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I have two Raspberry Pies and one DHCP server connected to a switch. Raspi #2 is configured to do netboot.

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I am sitting on Raspi #1 and doing

tcpdump 'ether host b8:27:eb:5d:4a:f6' -p -i eth0 -vv

I am also sitting on DHCP server and doing

tcpdump 'ether host b8:27:eb:5d:4a:f6' -p -i igc2 -vv

eth0 and igc2 are network interface names on respective devices.

When I turn Raspi #2 on, I am seeing it's DHCP requests, but I don't see DHCP server responses when sitting on Raspi #1.

When I sit on DHCP server itself, I see both requests and responses.

Why?

Does my switch filter packets? Why do I see other traffic then?

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1 Answer 1

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In DORA exchange client sends out Discover and Request -packets, server responds with Offer and ACK. Client's (Raspi #2) D and R are sent with destination MAC ff:ff:ff:ff:ff:ff i.e. broadcast MAC. Server's O and A are sent with destination MAC b8:27:eb:5d:4a:f6, Raspi #2 MAC.

Because Discover and Request are broadcast frames, the switch will flood them out of all connected interfaces and Raspi #1 will receive them. It will not receive unicast frames server sends directly to Raspi #2.

If you want to see both ends of the traffic in Raspi #1, you need to mirror either Raspi #2 port or server's port to the port connecting to Raspi #1.

See Wikipedia DHCP page

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