docs(networking): add Kube-OVN underlay multicast guide

[changluyi]

Summary

• add a Kube-OVN Underlay IPv4 multicast configuration guide for ACP 4.2
• document the web console workflow for enabling and disabling multicast snooping and viewing the resulting Subnet configuration
• link the guide from the subnet and Underlay physical network documentation
• clarify the IPv4-only scope and the physical switch, VLAN, IGMP, available IP, and multicast querier prerequisites

Validation

• yarn lint: 0 errors and 0 warnings
• yarn build: passed
• git diff --check: passed

The local build reports that Node.js 20.18.0 is below Rspack recommended 20.19+, but the lint and build commands both completed successfully.

Jira: https://jira.alauda.cn/browse/ACP-54443

[fanzy618]

Review findings

[P1] Require an IPv4 subnet

configure_underlay_multicast.mdx:15-18 describes IPv4 multicast, but the prerequisites only require an Underlay subnet and do not require spec.protocol: IPv4.

Kube-OVN allocates multicast querier addresses according to the subnet protocol. For IPv6/Dual, status.mcastQuerierIP is not an IPv4-only value (Dual can contain both addresses), while the OVN setting uses mcast_ip4_src. Please add an explicit IPv4 protocol prerequisite and clarify that IPv6/Dual subnets are not covered by this guide.

[P2] Add the disable procedure

The PR description says the guide covers enabling, verifying, and disabling multicast snooping, but the final document ends after the enable/verify steps and contains no disable procedure. Please add the web-console or CLI steps, for example:

kubectl patch subnet <subnet-name> --type=merge \\
  -p '{"spec":{"enableMulticastSnoop":false}}'

This is also needed to document releasing the multicast querier IP/MAC.

Fact-check notes

I verified that enableMulticastSnoop, status.mcastQuerierIP, and status.mcastQuerierMAC are present in Kube-OVN, and that enabling creates the querier while disabling releases its IP/MAC. The ACP version: 4.2 mapping should ideally include an ACP 4.2 release/Jira reference because it cannot be established from the upstream Kube-OVN source alone.

[changluyi]

Addressed both review items in commit 5bf4526:

• The prerequisites now explicitly require spec.protocol: IPv4 and state that IPv6 and dual-stack subnets are outside this guides scope.
• Added a concise Disable Multicast Snooping section, including the expected release of the querier IP and MAC address.

Validation completed successfully: yarn lint, yarn build, and git diff --check.