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Routing & Switching · Intermediate

Label Stack

The ordered set of MPLS labels on a packet; routers act only on the top label.

Definition

Stacks enable hierarchical services: an outer transport label to reach the egress PE, an inner VPN or pseudowire label to identify the service, and optional additional labels for TE, FRR or segment routing. The bottom-of-stack bit marks the last label.

Example

Push 16010 (transport) and 24 (VPN) at the ingress PE.

Exam tip

L3VPN typically uses two labels; segment routing can push several.

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