import org.apache.dubbo.common.logger.LoggerFactory;
import org.apache.dubbo.common.utils.CollectionUtils;
import org.apache.dubbo.common.utils.StringUtils;
import org.apache.dubbo.configcenter.ConfigChangeEvent;
import org.apache.dubbo.configcenter.ConfigChangeType;
import org.apache.dubbo.configcenter.ConfigurationListener;
import org.apache.dubbo.configcenter.DynamicConfiguration;
import org.apache.dubbo.rpc.Invocation;
import org.apache.dubbo.rpc.Invoker;
import org.apache.dubbo.rpc.RpcException;
import org.apache.dubbo.rpc.cluster.Router;
import org.apache.dubbo.rpc.cluster.router.AbstractRouter;
import org.apache.dubbo.rpc.cluster.router.tag.model.TagRouterRule;
import org.apache.dubbo.rpc.cluster.router.tag.model.TagRuleParser;
import java.util.List;
import java.util.function.Predicate;
import java.util.stream.Collectors;
import static org.apache.dubbo.common.Constants.FORCE_USE_TAG;
import static org.apache.dubbo.common.Constants.TAG_KEY;
/**
 * TagRouter, "application.tag-router"
 */
public class TagRouter extends AbstractRouter implements Comparable<Router>, ConfigurationListener {