Disable default route in dpu

[yue-fred-gao]

Description of PR

Summary:
Fixes #21392

Type of change

• Bug fix
• Testbed and Framework(new/improvement)
• New Test case
  • Skipped for non-supported platforms
• Test case improvement

Back port request

• 202205
• 202305
• 202311
• 202405
• 202411
• 202505

Approach

What is the motivation for this PR?

minigraph for smartswitch topo should not have gateway in DHCP_IPV4_SERVER table. f gateway is not present, "router" dhcp option won't be included in the response and client won't install default route. Please see PR sonic-net/sonic-buildimage#21462 for the design.

How did you do it?

Remove gateway option from generated golden config

How did you verify/test it?

Run "./testbed-cli.sh deploy-mg vms-kvm-t1-smartswitch-ha ./lab ./password.txt"
Verify "gateway" option is not in DHCP_IPV4_SERVER
Verified no default route over midplane in DPU by show ip route.

`admin@dpu00:~$ show ip route
Codes: K - kernel route, C - connected, S - static, R - RIP,
O - OSPF, I - IS-IS, B - BGP, E - EIGRP, N - NHRP,
T - Table, v - VNC, V - VNC-Direct, A - Babel, D - SHARP,
F - PBR, f - OpenFabric,
> - selected route, * - FIB route, q - queued route, r - rejected route

S>*0.0.0.0/0 [1/0] via 20.0.200.14, Ethernet0, 17:04:20
C>*20.0.200.0/28 is directly connected, Ethernet0, 17:04:20
K 20.0.200.0/28 [0/0] is directly connected, Ethernet0, 17:04:23
C>*169.254.200.0/24 [0/1024] is directly connected, eth0-midplane, 17:04:27
K 240.127.1.0/24 [0/0] is directly connected, docker0, inactive 17:04:27
admin@dpu00:~$ Connection to 169.254.200.1 closed by remote host.
`

Any platform specific information?

Specific to SmartSwitch

Supported testbed topology if it's a new test case?

Documentation

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[zjswhhh]

lgtm

[zjswhhh]

Hi @bingwang-ms - can you help merge please?

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[congh-nvidia]

Hi @yue-fred-gao , the show dhcp info command is not working without gateway:
https://github.com/sonic-net/sonic-buildimage/blame/master/dockers/docker-dhcp-server/cli/show/plugins/show_dhcp_server.py#L113
Would you mind raise another PR to fix the CLI?

tests.common.errors.RunAnsibleModuleFail: run module shell failed, Ansible Results =>
failed = True
changed = True
rc = 1
cmd = show dhcp_server ipv4 info
start = 2025-12-10 04:25:41.342223
end = 2025-12-10 04:25:41.709332
delta = 0:00:00.367109
msg = non-zero return code
invocation = {'module_args': {'_raw_params': 'show dhcp_server ipv4 info', '_uses_shell': True, 'expand_argument_vars': True, 'stdin_add_newline': True, 'strip_empty_ends': True, 'argv': None, 'chdir': None, 'executable': None, 'creates': None, 'removes': None, 'stdin': None}}
_ansible_no_log = False
stdout =
stderr =
Traceback (most recent call last):
  File "/usr/local/bin/show", line 8, in 
    sys.exit(cli())
             ^^^^^
  File "/usr/local/lib/python3.11/dist-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
         ^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/click/decorators.py", line 64, in new_func
    return ctx.invoke(f, obj, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/show/plugins/dhcp-server.py", line 113, in info
    table.append([interface, entry["mode"], entry["gateway"], entry["netmask"], entry["lease_time"], entry["state"]])
                                            ~~~~~^^^^^^^^^^^
KeyError: 'gateway'

[yue-fred-gao]

Hi @yue-fred-gao , the show dhcp info command is not working without gateway: https://github.com/sonic-net/sonic-buildimage/blame/master/dockers/docker-dhcp-server/cli/show/plugins/show_dhcp_server.py#L113 Would you mind raise another PR to fix the CLI?

tests.common.errors.RunAnsibleModuleFail: run module shell failed, Ansible Results =>
failed = True
changed = True
rc = 1
cmd = show dhcp_server ipv4 info
start = 2025-12-10 04:25:41.342223
end = 2025-12-10 04:25:41.709332
delta = 0:00:00.367109
msg = non-zero return code
invocation = {'module_args': {'_raw_params': 'show dhcp_server ipv4 info', '_uses_shell': True, 'expand_argument_vars': True, 'stdin_add_newline': True, 'strip_empty_ends': True, 'argv': None, 'chdir': None, 'executable': None, 'creates': None, 'removes': None, 'stdin': None}}
_ansible_no_log = False
stdout =
stderr =
Traceback (most recent call last):
  File "/usr/local/bin/show", line 8, in 
    sys.exit(cli())
             ^^^^^
  File "/usr/local/lib/python3.11/dist-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
         ^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/click/decorators.py", line 64, in new_func
    return ctx.invoke(f, obj, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/show/plugins/dhcp-server.py", line 113, in info
    table.append([interface, entry["mode"], entry["gateway"], entry["netmask"], entry["lease_time"], entry["state"]])
                                            ~~~~~^^^^^^^^^^^
KeyError: 'gateway'

Thanks for pointing this out. I have raised PR sonic-net/sonic-buildimage#24801. Please take a look.

[congh-nvidia]

Hi @yue-fred-gao , I have find another issue caused by this PR. Since the default route is removed, we are not able to ssh to the DPU from the switch, because there is no route to the NPU mgmt IP:

admin@sonic:~$ sudo tcpdump -i eth0-midplane tcp dst port 22 -vvv
[ 4334.346618] device eth0-midplane entered promiscuous mode
tcpdump: listening on eth0-midplane, link-type EN10MB (Ethernet), snapshot length 262144 bytes
06:06:03.992403 IP (tos 0x0, ttl 62, id 22881, offset 0, flags [DF], proto TCP (6), length 60)
10.210.24.178.37242 > 169.254.200.1.ssh: Flags [S], cksum 0x8334 (correct), seq 1884908377, win 64240, options

admin@r-SN4280-10:~$ show ip interface | grep eth0
eth0 10.210.24.178/22 up/up N/A N/A

May I ask where do you configure the route to the NPU mgmt IP?

[yue-fred-gao]

From the switch, you should have a subnet route 169.254.200.0/24 over eth0-midplane. So when you ssh to a DPU, eth0-midplane should be picked as the outgoing interface and its ip address (165.254.200.254) should be used as source IP, not the NPU mgmt IP. I think you can check in your setup why NPU mgmt IP is used instead.

[mssonicbld]

Cherry-pick PR to 202511: #22413