topic Re: DHCP Fwd-Path to Broadcast IP not working but unicast does in ExtremeSwitching (VSP/Fabric Engine)

DHCP Fwd-Path to Broadcast IP not working but unicast does

RobertD1 — Wed, 05 Apr 2023 15:43:22 GMT

Hello,

Moved the VLAN IP to a new Fabric Engine with 8.5.2 and DHCP Fwd-Path to a Broadcast IP does not work, it works fine on a VSP 4850 with 6.1.3.0. If change the address to the unicast IP of a DHCP Server in the Server subnet (which is on another BEB) then DHCP works.

What could the issue be here please?

Thanks,

Rob

Re: DHCP Fwd-Path to Broadcast IP not working but unicast does

MJS — Thu, 06 Apr 2023 12:15:50 GMT

I am a bit confused by this, "DHCP Fwd-Path to a Broadcast IP," so your fwd path is 192.168.0.255 or something like that? I've never used a broadcast address for the fwd path, it typically is the IP address of a DHCP server. If it was working prior, perhaps the client DHCP request was sent on the same vlan as your DCHP server so you got an address even though the forward IP was a broadcast IP?

Re: DHCP Fwd-Path to Broadcast IP not working but unicast does

RobertD1 — Thu, 06 Apr 2023 15:33:12 GMT

Hi MJS,

It is definitely an option and does work. It allows a customer to add additional DHCP Servers into the same VLAN without having to add another Fwd-Path to the unicast IP of the new server. It is working today like this on the older VSP 4850 but it may not be hardware or software issue.

I have tested a small setup in GNS3 with a PC in one VLAN with IP DHCP-Relay and a DHCP Server in another VLAN with a single 8.1 VM and it works with both unicast and broadcast IPs.

It could be that the remote switch where the DHCP Server resides across the Fabric (using IP shortcuts) needs ip directed-broadcast enabling on its VLAN?

Re: DHCP Fwd-Path to Broadcast IP not working but unicast does

RobertD1 — Thu, 06 Apr 2023 16:54:34 GMT

I expanded my testing further by using two switches with the PC on Switch 1 and the DHCP Server on Switch 2 and have an ISIS adjacency UP and this works with 8.1. Maybe it is a software issue.

Re: DHCP Fwd-Path to Broadcast IP not working but unicast does

RobertD1 — Fri, 07 Apr 2023 09:38:10 GMT

I also noticed ip directed-broadcast is no longer available in later VOSS software under VLAN interface. So, how can a BEB send a subnet broadcast across Fabric to another BEB from one subnet ie DHCP Relay agent 10.10.10.1 to DHCP Server subnet using 20.20.20.255 when the subnets are on different nodes?

Re: DHCP Fwd-Path to Broadcast IP not working but unicast does

MJS — Fri, 07 Apr 2023 14:31:29 GMT

Thanks for clarifying. I had no idea that was an option.