DHCP Relay 62xx Switches

Question

I have three 62xx (2-6248 and 1-6224) switches that serve as my core network.  Each switch is in a different building or different floor and they are connected using CX4 or XFP 10GB links.  I am new to VLANs and I am experimenting with a test VLAN at the moment.  Currently everything is in a flat architecture in the native VLAN 1. For this test I successfully created two VLANs on one 6248 (in the server room) and the 6224 in the downstairs wiring closet.  I put the 10Gb link between the two into general mode and allowed traffic from VLAN 1,40 and 125.  The 6248 in the server room will serve as my layer 3 device to route between the VLANs and I have routing enabled on that switch.  I put one port on the 6224 into access mode on VLAN 40.  I also moved the management VLAN for the 6224 to 125.  If I statically assign an IP address to a test machine connected to the single access port on the 6224, I can ping the gateway interface on the 6248 for both VLANs as well as the management interface of the 6224.  My problem comes in when I try to get DHCP to work on this box.  I can't get my test machine on the access port to get an IP address from our DHCP server.  I have created a scope on that server for that subnet and I have setup the helper-address to point to the DHCP server on the L3 device. When I look in the logs on the 6248 I see where the DHCP request arrives on VLAN 40 but the switch gives the following error. DHCP client message received on port 1/0/49 VLAN 40 from client 00:26:b9:d7:c6:ee has seconds field set to 0. Value is less than the minimum 60. Dropping. What does this mean and how would you go about troubleshooting this? My DHCP server is on VLAN 1 and I know that the 62xx switches do not route on VLAN 1.  Is that part of the problem or is there a configuration issue here?

DELL-Willy M · Answer

You need to take a look at the IP helper command it allows the relay of DHCP packets to the network where the DHCP server is located. Otherwise the DHCP traffic will be dropped if there is no DHCP server in that specific VLAN.

Page 711 starts the discussion about IP helper in the CLI User Guide found below.

ftp://ftp.dell.com/Manuals/all-products/esuprt_ser_stor_net/esuprt_powerconnect/powerconnect-6248_Reference%20Guide_en-us.pdf

Brad-Overstreet · Answer

I have tried the IP helper. I configured the helper globally to send DHCP to the DHCP server address. After that I configured it for just VLAN 40 and finally I configured it in both places. None of which worked. Below is a copy of the config from the 6248.

show run

!Current Configuration:

!System Description "PowerConnect 6248, 3.3.7.2, VxWorks 6.5"

!System Software Version 3.3.7.2

!Cut-through mode is configured as disabled

!

configure

gvrp enable

vlan database

vlan 40,125

vlan routing 40 1

vlan routing 125 2

exit

snmp-server location "Main Building - Second Floor - Room 201"

snmp-server contact "Brad Overstreet"

hostname "101-2-201-PC6248-1"

sntp unicast client enable

sntp client poll timer 1024

sntp server 192.168.200.17

clock summer-time recurring USA zone "CDT"

clock timezone -6 minutes 0 zone "CST"

stack

member 1 2

exit

ip address 192.168.200.83 255.255.255.0

ip default-gateway 192.168.200.90

ip domain-name automationtool.com

ip name-server 192.168.200.17

ip name-server 192.168.200.19

ip routing

bootpdhcprelay minwaittime 60

ip helper-address 192.168.200.17 time

ip helper-address 192.168.200.19 time

ip helper-address 192.168.200.17 dhcp

ip helper-address 192.168.200.17 ntp

ip helper-address 192.168.200.19 ntp

interface vlan 40

name "TEST"

routing

ip address 172.31.40.1 255.255.255.0

ip helper-address 192.168.200.17 dhcp

exit

interface vlan 125

name "Management"

routing

ip address 192.168.125.1 255.255.255.0

exit

ip ssh server

!

interface ethernet 1/g1

channel-group 9 mode auto

exit

!

interface ethernet 1/g2

channel-group 9 mode auto

exit

!

interface ethernet 1/g3

channel-group 11 mode auto

exit

!

interface ethernet 1/g4

channel-group 11 mode auto

exit

!

interface ethernet 1/g5

channel-group 10 mode auto

exit

!

interface ethernet 1/g6

channel-group 10 mode auto

exit

!

interface ethernet 1/g7

channel-group 10 mode auto

exit

!

interface ethernet 1/g8

channel-group 10 mode auto

exit

!

interface ethernet 1/g9

channel-group 8 mode auto

exit

!

interface ethernet 1/g10

channel-group 8 mode auto

exit

!

interface ethernet 1/g11

channel-group 8 mode auto

exit

!

interface ethernet 1/g12

channel-group 8 mode auto

exit

!

interface ethernet 1/g13

channel-group 7 mode auto

exit

!

interface ethernet 1/g14

channel-group 7 mode auto

exit

!

interface ethernet 1/g15

channel-group 7 mode auto

exit

!

interface ethernet 1/g16

channel-group 7 mode auto

exit

!

interface ethernet 1/g17

channel-group 6 mode auto

exit

!

interface ethernet 1/g18

channel-group 6 mode auto

exit

!

interface ethernet 1/g19

channel-group 6 mode auto

exit

!

interface ethernet 1/g20

channel-group 6 mode auto

exit

!

interface ethernet 1/g21

channel-group 5 mode auto

exit

!

interface ethernet 1/g22

channel-group 5 mode auto

exit

!

interface ethernet 1/g23

channel-group 5 mode auto

exit

!

interface ethernet 1/g24

channel-group 5 mode auto

exit

!

interface ethernet 1/g25

channel-group 4 mode auto

exit

!

interface ethernet 1/g26

channel-group 4 mode auto

exit

!

interface ethernet 1/g27

channel-group 4 mode auto

exit

!

interface ethernet 1/g28

channel-group 4 mode auto

exit

!

interface ethernet 1/g29

channel-group 3 mode auto

exit

!

interface ethernet 1/g30

channel-group 3 mode auto

exit

!

interface ethernet 1/g31

channel-group 3 mode auto

exit

!

interface ethernet 1/g32

channel-group 3 mode auto

exit

!

interface ethernet 1/g33

channel-group 2 mode auto

exit

!

interface ethernet 1/g34

channel-group 2 mode auto

exit

!

interface ethernet 1/g35

channel-group 2 mode auto

exit

!

interface ethernet 1/g36

channel-group 2 mode auto

exit

!

interface ethernet 1/g37

channel-group 1 mode auto

exit

!

interface ethernet 1/g38

channel-group 1 mode auto

exit

!

interface ethernet 1/g39

channel-group 1 mode auto

exit

!

interface ethernet 1/g40

channel-group 1 mode auto

exit

!

interface ethernet 1/g41

channel-group 13 mode auto

exit

!

interface ethernet 1/g42

channel-group 13 mode auto

exit

!

interface ethernet 1/g43

channel-group 13 mode auto

exit

!

interface ethernet 1/g44

channel-group 13 mode auto

exit

!

interface ethernet 1/g45

channel-group 12 mode auto

exit

!

interface ethernet 1/g46

channel-group 12 mode auto

exit

!

interface ethernet 1/g47

channel-group 12 mode auto

exit

!

interface ethernet 1/g48

channel-group 12 mode auto

exit

!

interface ethernet 1/xg1

gvrp enable

gvrp vlan-creation-forbid

gvrp registration-forbid

switchport mode general

switchport general allowed vlan add 40,125 tagged

exit

!

interface port-channel 2

description 'ATCSQL'

exit

!

interface port-channel 3

description 'ATCNS2'

exit

!

interface port-channel 4

description 'ATCACCT'

exit

!

interface port-channel 5

description 'PC5324-1'

exit

!

interface port-channel 6

description 'PC5324-2'

exit

!

interface port-channel 7

description 'PC5324-3'

exit

!

interface port-channel 8

description 'ATCMAIL2'

exit

!

interface port-channel 9

description 'ATCNS1'

switchport mode general

switchport general allowed vlan add 40 tagged

exit

!

interface port-channel 10

description 'NEWSQL'

exit

!

interface port-channel 11

description 'ATCCRM'

exit

!

interface port-channel 12

description 'ATCFPS2'

exit

!

interface port-channel 13

description 'ATCFPS1'

exit

snmp-server group DefaultRead v3 noauth notify Default read Default write Default

exit

DELL-Willy M · Answer

I currently do not see a port with access mode for VLAN 40 in the configuration that you provided.  You may need to set portfast on the port that you are trying to get DHCP addressing.  If the messages are timing out then portfast should help cut down on the STP convergence and allow the port to go into a forwarding state.

Brad-Overstreet · Answer

The port that is in access mode is on the 6224. It communicates with the 6248 over the VLAN trunk (1/xg1). The configuration of the 6224 is shown below. Note that port 1/xg1 is configured as the trunk port on the 6224 as well. Also note that 1/g23 is the port set to access mode.

!Current Configuration:

!System Description "PowerConnect 6224, 3.3.7.2, VxWorks 6.5"

!System Software Version 3.3.7.2

!Cut-through mode is configured as disabled

!

configure

vlan database

vlan 40,125

exit

snmp-server location "Main Building - First Floor - Room 117"

snmp-server contact "Brad Overstreet"

hostname "101-1-117-PC6224-1"

sntp unicast client enable

sntp client poll timer 1024

sntp server 192.168.200.17

sntp server 192.168.200.19 priority 2

clock summer-time recurring USA zone "CDT"

clock timezone -6 minutes 0 zone "CST"

stack

member 1 1

exit

ip address 192.168.125.85 255.255.255.0

ip default-gateway 192.168.125.1

ip address vlan 125

ip domain-name automationtool.com

ip name-server 192.168.200.17

ip name-server 192.168.200.19

logging file info

router rip

auto-summary

default-metric 1

exit

interface vlan 40

name "TEST"

exit

interface vlan 125

name "Management"

exit

!

interface ethernet 1/g1

channel-group 1 mode auto

exit

!

interface ethernet 1/g2

channel-group 1 mode auto

exit

!

interface ethernet 1/g3

channel-group 1 mode auto

exit

!

interface ethernet 1/g4

channel-group 1 mode auto

exit

!

interface ethernet 1/g7

channel-group 4 mode auto

exit

!

interface ethernet 1/g8

channel-group 4 mode auto

exit

!

interface ethernet 1/g9

channel-group 4 mode auto

exit

!

interface ethernet 1/g10

channel-group 4 mode auto

exit

!

interface ethernet 1/g11

channel-group 5 mode auto

exit

!

interface ethernet 1/g12

channel-group 5 mode auto

exit

!

interface ethernet 1/g13

channel-group 5 mode auto

exit

!

interface ethernet 1/g14

channel-group 5 mode auto

exit

!

interface ethernet 1/g15

channel-group 6 mode auto

exit

!

interface ethernet 1/g16

channel-group 6 mode auto

exit

!

interface ethernet 1/g17

channel-group 6 mode auto

exit

!

interface ethernet 1/g18

channel-group 6 mode auto

exit

!

interface ethernet 1/g23

spanning-tree portfast

switchport access vlan 40

exit

!

interface ethernet 1/xg1

gvrp enable

gvrp vlan-creation-forbid

gvrp registration-forbid

switchport mode general

switchport general allowed vlan add 40,125 tagged

exit

!

interface port-channel 1

description 'MB_1_117_PC5448_3'

exit

!

interface port-channel 4

description 'MB_1_117_PC5448_1'

exit

!

interface port-channel 5

description 'MB_1_117_PC5448_2'

exit

!

interface port-channel 6

description 'ATCBACKUP'

exit

Brad-Overstreet · Answer

Ok I have been using Wireshark to inspect the network traffic. I modified the configuration of the 6248 switch so that IP helper-address was defined only for the specific VLAN where it was needed. I confirmed via Wireshark that the DHCP discover broadcast is making it to the DHCP server and that the server is sending an offer out. However it appears that the offer is never making it back to the client computer.

Is this happening because the DHCP server is in the native VLAN and the 6248 will not route packets in this VLAN or do I have a configuration issue?

DELL-Willy M · Answer

From the interface ethernet 1/g23 can you ping the DHCP server successfully? When you have a static IP set for VLAN 40.

I do not see the IP helper set in the above configuration. The DHCP packets will not traverse outside off the switch

console(config)#ip helper enable

To relay both DHCP and DNS packets to 192.168.30.1, use the following commands:

console#config

console(config)#ip helper enable

console(config)#interface vlan 5

console(config-if-vlan5)#ip helper-address 192.168.30.1 dhcp

console(config-if-vlan5)#ip helper-address 192.168.30.1 dns

I typed the above out earlier and got delayed on posting it. I see your response came in about wireshark and VLAN 1. The management VLAN is not routable on the 6200 model switches. I would avoid using VLAN 1 for any traffic when you have multiple VLANs set up.

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