2 DHCP in 1 VLAN and IP helpers

[dmitry.pelikhovsky]

Good day!

There is:

2 cat4000 L3 switches with DHCP servers. DHCP servers hold one net 192.168.0.0/24 , ranges limited with excluded-addresses:

Resulting pools:

1. 192.168.0.10 - 128 

2. 192.168.0.129 - 254 

On every switch in vlan with DHCP there is ip helper for acronis PXE loader 192.168.1.200.

If only one DHCP server is active PXE works propertly , if both DHCP server works - ip helper does not works.

Why?

[Richard Burts]

With the limited information provided it is difficult to determine why it does not work when the second server is active. Perhaps some additional information including configurations might be helpful. Also the addresses of the DHCP servers.

HTH

Rick

[dmitry.pelikhovsky]

Good day, Richard!

DHCP servers addresses:

192.168.0.1 - HSRP

Primary addresses of interfaces:

R1 - 192.168.0.4 

R2 - 192.168.0.5 

[Richard Burts]

Are you indicating that the DHCP pools are configured on the L3 devices? If so please post configs.

HTH

Rick

[dmitry.pelikhovsky]

sw1:

ip dhcp excleded-address 192.168.0.1 192.168.0.128

no ip bootp server

interface vlan 301

ip address 192.168.0.4 255.255.255.0

standby 192.168.0.1

standby priority 2

no ip redirects

no ip proxy-arp

ip helper-address 192.168.1.200

!

ip dhcp pool test

network 192.168.0.0 255.255.255.0

default-router 192.168.0.1

sw2:

ip dhcp excleded-address 192.168.0.129 192.168.0.254

no ip bootp server

interface vlan 301 

ip address 192.168.0.5 255.255.255.0

standby 192.168.0.1

standby priority 1

no ip redirects

no ip proxy-arp

ip helper-address 192.168.1.200

!

ip dhcp pool test

network 192.168.0.0 255.255.255.0

default-router 192.168.0.1

[Richard Burts]

Thank you for posting the additional information. This partial config is pretty straightforward and I do not see anything yet that would cause the symptom you describe. So I would like to understand the symptom a bit better. You say that it works if one DHCP is active but does not work if both are active. Is it correct that it does work if SW1 is working and SW2 is down or is it that it works is SW1 is down and SW2 is working?

HTH

Rick

[dmitry.pelikhovsky]

Good day! 

If (sw1 DHCP is up and sw2 DHCP down ) OR (sw1 DHCP is down and sw2 DHCP is up) PXE works.

If sw1 DHCP is up and sw2 DHCP is up PXE does not works.

[paul driver]

Hello

You DONT require relays in your network if the dhcp server(s) are on the L3 Core 

res

Paul

[Richard Burts]

Paul

If I am understanding the discussion the issue is not about DHCP assigning IP addresses but is about sending traffic to a remote server for PXE.

To the original poster - I had hoped that it might be the case that it worked for one switch but not for the other. But you have clarified that it works for either switch as long as the other switch is not active. Which leads me to a follow up question. When you say it works only when a single switch is active are you saying that the other switch is down or just that the other switch has DHCP removed?

HTH

Rick

[dmitry.pelikhovsky]

Sw1 and sw2 - core double star switches and they work 24*7 with STP, HSRP and other services.

Interfaces vlan 301 of both switches was active all the time with routing, hsrp and all other services.

To switch off DHCP I printed

no ip dhcp pool test

So DHCP in Vlan 301 had no pool and was inactive.

Result of todays tests:

Sw1 DHCP on + helper off (hsrp active)

Sw2 DHCP off + helper on (hsrp passive)

=PXE Works

Any tests fails of both sw1 DHCP and sw2 DHCP works.

[Richard Burts]

Thanks for the additional information. This is quite odd. What it seems like is that if you enable DHCP on the switch that it disables processing of the helper address. I would certainly not expect that as normal behavior. So I wonder if there might be something in the config that is triggering this. As I look at what you have posted I wonder if this might be involved

no ip bootp server

Would you try turning on bootp server and see if it makes any difference? Assuming that it does not make a difference would you post a more complete config from one of the switches?

HTH

Rick

[dmitry.pelikhovsky]

Good morning, Richard!

If I enable bootp process on routers PXE do not works in any config. 

All over the world forums posting about acronis and dhcp troubles, but this command is the key to resolve it.

[Richard Burts]

I was not confident about the change that I suggested but thought it might be worth a try. Based on your information you certainly do not want to enable bootp process. There is a possibility that what is happening is the result of a bug in the code. Or it might be something else in the config. Is it worth it to post the config of one of the switches?

HTH

Rick