The BIER MPLS labels are downstream-assigned MPLS labels that are unique only to the BFR that advertises them. BIER MPLS labels can be advertised using IGP (IS-IS) extension sub-TLVs or BGP extension sub-TLVs.
Penultimate Hop Popping (PHP) is not supported by BIER-MPLS labels as the labels are used to identify the BIER forwarding table that packets need to be looked up in.
Figure: BIER MPLS encapsulation shows the BIER MPLS encapsulation label.
A BIER MPLS label is bound to the forwarding element class. A BIER label is assigned per BIER <SD, <BSL, SI>>. The SR OS supports only a BSL of 256.
Labels are chosen from the first available label in the label pool, and are only allocated locally when IGP advertises the BIER sub-TLVs.
When a packet arrives on a BFR the BIER forwarding table is identified using the MPLS label. BIER forwarding is then completed using the BIER header.