[libvirt] Network stops with CentOS 6.3 guest on Ubuntu 12.10

Hello, I'm trying to get CentOS 6.3 running under Ubuntu 12.10. When the interface comes up, I can ping another machine for 9 packets. After that it stops working. Bouncing the guest network interface gets it going again for another 10 packets. Monitoring the vnet0 interface on the host, I see ARP requests from the host to the guest are not getting replies. I cleared out iptables to eliminate that. No luck. I have Fedora 17 and Windows 7 running on the machine with no problems. Any ideas? Thanks, Andy