I've run into a problem on my KVM host where a single guest will be unreachable to other guests on the same host. This host has 2 bridged devices and guests assigned to each have the same issue. I've noticed that when I can't reach the problematic guest, the ARP entry for that system is incorrect. This issue seems to only be a problem about 75% of the time when making connections between the guests. I've checked the logs and haven't seen anything indicating a problem. Here's relavant information, if more is needed please let me know CentOS 6.2 x86_64 libvirt-0.9.4-23.el6_2.4 kernel-2.6.32-220.4.1.el6 # brctl show bridge name bridge id STP enabled interfaces br0 8000.001b21a1cf76 no eth3 vnet0 # Problematic host vnet1 vnet3 vnet4 vnet5 br1 8000.001b21a1ce4d no eth2 vnet6 virbr0 8000.525400a050ba yes virbr0-nic vnet2 # ARP entry when it's the correct MAC <omitted>.tamu.edu ether 52:54:00:e0:1e:6f C eth0 # ARP entry when it's wrong and ping or other connections time out <omitted>.tamu.edu ether 00:1e:c9:bb:cd:d4 C eth0 The MAC ending in cd:d4 does not exist on any system on this network. I've confirmed this with my campus IT who manages the switches and routers. # cat ifcfg-eth3 DEVICE="eth3" HWADDR="00:1B:21:A1:CF:76" NM_CONTROLLED="no" ONBOOT="yes" BRIDGE="br0" # cat ifcfg-br0 DEVICE="br0" ONBOOT="yes" TYPE=Bridge IPADDR="<omitted>" NETMASK="255.255.255.192" GATEWAY="<omitted>" DOMAIN="tamu.edu" # service iptables status Table: filter Chain INPUT (policy ACCEPT) num target prot opt source destination 13 ACCEPT tcp -- <omitted> 0.0.0.0/0 multiport dports 10050 /* 001 zabbix agent port */ 14 ACCEPT udp -- 0.0.0.0/0 0.0.0.0/0 udp dpt:53 15 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:53 16 ACCEPT udp -- 0.0.0.0/0 0.0.0.0/0 udp dpt:67 17 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:67 18 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:16509 19 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:16514 20 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpts:8002:8012 21 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:3493 22 ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 state RELATED,ESTABLISHED 23 ACCEPT icmp -- 0.0.0.0/0 0.0.0.0/0 24 ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 25 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:22 26 REJECT all -- 0.0.0.0/0 0.0.0.0/0 reject-with icmp-host-prohibited Chain FORWARD (policy ACCEPT) num target prot opt source destination 1 ACCEPT all -- 0.0.0.0/0 192.168.122.0/24 state RELATED,ESTABLISHED 2 ACCEPT all -- 192.168.122.0/24 0.0.0.0/0 3 ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 4 REJECT all -- 0.0.0.0/0 0.0.0.0/0 reject-with icmp-port-unreachable 5 REJECT all -- 0.0.0.0/0 0.0.0.0/0 reject-with icmp-port-unreachable 6 ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 PHYSDEV match --physdev-is-bridged 7 REJECT all -- 0.0.0.0/0 0.0.0.0/0 reject-with icmp-host-prohibited Chain OUTPUT (policy ACCEPT) num target prot opt source destination Table: mangle Chain PREROUTING (policy ACCEPT) num target prot opt source destination Chain INPUT (policy ACCEPT) num target prot opt source destination Chain FORWARD (policy ACCEPT) num target prot opt source destination Chain OUTPUT (policy ACCEPT) num target prot opt source destination Chain POSTROUTING (policy ACCEPT) num target prot opt source destination 1 CHECKSUM udp -- 0.0.0.0/0 0.0.0.0/0 udp dpt:68 CHECKSUM fill Table: nat Chain PREROUTING (policy ACCEPT) num target prot opt source destination Chain POSTROUTING (policy ACCEPT) num target prot opt source destination 1 MASQUERADE tcp -- 192.168.122.0/24 !192.168.122.0/24 masq ports: 1024-65535 2 MASQUERADE udp -- 192.168.122.0/24 !192.168.122.0/24 masq ports: 1024-65535 3 MASQUERADE all -- 192.168.122.0/24 !192.168.122.0/24 Chain OUTPUT (policy ACCEPT) num target prot opt source destination Thanks - Trey