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Routing & Switching · Intermediate

MPLS Label

A 32-bit shim header containing a 20-bit label value, 3-bit Traffic Class (EXP), 1-bit bottom-of-stack flag and 8-bit TTL.

Definition

Labels 0-15 are reserved: 0 IPv4 explicit null, 1 router alert, 2 IPv6 explicit null, 3 implicit null (signals PHP and never appears in packets), 13 GAL. Multiple labels form a label stack, with the S bit set on the bottom label. Each label adds 4 bytes, so MTU must be increased in the core.

Example

An L3VPN packet carries a transport label on top and a VPN label at the bottom.

Exam tip

Label 3 (implicit null) is advertised but never seen in a packet; label 0 (explicit null) is carried to preserve EXP.

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