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I would agree that having a active/standby for DHCP agent makes a lot of
sense. We might want to leverage the VRRP infastructure for that.
I am not sure I understand clearly the need to have the DHCP agents sit
behind the load balancers. What are we trying to load balance here? The
amount of DHCP intermittent and transient to say the least with a heavy
bias towards more traffic at the time of a VM booting up.

If we were to truly load balance we would need to keep the state of the
DHCP servers in sync (dynamically) as they would be allocating from a
common pool of resources. That might not be a problem that we would want to
inherit.

On the other hand, your suggestion to use VRRP would be a great idea for
those use cases where the L3 agent and the DHCP agent would be co-located.
The problem of keeping the state in sync would still have to be dealt with
but is not as severe as the load balancing case.

Just my thoughts.
Vinay

On Sun, Nov 25, 2012 at 5:56 AM, Gary Kotton <gkotton at redhat.com> wrote:

>  Hi,
> There were two ideas discussed at the summit the first is the LBaaS and
> the second was improvements for the DHCP agent (multinode). I think that we
> can leverage the LBaaS to support a highly available and robust Quantum
> DHCP service.
>
> This can be achieved as follows:
>
> 1. For each network that supports a DHCP service there will be a VIP for
> the DHCP address (this will also have the relevant health checks etc.)
> 2. Each DHCP running agent will be registered as a member (I hope that I
> have the terminology correct here). Basically vip = {dhcps1, dhcps2, ...}
> 3. All of the DHCP requests and lease updates will be sent via the VIP for
> the DHCP. The load balcner will select a DHCP server if this is the first
> time a request from the client has been made or it will forward to a
> existing server entry.
>
> Please see the diagram below. This will enable a cluster of hosts on the
> same network tenant to get a highly available DHCP service - the DHCP
> server IP is the virtual IP (it is ideal to have an active backup load
> balancing pair to ensure HA - this could either be by VRRP or some
> propriatery method that any of the vendors support). My thinking is that if
> we can use this for the first LBaaS integration example then we are
> certainly moving in the right direction and we have killed two birds with
> one stone.
>
> In the example below there will be 2 DHCP agents. The traffic will be load
> balanced by the active load balancer (in an active back configuration the
> persistent sessions will be maintained :)).
>
> A few minor changes may be required when Nova receives the DHCP address -
> we should return the VIP address.
>
>
>
> Ideas, comments or thoughts?
>
> Thanks
> Gary
>
>
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>
>


-- 
Vinay Bannai
Email: vbannai at gmail.com
Google Voice: 415 938 7576
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