What is the MTU impact on the SDP path-mtu if fast reroute facilities-mode is implemented on the LSP?

When MPLS Fast Reroute (FRR) with facility backup is enabled on an LSP, the Point of Local Repair (PLR) pre-establishes a bypass tunnel to the Merge Point (MP). Upon a primary path failure, the PLR pushes one additional MPLS label onto the packet's label stack. This new label directs the packet through the bypass tunnel. This extra label adds 4 bytes of overhead to the packet size. To prevent fragmentation during a reroute event, the Service Distribution Path (SDP) using this LSP must account for this worst-case scenario. Therefore, the effective path MTU for the service is reduced by 4 bytes.

A. An extra MPLS label is pushed onto the stack during rerouting, which directly impacts the available MTU.
C. The path MTU decreases to accommodate additional overhead; it does not increase.
D. Facility backup adds only one extra label (4 bytes), not two (8 bytes). An 8-byte reduction might be seen in other more complex scenarios, but not standard facility FRR.
E. The path MTU decreases to accommodate additional overhead; it does not increase.

1. Nokia 7450 ESS, 7750 SR, 7950 XRS, and VSR MPLS Guide, Release 21.10.R1: In the "MPLS Fast Reroute (FRR)" chapter, the section on "Facility Backup Method" describes the label stack manipulation. It details that the PLR pushes a new label for the bypass LSP on top of the original label stack. This confirms the addition of a single 4-byte label.

2. Nokia 7750 SR OS Services Guide, Release 21.10.R1: In the "Service Distribution Paths (SDPs)" chapter, the section on "SDP Path MTU" explains how the path MTU is calculated. It states, "When an LSP with FRR is configured on an SDP, the configured path MTU is reduced by 4 bytes to accommodate the extra label." This directly confirms the 4-byte reduction.