From c33e993f187f20407f2258ca08e0447c78aba885 Mon Sep 17 00:00:00 2001
From: Jonathan Voss <jvoss@onvox.net>
Date: Sun, 18 Oct 2026 15:09:02 +0000
Subject: isis: T9485: only enable SR-MPLS when MPLS segment-routing options
 are set

"segment-routing on" was rendered whenever the "protocols isis
segment-routing" node existed. In FRR that statement enables SR-MPLS only,
so an SRv6-only configuration also turned on SR-MPLS: isisd allocated
adjacency labels and zebra logged LSP install failures on systems without
MPLS label space.

"segment-routing on" is now rendered only if global-block, local-block,
prefix or maximum-label-depth is configured. SRv6-only configurations no
longer enable SR-MPLS; SRv6 rendering is unchanged.
---
 data/templates/frr/isisd.frr.j2              |  4 +++
 smoketest/scripts/cli/test_protocols_isis.py | 43 ++++++++++++++++++++++++++++
 2 files changed, 47 insertions(+)

diff --git a/data/templates/frr/isisd.frr.j2 b/data/templates/frr/isisd.frr.j2
index b385e896b..f94085d33 100644
--- a/data/templates/frr/isisd.frr.j2
+++ b/data/templates/frr/isisd.frr.j2
@@ -202,7 +202,11 @@ advertise-passive-only
 {%             endif %}
 {%         endfor %}
 {%     endif %}
+{#     "segment-routing on" enables SR-MPLS only, SRv6 does not need it #}
+{%     set sr_mpls_options = ['global_block', 'local_block', 'maximum_label_depth', 'prefix'] %}
+{%     if segment_routing | select('in', sr_mpls_options) | list %}
  segment-routing on
+{%     endif %}
 {% endif %}
 {% if spf_delay_ietf.init_delay is vyos_defined %}
  spf-delay-ietf init-delay {{ spf_delay_ietf.init_delay }} short-delay {{ spf_delay_ietf.short_delay }} long-delay {{ spf_delay_ietf.long_delay }} holddown {{ spf_delay_ietf.holddown }} time-to-learn {{ spf_delay_ietf.time_to_learn }}
diff --git a/smoketest/scripts/cli/test_protocols_isis.py b/smoketest/scripts/cli/test_protocols_isis.py
index 8296e2732..d190c23d4 100755
--- a/smoketest/scripts/cli/test_protocols_isis.py
+++ b/smoketest/scripts/cli/test_protocols_isis.py
@@ -462,6 +462,8 @@ class TestProtocolsISIS(VyOSUnitTestSHIM.TestCase):
         self.assertIn(f' net {net}', tmp)
         self.assertIn(f' segment-routing srv6', tmp)
         self.assertIn(f'  locator {locator}', tmp)
+        # SRv6 alone must not enable SR-MPLS
+        self.assertNotIn(' segment-routing on', tmp)
 
         # Commit for isis
         self.cli_commit()
@@ -530,6 +532,8 @@ class TestProtocolsISIS(VyOSUnitTestSHIM.TestCase):
         self.assertIn(' segment-routing srv6', tmp)
         self.assertIn(f'  locator {locator}', tmp)
         self.assertIn(f'  interface {dum_iface}', tmp)
+        # SRv6 alone must not enable SR-MPLS
+        self.assertNotIn(' segment-routing on', tmp)
 
         # Test node-msd configuration
         self.cli_set(srv6_base_path + ['node-msd', 'max-end-d', '40'])
@@ -544,6 +548,7 @@ class TestProtocolsISIS(VyOSUnitTestSHIM.TestCase):
         self.assertIn('   max-end-pop 50', tmp)
         self.assertIn('   max-h-encaps 60', tmp)
         self.assertIn('   max-segs-left 70', tmp)
+        self.assertNotIn(' segment-routing on', tmp)
 
     def test_isis_15_distribute_link_state(self):
         interface = 'lo'
@@ -603,6 +608,44 @@ class TestProtocolsISIS(VyOSUnitTestSHIM.TestCase):
         self.assertIn(f' mpls-te router-address {te_address}', tmp)
         self.assertIn(' mpls-te export', tmp)
 
+    def test_isis_16_segment_routing_srv6_and_mpls(self):
+        locator = 'TEST'
+        msd = '5'
+        sr_base = ['protocols', 'segment-routing']
+        self.cli_set(sr_base + ['srv6', 'locator', locator, 'prefix', '2001:db8::/64'])
+        self.cli_set(sr_base + ['interface', 'lo'])
+
+        # The dummy interface used to install SRv6 SIDs in the Linux data plane
+        dum_iface = 'dum6'
+        self.cli_set(['interfaces', 'dummy', dum_iface])
+
+        self.cli_set(base_path + ['net', net])
+        self.cli_set(base_path + ['interface', 'lo'])
+
+        # SRv6 and SR-MPLS side by side
+        sr_path = base_path + ['segment-routing']
+        self.cli_set(sr_path + ['srv6', 'locator', locator])
+        self.cli_set(sr_path + ['srv6', 'interface', dum_iface])
+        self.cli_set(sr_path + ['maximum-label-depth', msd])
+        self.cli_commit()
+
+        tmp = self.getFRRconfig(f'router isis {domain}', stop_section='^exit')
+        self.assertIn(' segment-routing on', tmp)
+        self.assertIn(f' segment-routing node-msd {msd}', tmp)
+        self.assertIn(' segment-routing srv6', tmp)
+        self.assertIn(f'  locator {locator}', tmp)
+        self.assertIn(f'  interface {dum_iface}', tmp)
+
+        # Removing the only SR-MPLS option must disable SR-MPLS again. Not
+        # global-block: FRR rejects the "no" form frr-reload.py derives for it
+        self.cli_delete(sr_path + ['maximum-label-depth'])
+        self.cli_commit()
+
+        tmp = self.getFRRconfig(f'router isis {domain}', stop_section='^exit')
+        self.assertNotIn(' segment-routing on', tmp)
+        self.assertNotIn(' segment-routing node-msd', tmp)
+        self.assertIn(' segment-routing srv6', tmp)
+
 
 if __name__ == '__main__':
     unittest.main(verbosity=2, failfast=VyOSUnitTestSHIM.TestCase.debug_on())
-- 
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