SX550X - issue with ip helper-address to PXE Booting

topalotaNo · ‎02-27-2023

Hello,

I just joined a new company where PXE Booting never worked, i'm trying to fix it but I am stuck at some point. The switches are Cisco SX550X and L3 enabled for VLAN Routing Management.

There is 3 VLANS in my case.

VLAN 115 - DHCPVLAN - 192.168.20.0/24

VLAN 116 - PXEVLAN - 192.168.30.0/24

VLAN 117 - WORKSTATIONVLAN - 192.168.40.0/24

DHCP Server = 192.168.20.1

PXE Server = 192.168.30.1

Clients workstations in VLAN 116 or VLAN 117 can obtain DHCP IP from DHCP Server.

When I try to boot from network from a new workstation, I do not see the broadcast request on the PXEServer (tcpdump filtered with port 67/68/69/4011). It means that the packets are not forwarded to the VLAN 116

show ip dhcp relay

DHCP relay is Enabled
Option 82 is Enabled
Maximum number of supported VLANs without IP Address is 256
Number of DHCP Relays enabled on VLANs without IP Address is 0
DHCP relay is not configured on any port.
DHCP relay is enabled on Vlans: 116-117
Active: 116-117
Inactive:
Servers: 192.168.20.1

show ip helper-address

Interface       Helper Address     Udp Port
---------   -------------    ------------
192.168.20.1      192.168.30.0    37,42,49,53,137,138
192.168.20.1      192.168.40.0    37,42,49,53,137,138

I suspect first that the ip helper-address command was misconfigured, the previous employee inversed the "interface" and the "helper address" from the command line.

I can not provide the config file as I am in physical air gap environment.

Can you confirm that the command should be:

conf t
ip helper-address 192.168.40.254 192.168.20.1 37 42 49 53 137 138
ip helper-address 192.168.40.254 192.168.30.1 37 42 49 53 69 137 138 4011

Do I need to dhcp relay enable the PXE IP address, as udp 67/68 are forwarded through dhcp relay and PXE will not receive the broadcast otherwise ?

Thank you for you help.

marce1000 · ‎02-27-2023

- You may find this thread useful : https://community.cisco.com/t5/switching/dhcp-pool-and-helper-address-for-pxe/td-p/2868337

M.

-- Each morning when I wake up and look into the mirror I always say ' Why am I so brilliant ? '
When the mirror will then always repond to me with ' The only thing that exceeds your brilliance is your beauty! '

topalotaNo · ‎02-28-2023

I tried adding IP Helper for the PXE Server but I can not see the DHCP Requests coming from the clients VLAN.

ip helper-address X.X.X.X does not work

I need to type ip helper-address ip_interface ip_helper_address udp_port

e.g: ip helper-address 192.168.40.254 192.168.30.1 42 49 53 69 137 138 4011 ---> ip helper for the Deployment Server ip helper-address 192.168.40.254 192.168.20.1 42 49 53 137 138 ---> ip helper for the DHCP Server

192.168.40.254 is the gateway (Interface IP of the VLAN 117)

The DHCP is installed on Windows domain controller and DHCP options is not supported for UEFI. The deployment server is a TFTP installed on an Oracle Linux 8.

Do I need to add the PXE IP_address as DHCP Relay ? Or the ip helper-address is sufficient just for the PXE. There is some DHCP snooping that is activated, could it be theses settings that could prevent the PXE to work ?