DUP MESSAGE

[ray_stone]

Hello Experts:

Our server backup team is reporting an issue of performance with Backup Server placed on V-lan 50 (192.168.1.1/24) as it's taking a backup of another server placed on other Vlan 100 (192.168.2.1/24) and please note both V-lan being managed by same Cisco l3 switch. For further troubleshooting, I asked to provide the ping and trac route output from backup server to the other server and we are recieving DUP message as you can see in the logs given below:

It's a UNIX Server.

PING testing.testing.com: (192.168.2.20): 56 data bytes
64 bytes from 192.168.2.20: icmp_seq=0 ttl=63 time=0 ms
64 bytes from 192.168.2.20: icmp_seq=0 ttl=63 time=0 ms (DUP!)
64 bytes from 192.168.2.20: icmp_seq=1 ttl=63 time=0 ms
64 bytes from 192.168.2.20: icmp_seq=1 ttl=63 time=0 ms (DUP!)

----testing.testing.com PING Statistics----
17 packets transmitted, 17 packets received, +16 duplicates, 0% packet loss
round-trip min/avg/max = 0/0/0 ms

Can anybody advice on this. there is no CRC or input errors found and connection is also ok if i check via switch for servers. what is a next step as it seems to have some issue of looping so help me for further investigation.

Thanks.

[Peter Paluch]

Ray,

Receiving duplicates is quite uncommon. Usually, they signify an IP address conflict (two nodes sharing the same IP address), inappropriate retransmissions at the link layer (even possible loops), perhaps issues with the active/backup configuration of these servers.

The first thing I would try is to use the arping UNIX utility to find out if two or more stations in the network share the same IP. The arping sends out ARP Request packets and awaits responses. If two or more different MAC addresses are displayed in the output then it means that more than one station is considering the pinged IP address as its own.

If only one station responds then I would personally follow the path from the pinging server towards the 192.168.2.20 and check each cable and each device on the path for any abnormal behavior or configuration. Duplicate packets do not have a single reason for occurence so we are looking for anything that could cause this behavior. Look for spanning tree issues, flapping ports or duplicate MAC addresses being reported, EtherChannel misconfigurations, ... it is hard to enumerate, considering the plethora of diverse technologies you may be using. You will need to perform quite a thorough check.

Best regards,

Peter