Nexus 9K dhcp and arp issue


I’ve got ARP issues with our Nexus 9k.

I’ve got a couple of fiber Huawei OLT L2 connected to the Nexus 9k. Behind OLT we have got around 5000k ONT/ONU + tenda/tp-link home routers.

Randomly around some routers start to re-request IP
On the DHCP server log, I see continuous DHCP ACK for these routers with same IP and same IP address.

Note that I’m using ISC DHCP with Linux Debian 11.

The only way I’ve found to fix is clearing ip arp cache on the Nexus.
Any idea of what occurs here?


Hello Sebastien

Based on your description, it seems like there is indeed an ARP issue in your network that is causing some routers to continuously re-request IP addresses. Clearing the ARP cache on the Nexus 9k, as you suggest, temporarily resolves the issue, but of course, it’s not a permanent solution.

There are a few things that come to mind that may help you zero in on the problem during your troubleshooting:

  1. Examine the Nexus device to see if you’re getting an ARP table overflow. If this is the case you may want to consider increasing the ARP table size or implementing ARP suppression.
  2. If for some reason duplicate IP addresses are being assigned, this can cause ARP instability as well as continuous DHCP ACKs.
  3. If you have flapping interfaces on the Nexus or the Huawei OLTs, you may see ARP entries being learned and removed continuously
  4. Check the ARP aging timers. These are by default on Cisco devices set to 4 hours. Check them on the OLT as well to ensure that they are not too aggressive causing many ARP requests…
  5. DHCP server config - check things like lease times, overlapping scopes, or DHCP conflicts.

I hope this is a good start to get you looking at some possible troubleshooting approaches. Let us know how you get along!

I hope this has been helpful!