Cisco NX-OS: Layer3-to-Layer2 gNMI update resets unchanged interface MTU to 1500

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难度
4/5
预计耗时
3-5 天
新手友好度
48/100
Issue 类型
缺陷
描述清晰度
基本清楚
活跃度
活跃
技术栈
go, kubernetes
领域
backend, networking

调研方向

Start by reading the discussion linked from the issue, then trace how the operator builds and sends gNMI updates for physical interface layer and MTU changes. Look for the relevant reconciliation or update tests and reproduce the Layer3-to-Layer2 case with an unchanged MTU. Done means the requested MTU remains 9216 after the transition and the regression is covered by a test.

由索引模型根据 Issue 内容生成。

描述

area/switch-automation firmware-bug platform/nx vendor/cisco

Problem Statement

On Cisco NX-OS, changing a physical interface from Layer 3 to Layer 2 through a gNMI Update resets its MTU to 1500, even when the request explicitly keeps the MTU unchanged!

This was reproduced on NX-OS 10.3(9) using an N9Kv. The issue is specific to the Layer3-to-Layer2 transition with the same MTU value. Layer2-to-Layer3 transitions preserve the MTU, and changing the MTU while changing the layer also preserves the requested MTU.

This breaks the operator's interface reconciliation. For example, when a routed member with MTU 9216 joins a Layer 2 port-channel, the operator sends layer: Layer2 and mtu: 9216. NX-OS leaves the member at MTU 1500, causing the port-channel configuration to fail:

Port-channel MTU 9216 doesn't match the interface MTU 1500

Reproduction

Configure the physical interface as Layer 3 with a jumbo MTU:

gnmic -a <device-address> --port <gnmi-port> -u <username> -p <password> \
  --skip-verify --encoding JSON set \
  --update-path 'System/intf-items/phys-items/PhysIf-list[id=eth1/1]' \
  --update-value '{"layer":"Layer3","mtu":9216}'

Change it to Layer 2 while preserving the same MTU:

gnmic -a <device-address> --port <gnmi-port> -u <username> -p <password> \
  --skip-verify --encoding JSON set \
  --update-path 'System/intf-items/phys-items/PhysIf-list[id=eth1/1]' \
  --update-value '{"layer":"Layer2","mtu":9216}'

Read back the configured MTU:

gnmic -a <device-address> --port <gnmi-port> -u <username> -p <password> \
  --skip-verify --encoding JSON get \
  --path 'System/intf-items/phys-items/PhysIf-list[id=eth1/1]/mtu' \
  --type config --values-only

Actual result:

[
  1500
]

Expected Behavior

The configured MTU must remain 9216, because it was included unchanged in the Layer3-to-Layer2 gNMI update payload as part of the request.

Observed Scope

  • Affected: Layer3 → Layer2 with an unchanged MTU (9216 → 9216)
  • Not affected in testing: Layer2 → Layer3 with an unchanged MTU
  • Not affected in testing: Layer3 → Layer2 while changing the MTU (for example, 9000 → 9216)

Impact / Workaround

The operator must issue an additional patch after changing the interface layer to restore the requested MTU. Without it, interfaces intended to join a jumbo Layer 2 port-channel remain at MTU 1500 and cannot be added successfully.

Related Discussion

https://github.com/ironcore-dev/network-operator/pull/667#discussion_r4452687541

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