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Cannot get DHCP lease through connection through switch sending spanning tree protocol BPDUs

stevebrandlistevebrandli Posts: 1 Newcomer
in Connectivity Nov 24, 2022

This is not really a question. I am posting this for anyone else that has this problem.

It appears that Spectrum will not respond to DHCP Discover packets after having received a BPDU (part of the spanning tree protocol). This is a protocol used by switches to avoid loops in switch topology. Managed switches have this turned on by default.

Situation: I have two managed switches in my network. I connect the Spectrum modem to one of the switches (a Cisco SG250-26) and my router to the same switch. The modem and the router are in their own VLAN. This has worked for more than a year.

The other day, this stopped working. (I am not entirely sure what changed although I have some theories.) A packet sniffer confirmed that my router was sending out DHCP Discover packets and that Spectrum was not responding. If I plugged the router directly into the modem, DHCP worked. Plugging my Linux and Windows machines in place of the router (both on and off the switch) gave me the same results.

I noticed that the switch was emitting BPDUs. Spanning tree was disabled on my switch. However, the switch has a feature that, if a BPDU is received on port on which spanning tree is disabled, it will by default transmit that BPDU on all of the other ports. This is called BPDU Flooding. When I changed the switch configuration to BPDU Filtering, which disables this reflection feature, my router was able to use DHCP through the switch. My theory is that my new second switch was sending BPDUs whereas the switch it replaced (an unmanaged switch) did not. My first switch was retransmitting those BPDUs.

An important points: I verified this with two different Spectrum modems. Both modems run a DHCP server when they are not connected to Spectrum. The leases it serves are very short term--a few seconds. Once the modem connects, it the modem stops responding to DHCP requests, causing the router to do another DHCP Discover, which goes upstream to a Spectrum server. My router was able to obtain a DHCP lease from the modem, but as soon as the modem connected, it could not obtain the lease upstream from Spectrum. I suspect that the modem is transmitting the BPDUs upstream and that this is causing the Spectrum equipment to not respond to the DHCP Discover packets. (It appears that, once I had an IP address, BPDUs did not cause any problems. So the problem may depend on timing.)

Spectrum would not help me diagnose because they claimed it was my problem since direct connection with the modem worked. However, I believe this is Spectrum's problem. Their equipment should ignore the BPDUs. I did see one mention of this problem on the internet, which leads me to believe others have had this problem.

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