T1-T0 router link IP change behavior in NSX

Products

• VMware NSX

Issue/Introduction

• The T1-T0 router link IP is changed.
• If any static routes configured on T0 gateway are pointing to the router link IP of T1 as next hop, the communication will get impacted.
• Below logs are observed in NSX Manager syslog indicating a disconnection occurred between T1 and T0:
  
  2026-08-10T14:52:01.088Z  INFO providerTaskExecutor-1-14 NetworkProviderNsxT 77786 POLICY [nsx@6876 comp="nsx-manager" level="INFO" subcomp="manager"] Connecting Tier1 /infra/tier-1s/#######-####-####-####-######## to Tier0 path null, old Tier0 path /infra/tier-0s/T0-gateway
2026-08-10T14:52:01.088Z  INFO providerTaskExecutor-1-14 NetworkProviderNsxT 77786 POLICY [nsx@6876 comp="nsx-manager" level="INFO" subcomp="manager"] Disconnecting Tier1 /infra/tier-1s/########-####-####-####-######## from Tier0 /infra/tier-0s/T0-gateway

• A PUT call is observed for this disconnection in localhost_access_log file on NSX Manager:
  
  2026-08-10T14:51:57.971Z 127.0.0.1 - "PUT /policy/api/v1/infra/tier-1s/#######-####-####-####-######## HTTP/1.1" 200 - 0 0

• In the nsx-audit.log, payload of the PUT call is observed with Operation="CreateOrReplaceTier1". Old value can be observed with T0 path, New value can be observed without T0 path.

Environment

VMware NSX

Resolution

This is expected behavior.
When a T1 gateway is disconnected from a T0 gateway, the /31 IP pool (100.64.x.x/31) allocated to the transit router link ports is released. Upon reconnecting the T1 to the T0 gateway, a new IP pair from the available 100.64.0.0/16 subnet is automatically reassigned to those link ports.