Description of problem:

When arp_proxy is configured at a LSP of router type it may work at a default gw at VMs, with IPv4 configuring a DHCPv4Options with the arp_proxy as gateway is enoguh but with IPv6 the router advertisment should be configured to return the arp_proxy address.

One option would be to add another field at "ipv6_ra_configs" to override the LRP network address, something like "override_ra_src_ip" there one can configure the address, this can go a step further and use the arp_proxy field to discover what to override.

It will need to change src_ip of the RA so the dhcpv6 stateful will work as expected and the client will assign the RA src-ip as default gw.

The RA src-ip is configured here https://github.com/ovn-org/ovn/blob/main/northd/northd.c#L12813

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How reproducible: Always


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