[nsp] RE: No replies on alternate packets of traceroutes

Bruce Pinsky bep at whack.org
Mon Nov 17 15:12:19 EST 2003

Hash: SHA1

Kashif.Khawaja at Broadwing.com wrote:

| Hi All,
| I was wondering if somebody could shed light on whether it is normal
behaviour to not see
| replies on alternate packets in traceroutes from one cisco router to
another e.g.
| Lab#trace ip
| Target IP address:
| Source address:
| Numeric display [n]:
| Timeout in seconds [3]:
| Probe count [3]: 10
| Minimum Time to Live [1]:
| Maximum Time to Live [30]:
| Port Number [33434]:
| Loose, Strict, Record, Timestamp, Verbose[none]:
| Type escape sequence to abort.
| Tracing the route to L0.lab (
|   1 P4-0.lab ( 0 msec *  0 msec *  88 msec *  60 msec *  0 msec *
| I have verified that layer one is clean. Pings are fine. No drops there.
No particular
| process is shooting up really high. About the only one I see is the IP
Input process
| showing some activity but I guess that is to be expected since these
packets are probably
| being Process switched. Basically, there are a few stable BGP sessions on
each router and
| not a lot of traffic passing through the link or the routers.
| Any pointers (even to documents) would be helpful!

By default, cisco routers restrict the number of ICMP responses per second.
~ IIRC, unreachables are throttled at two per second.  In some versions, you
can disable or adjust the throttling with the command "no ip icmp
rate-limit unreachable"

R1(config)#ip icmp rate-limit unreachable ?
~  <1-4294967295>  Once per milliseconds
~  DF              code 4, fragmentation needed and DF set

However, keep in mind that this throttling is there to help protect against
DOS attacks so be careful if you tinker.


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