[dhcp_server] Fix the issue with "kea-dhcp4.conf" file generation for Smart Switch.

[oleksandrivantsiv]

Why I did it

The configuration generated from the template for the Smart Switch contained incorrect data in the "subnet4:id" field. For regular cases, the subnet ID is deduced from the VLAN name. For the Smart Switch, there is always one subnet, and the ID is set to 0.

"subnet4": [
        {
            "id": bridge-midplane,
            "subnet": "169.254.200.0/24",
            "pools": [
                {
                    "pool": "169.254.200.1 - 169.254.200.1",
                    "client-class": "sonic:dpu0"
                },
...

Work item tracking

• Microsoft ADO (number only):

How I did it

Fix configuration generation for Smart Switch device.

How to verify it

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111
• 202205
• 202211
• 202305

Tested branch (Please provide the tested image version)

Description for the changelog

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)

[yaqiangz]

Thanks for fix!

[yaqiangz]

Hold on...
This change for setting subnet id is for support lease update (From kea-lease file to state_db)
dhcp_lease.py#L123
If this part doesn't corresponding support, lease information will be incorrect for smart swith..
Example:
In state_db, key would be stored like "DHCP_SERVER_IPV4_LEASE|Vlan0|aa:bb:cc:dd:ee:ff"
Could you add correspond support in it?
We can check add flag indicate whether it's smart switch via is_smart_switch in KeaDhcp4LeaseHandler-init and construct redis key by the flag in here dhcp_lease.py#L123

(Just my suggestion, if you have any other insights, please feel free to point out)

[oleksandrivantsiv]

@yaqiangz I updated the implementation. Please check.

[yaqiangz]

LGTM

[yaqiangz]

@yxieca Could you please help to merge this PR?

[oleksandrivantsiv]

@yxieca kindly reminder. Can you please merge the PR?

[yaqiangz]

Hi @yxieca could you please help to merge this PR?

[yaqiangz]

Hi @yxieca could you please help to merge this PR?

[yaqiangz]

Hi @bingwang-ms, could you please help to approve the backport request?

[mssonicbld]

Cherry-pick PR to 202405: #19519