為什麼mpls的ingrees設備去往一條fec會有入方向的標簽

好比拓撲是  pc1---R1---R2---R3---R4---PC2，像這種拓撲R1和R4連的都是pc 配置mpls ldp的時候卻發現這類邊緣pe居然會有入方向的標簽，比如r1去往pc2的fec就會分配到null/1027和1027/1027這兩種標簽 邊緣設備還會有入方向的標簽？不是隻有出方向的標簽而已嗎？有沒有大神解釋解釋的 

 

 

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