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On which router B bit is added in Label?

On which router B bit is added in Label?

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New Member

Which router in MPLS? In

Which router in MPLS? In other words How does label forwarding is doing and how many label are their in initial packet which is generated by PE router.

Cisco Employee

Ashwin,I hope you are


I hope you are referring "Bottom of stack" bit?. If yes, please see below,


When LER (Label Edge Router) receives the IP packet and if the egress is LSP based, it pushes one or more labels. The number of label to be pushed depends on the service and the transport path. For example, when the SP is providing L2/L3 VPN service, the LER will push 2 labels. The bottom label to identify the service and the top label to identify the remote edge router. If there is TE tunnel established along the path and if the TE is not end-to-end, you may see 3 labels pushed. 


When LER pushes the labels, it set the S bit as 1 in the bottom label. This is required so the ASIC can understand if the ethertype is to be set to 0800 (IP packet) or 8847 (MPLS packet).



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