为什么mpls的ingrees设备去往一条fec会有入方向的标签

好比拓扑是  pc1---R1---R2---R3---R4---PC2，像这种拓扑R1和R4连的都是pc 配置mpls ldp的时候却发现这类边缘pe居然会有入方向的标签，比如r1去往pc2的fec就会分配到null/1027和1027/1027这两种标签 边缘设备还会有入方向的标签？不是只有出方向的标签而已吗？有没有大神解释解释的 

 

 

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