Pings erratic on routers back to back via ethernet yet no line errors

Posted on 2006-05-08
Last Modified: 2010-04-17
How can I isolate / identify the cause of why Pings between to routers spike from 1 ms to 17ms (or sometimes dropped)? The Fast ether interfaces on both routers show no errors, drops, etc? What are the possible causes to look for consider? Both routers are doing OSPF and QOS (but not on this interface). These are Avaya Cajun routers but use many of the Cisco commands.
Question by:murphymail
    LVL 28

    Accepted Solution

    Are they half duplex or full duplex? Does every interface match? Half duplex means occasional collisions which will slow things down in a random way, depending on load. Mismatches will cause lots of collisions with negative results.
    LVL 12

    Expert Comment

    Rule #1: don't ping routers.
    Rule #2: if you insist on breaking rule #1, don't use the results as a measure of performance.

    Ping a host just beyond the router (from a host just before the router, or at least from a host), and see what you get.

    Routers are made to FORWARD traffic at layer 3.  They aren't made to PROCESS traffic at layer 7, at least at line rate and/or in the same manner that it's forwarding traffic.  If the CPU is busy doing something else (routing update, periodic task, whatever), it'll be slow to answer a ping.  Oh well, nothing to worry about.

    The traceroute utility at, for example, looks at per-hop latency.  If hop N shows lower latency than any previous hop, the utility automatically goes back and lowers that previous hop's RTT to reflect the RTT to hop N, since clearly the previous hop is forwarding traffic at least that quickly.  This is proof that pinging a router or tracerouting to it is NOT a good measure of performance.

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