Question re multiple l3 and dhcp agents Blueprint

Asked by Wojciech Dec


while I can understand the desire cast in this blueprint to allow multiple L3 and dhcp agents, what strikes me is that this setup leads to numerous routing side issues that are not mentioned, or at least not clearly in the blueprint, and which potentially can result in the whole scheme not working:

Eg. Host A is hooked up to 2 L3 agents (one being the default, one backup). By definition the L3 agents need to route traffic to the same pool of IP addresses, and do so based on their active/backup designation. That said however, in case of NAT a simple switch of active/standby is insufficient as the IP tables entries need to be carried over. What more, this simplistic approach probably doesn't work beyond the 1+1 redundant case.

Would welcome some additional clarification on how the multiple agents setup is meant to actually operate.


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yong sheng gong (gongysh) said :

Current multiple DHCP and L3 agents will help to lead the traffic from VMs on a host to a certain l3 agent with the same host name.
default l3 agent is for the Vms on a host for which there is no L3 agent with the same host name.

To setup a such env, for example we have two compute hosts, hosta and hostb. we have 2 l3 agents with host name hostc and hosta. and we have 2 dhcp agents with host name hosta and hostb. If we let l3 agent on hosta host quantum routes for Vms on hosta, we will have the 'multi-host' like effect for hosta. All cross subnet traffic out from hosta's VMs will pass hosta's l3 agent.

multi l3 agents do not run as active/back way. it is for distribute traffic among l3 agents according to their hosting quantum routers.

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Wojciech Dec (wdec-ietf) said :

Ok, so this clarifies things, but opens up a followup clarification:

From the example you give, if hosta's L3 agent has IP A, and hostb's L3 agent IP B, then hostA and hostB must each get IP A or IP B as their default gateway passed from the DHCP agent. Any other host, say host C without its own L3 agent, could then get either IP A or IP B also as their default gateway - making things not quite in line with the idea of "all other hosts using the default L3 agent". I.e. either A or B could end up being default L3 agents, and there isn't really any more one default agent.

The intended operation of the separate DHCP agents also should be clarified: How will the IP address pool for a shared subnet be divided or managed by multiple dhcp agents?

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Wojciech Dec (wdec-ietf) said :

Can we clarify things a bit more?

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Launchpad Janitor (janitor) said :

This question was expired because it remained in the 'Open' state without activity for the last 15 days.

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yong sheng gong (gongysh) said :

DHCP agents host netwroks in the unit of networks. I think there is no way to assign one subnet of a network to dhcp agent a, another subnet of this network to dhcp agent b.
in that BP, we can allow multiple DHCP agents host the shared network at the same time