two linux machines A and B , they are in one network: 192.168.160.0 A has two netcards: eth0 Link encap:Ethernet HWaddr 00:0C:29:F9:DD:29 inet addr:192.168.160.11 Bcast:192.168.160.255 Mask:255.255.255.0 inet6 addr: fe80::20c:29ff:fef9:dd29/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:448 errors:0 dropped:0 overruns:0 frame:0 TX packets:361 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:42722 (41.7 Kb) TX bytes:46887 (45.7 Kb) eth1 Link encap:Ethernet HWaddr 00:0C:29:F9:DD:33 inet addr:192.168.160.21 Bcast:192.168.160.255 Mask:255.255.255.0 inet6 addr: fe80::20c:29ff:fef9:dd33/64 Scope:Link UP BROADCAST MULTICAST MTU:1500 Metric:1 RX packets:73 errors:0 dropped:0 overruns:0 frame:0 TX packets:7 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:9926 (9.6 Kb) TX bytes:578 (578.0 b) B has one netcard: eth0 Link encap:Ethernet HWaddr 00:0C:29:A9:5F:E2 inet addr:192.168.160.12 Bcast:192.168.160.255 Mask:255.255.255.0 inet6 addr: fe80::20c:29ff:fea9:5fe2/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:793 errors:0 dropped:0 overruns:0 frame:0 TX packets:649 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:74366 (72.6 Kb) TX bytes:80645 (78.7 Kb) Interrupt:17 Base address:0x2000 now on B :ping 192.168.160.11 and 192.168.160.21, both are ok run arp -a,result is as follows:? (192.168.160.21) at 00:0C:29:F9:DD:29 [ether] on eth0? (192.168.0.59) at 00:0C:29:F9:DD:1F [ether] on eth1? (192.168.160.1) at 00:50:56:C0:00:01 [ether] on eth0? (192.168.0.1) at 00:11:95:CD:35:66 [ether] on eth1Vsles11sp1 (192.168.160.11) at 00:0C:29:F9:DD:29 [ether] on eth0 then I unplug eth1 , ethtool eth1 show "Link detected: no", now on B :ping 192.168.160.11 and 192.168.160.21, both are ok, too. I was confused that things. Google , I has understanded a little . on A , I run "echo 1 >/proc/sys/net/ipv4/conf/all/arp_ignore",then on B , ping ping 192.168.160.11 and 192.168.160.21, both still are ok Can you give me advise ? What will I do for on B pinging 192.168.160.21 is not ok?