Ping Latency Issues on hosts connected through 3550 Cisco Switches

Posted on 2007-10-19
Last Modified: 2010-05-19
Hi All Please observer the following illustration:

Linux Host A ---Path A---Cisco Switch 3550 on Rack A---Gigabit Path B---Cisco Switch 3548 on Rack B---Path C-- Linux Host C

The thing is that I am concerned about the ping latency when pinging from host to host and from host to switches please consider the following :

I used ping -c 100 -s 1024 hostIP to get the data below

Case A : From Host C to Switch B         min/avg/max/mdev = 1.884/2.676/14.752/1.865 ms
Case B : From Host A to Host C            min/avg/max/mdev = 0.825/0.901/1.027/0.046 ms

Note that a ping from Host C to Host A has to travel through Switch B then Switch A and then reach Host A and backwards, while a ping from Host C to Switch B has only to travel to Switch B  and back "the same ethernet segment" so how can it be that that the average ping of case A took more than the average ping of Case B although the round trip time of Case B is shorter and they both travelled through the same paths.

Case C: From Host A to Switch B      min/avg/max/mdev = 2.248/8.730/56.952/13.380 ms

Now in this case the round trip time from Host A to switch B (Case C) is shorter than the round trip time from Host A to Host C (Case B). However Case C has an higher average time than Case B.

How can this be ?

The only thing that I might think about is that it has to have something to do with the CPU of the Switch but i do not know what exactly..

Question by:http://
    LVL 50

    Accepted Solution

    >The only thing that I might think about is that it has to have something to do with the CPU of the Switch

    You're correct. Switches are designed to forward frames as quickly as possible. Since only source and destination MAC addresses checked (by ASIC's), the frames are forwarded very fast.

    When you communicate with the switch itself, it (the CPU) will have to do more work. The CPU's in these switches aren't the optimized for this type of operation.

    Expert Comment

    Is there a VLAN infrastructure and are the Cisco devices and Linux hosts in diferent VLANs?
    LVL 19

    Author Comment

    Nope, default VLAN settings..
    LVL 3

    Expert Comment

    donjohnston is right on. Ping generally should not be used as a tool to measure performance (i.e. response time) reliably, but simply as a tool for verifying reachability which is what ping was created to do.

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