cent OS 6.3 : eth0 shows ip but does not ping to other machines

pankajd · 11-10-2012, 07:09 AM

hi,
i have cent os 6.3 (2.6.32-279.el6.x86_64 #1 SMP), 64 bit AMD machine. It has 2 ethernet interfaces and 1 dual port ib card. initially eth0 had 192.168.16.61 as ip. we then changed it to 172.28.2.61 and ifconfig show the new ip address. Now if we try to ping any other machine of 172.28.2.0 network, it does not ping. tcpdump -i eth0 shows that it still tries to contact 16.0 network machines.
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tcpdump -i eth0
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
18:35:22.595192 IP 192.168.16.37.mdns > 224.0.0.251.mdns: 0 SRV (QM)? CanonMF4500w._http._tcp.local. (47)
18:35:22.906188 IP 192.168.16.6.mdns > 224.0.0.251.mdns: 0*- [0q] 1/0/1 (Cache flush) SRV Canonb0480b.local.:80 0 0 (89)

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There is no iptables/selinux. eth1 is 172.29.2.671 which is working absolutely fine.

#ifconfig eth0

ifconfig eth0
eth0 Link encap:Ethernet HWaddr 00:25:90:58:FB:F6
inet addr:172.28.2.61 Bcast:172.28.3.255 Mask:255.255.252.0
UP BROADCAST MULTICAST MTU:1500 Metric:1
RX packets:109510 errors:0 dropped:0 overruns:0 frame:0
TX packets:774 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:14912536 (14.2 MiB) TX bytes:47923 (46.7 KiB)
Memory:dfd60000-dfd80000

ping 172.28.2.200
PING 172.28.2.200 (172.28.2.200) 56(84) bytes of data.
From 172.28.2.61 icmp_seq=2 Destination Host Unreachable
From 172.28.2.61 icmp_seq=3 Destination Host Unreachable

what can be the issue?

i have checked the cable and switch port, both are good.

ggeminii · 11-10-2012, 11:47 AM

I guess your routing table is not getting updated.
Make sure "route -n" shows the right route.
If it doesn't then manually add the route so that eth0 is used for 172.28.2.0 network.
Do something like this
1.sudo route del -net 172.28.2.0 netmask 255.255.0.0
2.sudo route add -net 172.28.2.0 netmask 255.255.0.0 metric 0 dev eth0.
I am not sure what your netmask should be.Replace it with what is right for you.Also lower metric value means higher preference for using eth0 over eth1.