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pnp startup-vlan and DHCP clients not getting default gateway

Attempting an initial go at some basic LAN Automation tasks utilising pnp on a C3560CX to do a day 0 build.  Having issues with the switch not getting an default gateway via dhcp when using the 'pnp startup-vlan' on the upstream switch (C3850) the 3560 is being connected to.


Utilising DHCP option 43 and having the 3560 dhcp off vlan 1 (no pnp startup-vlan configured) sees a successful HELLO being sent to DNAC and the device being ready to claim in the Web UI.  The upstream port is configured as a switchport in a VLAN (not 1) with a DHCP Scope configured.  Breaking the PNP Boot process by logging into the switch confirms that a default gateway is operating (via 'sh ip default-gateway')


Changing the upstream port to a trunk and configuring pnp startup-vlan on the upstream switch sees the 3560 boot and create the pnp startup vlan successfully.  The SVI then proceeds to receive a DHCP Address but fails to contact DNAC.  Breaking the PNP process by logging in locally shows no default gateway address configured (again via 'sh ip default-gateway'). The pnp startup-vlan is the same access vlan used in the previous step with successful DHCP so I have a working DHCP Config.


This an issue seen by anyone else.  Still trying to isolate exactly the conditions via some other testing but any pointers would be awesome.

VIP Advocate

What version of DNAC are you running? Can you share your option43 config from DHCP server?  An example is as follows:

5A1N - specifies plug and play
B2 - ipv4 type
I192.X.X.X - connect to dnac ip
J443 - port # to use

Cheers for the reply.


ISE Version

PnP via Option43 Ipv4 (A1D;B2;I10.x.x.x;J80;K4)


This would actually appear to be an issue with the IOS version 15.2(7)E4 on the C3560CX-8PC-S I've been testing with.  Earlier version of the same train (E3,E2 etc) all work without issue.  Full build from automation is successful while using the 'pnp startup-vlan' command on the upstream switch.  I'm going to suggest there's issues generally with PnP and this version of code.   Also seeing a PKI Signing failure with the E4 release which I can't replicate when using PnP on the E3 and E2 releases.

Cheers for the reply.

Interesting.  Out of curiosity what is the A1D in your option43? AFAIK it should be the following for PnP:

  • 5A1N;—Specifies the DHCP suboption for Plug and Play, active operation, version 1, no debug information. It is not necessary to change this part of the string.

Have you attempted to test that IOS with 5A1N?  Just wondering if you would have different test results.

Apologies, I'd pasted from the string shown in the output from the switch as it booted.  It strips the '5' from the start of the string.  Actual string used in Opt43 was "5A1D;B2;I10.x.x.x;J80;K4".  I'd used the D in place of the N as the 4th character to enable debugging (on the assumption that it would give me some more pointers.  Will test in a sec with the amended string of "5A1N;B2;I10.x.x.x;J80;K4" and report  back.  Not holding breath at this point as it would appear the issue has been introduced in the E4 release of 15.2(7).  Had no issues without changing the string on E3 and E2 releases.

Same symptoms when using DHCP and the startup-vlan option.  Switch receives the DHCP address ok but doesn't appear to pull in the default gateway option.  All PnP requests to servers fail (unsurprisingly).


Going to put this one down to E4 at the moment.


As a side item, the 'D' value does provide additional debug details on the switch console when booting.  A much more condensed version of logs are shown with the 'N' set.