DHCP failover setup different IP offering with 2 helper address

[Alp Eren Kose]

Hello,

We have setup a DHCP failover configuration, with Wireless LAN Controllers having multiple ip-helper-address statements. The controllers are Cisco and Aruba branded. As I understand it, Controllers relay DHCP discover messages to both DHCP servers, however sometimes we see different IP addresses are OFFERED for the same request by both DHCP servers. Is it the normal behavior?

Could you also point some documentation that I can read to understand how exactly failover works in ISC DHCP version 4.2.8? Especially how multiple discovery and request packets are handled from multiple ip-helper address statements.

Note: Please forgive me if it has been asked before or is really a basic question. I did have a quick look and did not come up with an answer, I wouldn't be posting without more search if it wasn't urgent.

Thanks in advance.
Alp

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[glenn.satchell]

Hi Alp

There's a rather long section in the dhcpd.conf man page. Scroll down to
the section "DHCP FAILOVER" and follow from there.

It's quite normal for the dhcp servers to offer different IP addresses.
The pool balancing splits the free IP addresses between the two servers,
so they offer an IP out of their portion of the free addresses. Unless
they've already seen that client before in which case they will offer a
previous address.

The dhcp clients should be able to cope with multiple offers, and will
select a suitable one, often the first received.

Make sure the dhcp servers have their time synchronised, most easily done
by running ntp.

regards,
-glenn

On Sun, June 28, 2015 10:33 pm, Alp Eren Kose wrote:

> Hello,
>
> We have setup a DHCP failover configuration, with Wireless LAN Controllers
> having multiple ip-helper-address statements. The controllers are Cisco
> and
> Aruba branded. As I understand it, Controllers relay DHCP discover
> messages
> to both DHCP servers, however sometimes we see different IP addresses are
> OFFERED for the same request by both DHCP servers. Is it the normal
> behavior?
>
> Could you also point some documentation that I can read to understand how
> exactly failover works in ISC DHCP version 4.2.8? Especially how multiple
> discovery and request packets are handled from multiple ip-helper address
> statements.
>
> Note: Please forgive me if it has been asked before or is really a basic
> question. I did have a quick look and did not come up with an answer, I
> wouldn't be posting without more search if it wasn't urgent.
>
> Thanks in advance.
> Alp
> _______________________________________________
> dhcp-users mailing list
> [hidden email]
> https://lists.isc.org/mailman/listinfo/dhcp-users

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[Gero Palacio]

Hi Alp,

We have setup a DHCP failover configuration, with Wireless LAN Controllers having multiple ip-helper-address statements. The controllers are Cisco and Aruba branded. As I understand it, Controllers relay DHCP discover messages to both DHCP servers, however sometimes we see different IP addresses are OFFERED for the same request by both DHCP servers. Is it the normal behavior?

It is the expected behavior when the "sec" field in the DHCP client's message is grater than the "mclt" defined in the primary DHCP server. Or if the DHCP server are *not* in normal state. If that's not the case (both servers in normal state and "sec" < "mclt") both servers will receive the DHCP DISCOVERY message but only one of them will reply back with an offer. There's a thread describing a similar problem I had in this mail list, subject "DHCP Relay agent not forwarding messages to the client" first message from June 15.

Could you also point some documentation that I can read to understand how exactly failover works in ISC DHCP version 4.2.8? Especially how multiple discovery and request packets are handled from multiple ip-helper address statements.

You can check out this link <a href="https://tools.ietf.org/html/draft-ietf-dhc-failover-12 and ">DHCP Failover Protocol Draft IEFT and this DHC Load Balancing Algorithm RFC 3074. Always keep in mind this tip which I got from another thread in this mail list:

The split *does not* determine the share of the pool. The pool is
 *always* balanced 50/50. The split value determines which peer will
 respond to the client based on the hashed value of the client
 identifier (MAC address).

Hope this helps!
Gero. 

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[Gero Palacio]

I need to correct myself. I did not meant the "mclt", I meant "load balance max secs" option in the DPCH servers, sorry about that.

It is the expected behavior when the "sec" field in the DHCP client's message is grater than the "load balance max seconds" defined in the primary DHCP server. Or if the DHCP server are *not* in normal state. If that's not the case (both servers in normal state and "sec" < "load balance max seconds") both servers will receive the DHCP DISCOVERY message but only one of them will reply back with an offer

From man dhcpd:

load-balance-max-secs integer examine

Indicates the maximum value for the secs field in a client request before load balancing is bypassed.

Cheers.

On Sun, Jun 28, 2015 at 3:04 PM, Gero Palacio <[hidden email]> wrote:

Hi Alp,

We have setup a DHCP failover configuration, with Wireless LAN Controllers having multiple ip-helper-address statements. The controllers are Cisco and Aruba branded. As I understand it, Controllers relay DHCP discover messages to both DHCP servers, however sometimes we see different IP addresses are OFFERED for the same request by both DHCP servers. Is it the normal behavior?

It is the expected behavior when the "sec" field in the DHCP client's message is grater than the "mclt" defined in the primary DHCP server. Or if the DHCP server are *not* in normal state. If that's not the case (both servers in normal state and "sec" < "mclt") both servers will receive the DHCP DISCOVERY message but only one of them will reply back with an offer. There's a thread describing a similar problem I had in this mail list, subject "DHCP Relay agent not forwarding messages to the client" first message from June 15.

Could you also point some documentation that I can read to understand how exactly failover works in ISC DHCP version 4.2.8? Especially how multiple discovery and request packets are handled from multiple ip-helper address statements.

You can check out this link DHCP Failover Protocol Draft IEFT and this DHC Load Balancing Algorithm RFC 3074. Always keep in mind this tip which I got from another thread in this mail list:

The split *does not* determine the share of the pool. The pool is
 *always* balanced 50/50. The split value determines which peer will
 respond to the client based on the hashed value of the client
 identifier (MAC address).

Hope this helps!
Gero. 

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[John Wobus]

On Jun 28, 2015, at 8:33 AM, Alp Eren Kose wrote:
> We have setup a DHCP failover configuration, with Wireless LAN  
> Controllers having multiple ip-helper-address statements. The  
> controllers are Cisco and Aruba branded. As I understand it,  
> Controllers relay DHCP discover messages to both DHCP servers,  
> however sometimes we see different IP addresses are OFFERED for the  
> same request by both DHCP servers. Is it the normal behavior?

As far as I know, DHCP redundancy works by having the servers offer  
different addresses and the client picking one.
ISC DHCP servers reduce the obvious duplicated effort by having one of  
the two DHCP servers wait a few seconds to see if
the other server makes an offer.  But if the one server doesn't pick  
up on it's peer's offer, the address it
offers itself is always a different one.

John Wobus
Cornell IT

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