data/templates/frr/isisd.frr.j2 renders segment-routing on whenever the protocols isis segment-routing node exists.

In FRR, segment-routing on enables SR-MPLS only; SRv6 is configured independently by the segment-routing srv6 block. An SRv6-only configuration therefore also turns on SR-MPLS.

isisd then allocates MPLS Adjacency-SIDs from the SRLB (15000+), advertises SRGB/SRLB and Adjacency-SID sub-TLVs, and asks zebra to install the labels. On a router without MPLS label space (net.mpls.platform_labels = 0, the default when MPLS is not configured) every adjacency or interface-flags event, even running tcpdump (promiscuous mode toggle), logs a burst per adjacency.

There is no CLI way to avoid it: a manual no segment-routing on in vtysh is reverted by the next commit.

In FRR's isisd, SR-MPLS is gated by area->srdb.enabled and SRv6 by area->srv6db.config.enabled (Router Capability sub-TLVs, SRv6 Locator TLV, End.X SIDs and zebra SID installation), checked at frr-10.5.2 and frr-10.7.1. SRv6 does not depend on SR-MPLS.

Steps to reproduce

set interfaces dummy dum6
set protocols segment-routing srv6 locator MAIN prefix 2001:db8::/48
set protocols isis net 49.0001.1921.6800.1002.00
set protocols isis interface eth0
set protocols isis segment-routing srv6 locator MAIN
set protocols isis segment-routing srv6 interface dum6
commit

Form an IS-IS adjacency on eth0, then run tcpdump -i eth0 or flap the link.

Expected behavior

SRv6 only: no segment-routing on in router isis, no MPLS labels allocated or installed, no zebra errors.

Actual behavior

show running-config (vtysh) contains segment-routing on; zebra logs, per adjacency:

zebra: Extended Error: Label >= configured maximum in platform_labels
zebra: [EC 4043309093] netlink-dp (NS 0) error: Invalid argument, type=RTM_NEWROUTE(24) ...
zebra: [EC 4043309109] LSP Install Failure: in-label 15001

Lab verification

5 routers, VyOS 1.5 rolling, FRR 10.7.1, IS-IS L2 + SRv6 uSID locators, BGP L3VPN over SRv6.

no segment-routing on via vtysh on one router, then on all five: SRv6 capability, locators, End.X SIDs, seg6local routes, VPN routes with encap seg6 and v4/v6 VRF transit were unchanged. The MPLS table and SR-MPLS LSDB entries dropped to 0 and the LSP install errors stopped. Adjacency loss and re-formation reconverged normally; re-enabling restored SR-MPLS cleanly.