Managed switch filters unicast DHCPOFFERs, but allows broadcast DHCPOFFERs?

rikinet

1/5/24, 12:10 AM

I have a network containing N managed switches. Each of them is configured as a DHCP server and should assign predefined IP addresses in a port-based manner:

Switch 1 assigns 172.17.1.1 to the device at its port 1,
Switch 2 assigns 172.17.2.1 to the device at its port 1,
...
Switch N assigns 172.17.N.1 to the device at its port 1.

The devices attached to the ports 1 are configured to try PXE booting.

When DHCP'ing in the PXE-ROM stage, they broadcast DHCPDICSOVERs with the "broadcast flag" set, which means they expect the DHCP server(s) to reply with layer-2 broadcasts (to ff:ff:ff:ff:ff:ff).
When DHCP'ing in the OS stage, they broadcast DHCPDICSOVERs with the broadcast flag not set, which means they expect the DHCP server(s) to reply with layer-2 unicasts (to the client’s MAC).

What I observe is:

After the DHCPDISCOVER from the PXE-ROM stage, the client receives broadcast DHCPOFFERs from each of the N switches. Interestingly, none of them is an offer for the designated IP for a port 1 (172.17.N.1) - they are all random IPs from the configured pool range.
After the DHCPDISCOVER from the OS stage, the client receives only a single unicast DHCPOFFER, only from the switch it is directly connected to. Interestingly, this is an offer for the designated IP (172.17.<local>.1).

To me, that means that the unicast DCHPOFFERs from all the other switches must have been filtered out on the way.

Why would a switch do that? Is this a typical configuration option?

I have no information about or access to the switches’ configuration, capabilities etc., but I was told about some "DHCP deny" option – maybe that’s a hint.

0 + 0

dhcp

switch

ip-address

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: Managed switch filters unicast DHCPOFFERs, but allows broadcast DHCPOFFERs?

Post an answer

Most people don’t grasp that asking a lot of questions unlocks learning and improves interpersonal bonding. In Alison’s studies, for example, though people could accurately recall how many questions had been asked in their conversations, they didn’t intuit the link between questions and liking. Across four studies, in which participants were engaged in conversations themselves or read transcripts of others’ conversations, people tended not to realize that question asking would influence—or had influenced—the level of amity between the conversationalists.