Determining DHCP Relay Option 82 Ciruit ID - Nexus 9K

[Edward Clear]

So I am setting up DHCP Relay  on some Nexus 9500s (running non-ACI NXOS 9.3(3)) to work with a Windows 2019 DHCP Server.   I've been following the Cisco "Configuring Microsoft Windows Server 2012 to provide DHCP services in an eVPN Scenario" document, but I'm actually putting the relay on standard SVI interfaces.   I had it working when I was dealing with the first VLAN and a single [non-loopback] DHCP Scope.   When I tried to add the second VLAN, I found it was leasing addresses from the wrong scope.  I realized I hadn't set up the policy to match the Circuit ID for each scope.  

That presents the problem of how to determine the Circuit ID for each VLAN or SVI.   In the document it tells how to calculate the DHCP Option 82 Circuit ID from the vn-segment setting.  I don't have that on a standard SVI.    So is there a formula out there on how NXOS maps the SVI or VLAN information to the Circuit ID, or am I stuck with snooping the first DHCP DISCOVER packet for each VLAN to discover the Circuit ID?

[Georg Pauwen]

Hello,

I found the below in the document linked:

--> When the Cisco NX-OS device receives the DHCP request, it adds the Option 82 information in the packet. The Option 82 information contains the device MAC address (the remote ID suboption) and the port identifier ifindex (for non-VXLAN VLANs)

So it looks like for standard SVIs, these two options are needed.

https://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus9000/sw/6-x/security/configuration/guide/b_Cisco_Nexus_9000_Series_NX-OS_Security_Configuration_Guide/b_Cisco_Nexus_9000_Series_NX-OS_Security_Configuration_Guide_chapter_01101.html#conce...

[Edward Clear · Accepted solution]

Thanks for the reply.  That's the first half of the answer.  I actually found the rest a while after posting.  The key being it's the SNMP port index, which you can display with "show interface snmp-ifindex".

https://community.cisco.com/t5/server-networking/snmp-ifindex-nx-os/td-p/1600219

For the use case of working with Microsoft 2012 or 2019 DHCP Server, you set a Scope Policy with the condition that the Relay Agent Circuit ID matches the hexadecimal  (0x) value displayed with "show interface snmp-ifindex".