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bvanderhaar
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Joined: Wed Jul 21, 2021 8:26 pm

Inconsistent static DHCP with SFP+/DAC

Fri Sep 17, 2021 12:00 am

Clients connected via ethernet and through SFP+ S+RJ10 ports have 100% success properly receiving static DHCP assignments. However, recently I added a client with SFP+ DAC connection, which is not receiving the static DHCP assignment consistently. Upon a single reboot, the client properly receives the correct IP. However, after a subsequent reboot 12 hours later, a new IP from the pool was assigned. Looking at the 'Leases' page in the web interface, the MAC address is the same, yet the DHCP server issues a new IP from the pool, rather than the statically assigned IP. To fix, I delete the previous static assignment and re-assign the new entry to be static.
Hardware/software details: RB4011 with RouterOS v6.48.4 is the DHCP server; CRS328 running SwOS 2.13 is the switch with the SFP+ port connected to a Mellanox X-3 PCIe card. I'm using MikroTik XS* DAC cable. Any help is appreciated - thanks in advance.
 
biomesh
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Re: Inconsistent static DHCP with SFP+/DAC

Fri Sep 17, 2021 4:33 am

Is the client Id the same in the lease? This is really the unique identifier and if you don't have one on the lease, it falls back to the Mac address.

I would just check to see if the device connected is trying to send a different client Id or let's say one with all zeros.
 
bvanderhaar
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Re: Inconsistent static DHCP with SFP+/DAC

Sun Sep 19, 2021 8:35 pm

Is the client Id the same in the lease? This is really the unique identifier and if you don't have one on the lease, it falls back to the Mac address.

I would just check to see if the device connected is trying to send a different client Id or let's say one with all zeros.
Thanks for responding - it is a different client id, so the DHCP server is functioning as designed. How/where is the Client ID generated? Does the client provide this on the initial DHCP broadcast or does the server generate one and provide one on the DHCP response to a DHCP request? I'm looking to fully understand the DHCP protocol, since I incorrectly assumed it was a simple MAC/IP table, looking for a quick tip/more reading to complete my understanding.
 
sindy
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Re: Inconsistent static DHCP with SFP+/DAC

Sun Sep 19, 2021 8:46 pm

The ultimate resource is always the relevant standard, which is the RFC in DHCP case. But the client id value is generated by the client, and it need not necessarily be based on the client's MAC address.
 
bvanderhaar
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Re: Inconsistent static DHCP with SFP+/DAC

Thu Sep 23, 2021 7:22 pm

The ultimate resource is always the relevant standard, which is the RFC in DHCP case. But the client id value is generated by the client, and it need not necessarily be based on the client's MAC address.
Thanks! I appreciate your response with the answer, I recognize it took effort on your part, while I have resources to find my own answer - reading the the RFC and examining the broadcast traffic.

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