docs: add critical note for same-subnet requirement in MetalLB underlay

[changluyi]

VIP and backend Pod IPs must be in the same Underlay subnet, otherwise ARP and DNAT routing will fail.

Summary by CodeRabbit

• Documentation
  • Added a critical warning clarifying that the MetalLB LoadBalancer VIP and backend Pod IPs must reside in the same underlay subnet. If they are on different subnets, ARP-based VIP advertisement and DNAT-based forwarding will not align, causing service traffic to fail and services to become unreachable.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 1bd7e995-28ce-4a86-905d-972bb6e884f3

📥 Commits

Reviewing files that changed from the base of the PR and between 40d5724 and 830e664.

📒 Files selected for processing (1)

• docs/en/configure/networking/how_to/kube_ovn/config_metallb_underlay.mdx

✅ Files skipped from review due to trivial changes (1)

• docs/en/configure/networking/how_to/kube_ovn/config_metallb_underlay.mdx

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Walkthrough

A documentation update adding a critical warning to MetalLB Underlay configuration guidance, stating MetalLB LoadBalancer VIPs and backend Pod IPs must be in the same underlay subnet for ARP advertisement and Kube-OVN DNAT to function.

Changes

Cohort / File(s)	Summary
Documentation - MetalLB Underlay Configuration docs/en/configure/networking/how_to/kube_ovn/config_metallb_underlay.mdx	Inserted a Critical callout requiring the MetalLB LoadBalancer VIP and backend Pod IPs to reside in the same underlay subnet; explains ARP (MetalLB L2) and DNAT (Kube-OVN) interaction and failure mode when cross-subnet.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Possibly related PRs

• config metallb underlay #415 — Adds the MetalLB underlay documentation file; closely related to this insertion.
• add warn for metallb underlay #443 — Edits the same MetalLB underlay doc to add connectivity warnings.
• docs: add MetalLB underlay configuration doc for ACP >= 4.2.2 #541 — Updates the same document with pool naming, IPv4 notes, and migration steps.

Suggested reviewers

• fanzy618
• oilbeater

Poem

🐰 I hopped through docs with nibbling care,
VIPs and Pods must share the same square,
ARP sings loud, DNAT guides the way,
Keep them together — and services will play! 🥕

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title directly and accurately describes the main change: adding a critical note about the same-subnet requirement for MetalLB underlay configuration.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/add-same-subnet-warning

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