dhcp scheduled to node without l2 segment access

[Allain Legacy]

In a past release a feature was introduced to make DHCP scheduling physical network aware [1]. While that commit addressed the simple case of each node only having a single L2 agent running it makes it clear in the description that it does not support cases where two L2 agents (e.g., OVS + SRIOV) exist on the same node. In such a configuration it is possible for the SRIOV mechanism driver to report that it has access to a segment while the OVS mechanism driver reports that it does not. Since the ML2 plugin [2] assumes that a host has access to a segment as long as one mechanism driver reports that it does it will cause the DHCP scheduler to create a binding on a host that potentially does not have L2 access to that segment.

We have run it to this problem in stable/newton but the most recent code in master appears to also have this same limitation.

The configuration to reproduce this is simple. Create a compute node where the OVS agent reports interface mappings to physnet0 and the SRIOV agent reports device mappings to physnet1. Then create a network with provider:physical_network=physnet1 with a DHCP enabled subnet and the DHCP scheduler will allow scheduling of that network to the node when it should not because the OVS agent does not have access to physnet1.

[1] https://git.openstack.org/cgit/openstack/neutron/commit/?id=0267c6a5acdcb68ea7e83ecb980362c4235ed1d7
[2] neutron.db.agents_db.AgentDbMixin.filter_hosts_with_network_access

[Brian Haley (brian-haley)]

This bug seems to be referenced in the commit message of that change:

"Such host filtering has some limitations if a dhcp-agent is on a host handled by multiple l2 mechanisms with one mechanism claiming network reachability but not the one handling dhcp-agent ports. Indeed the host is able to reach the network but not dhcp-agent ports! Such limitation will be handled in a follow-up change using host+vif_type filtering."

And the code in filter_hosts_with_segment_access() has this comment:

    This method returns all hosts from candidate_hosts with access to a
    segment according to at least one driver.

[Rodolfo Alonso (rodolfo-alonso-hernandez)]

Bug closed due to lack of activity, please feel free to reopen if needed.