quantum single flat network, but cannot ping instances

Can you check via VNC if VMs get IP addresses on their interfaces?

No, it does not from the console.logs (an excerpt):

[ OK ]^M
Bringing up loopback interface: [ OK ]^M
Bringing up interface eth0:
Determining IP information for eth0... failed.
[FAILED]^M
Starting system logger: [ OK ]^M
Starting kernel logger: [ OK ]^M
iscsid (pid 1231) is running...
Setting up iSCSI targets: iscsiadm: No records found
[ OK ]^M

I just checked the ovs-ofctl on the quantum network node:

# ovs-ofctl show br-int
OFPT_FEATURES_REPLY (xid=0x1): ver:0x1, dpid:0000d6b390e5324c
n_tables:255, n_buffers:256
features: capabilities:0xc7, actions:0xfff
 1(int-br-eth0): addr:ee:ae:e4:44:7d:3b
     config: 0
     state: 0
     current: 10GB-FD COPPER
 3(tapdd91d332-c6): addr:e7:00:00:00:00:00
     config: PORT_DOWN
     state: LINK_DOWN
 LOCAL(br-int): addr:d6:b3:90:e5:32:4c
     config: 0
     state: 0
OFPT_GET_CONFIG_REPLY (xid=0x3): frags=normal miss_send_len=0

And it seems to think that the tap is down. Is that ok? If not, how can I get the link state up? The dhcp agent is running or at least there are two processes for it:

nobody 24743 0.0 0.0 28820 1096 ? S Mar17 0:00 dnsmasq --no-hosts --no-resolv --strict-order --bind-interfaces --interface=tapdd91d332-c6 --except-interface=lo --domain=openstacklocal --pid-file=/var/lib/quantum/dhcp/6f12748f-2cae-4746-815d-66338e267b2f/pid --dhcp-hostsfile=/var/lib/quantum/dhcp/6f12748f-2cae-4746-815d-66338e267b2f/host --dhcp-optsfile=/var/lib/quantum/dhcp/6f12748f-2cae-4746-815d-66338e267b2f/opts --dhcp-script=/usr/bin/quantum-dhcp-agent-dnsmasq-lease-update --leasefile-ro --dhcp-range=set:tag0,1.1.1.0,static,120s
root 24744 0.0 0.0 28792 244 ? S Mar17 0:00 dnsmasq --no-hosts --no-resolv --strict-order --bind-interfaces --interface=tapdd91d332-c6 --except-interface=lo --domain=openstacklocal --pid-file=/var/lib/quantum/dhcp/6f12748f-2cae-4746-815d-66338e267b2f/pid --dhcp-hostsfile=/var/lib/quantum/dhcp/6f12748f-2cae-4746-815d-66338e267b2f/host --dhcp-optsfile=/var/lib/quantum/dhcp/6f12748f-2cae-4746-815d-66338e267b2f/opts --dhcp-script=/usr/bin/quantum-dhcp-agent-dnsmasq-lease-update --leasefile-ro --dhcp-range=set:tag0,1.1.1.0,static,120s

Ok, so, apparently the tap device cannot be found, according to the dhcp-agent.log on the network node:

Stderr: 'Cannot find device "tapdd91d332-c6"\n'
2013-03-18 07:37:52 ERROR [quantum.agent.dhcp_agent] Unable to enable dhcp.
Traceback (most recent call last):

How do can I make it be found?

I actually rebooted the quantum network node (which has the dhcp agent) and now the tap interface is present, but strangely I cannot access or ping the quantum server (on the nova controller) from the network node. The quantum server also has the rabbitmq service and quantum database. I will provide more information after I debug a bit more.

it seems likely that the quantum-ovs-cleanup script is not being run on reboot.

you could try running it manually, and then restarting the dhcp agent.