Refusing Pings on Core Routers??? A new trend?

Rubens Kuhl Jr. rubensk at
Fri Oct 20 02:14:07 UTC 2006

> template response -- I hear is "Well, you can't rely on traceroute
> because of ICMP prioritisation".  When you start to explain how
> traceroute actually works (both ICMP-based and UDP-based (which
> still relies on ICMP responses, of course!)), and that ICMP prio
> should only affect the IP of which the router listens on (and not
> hops beyond or at the dest), most NOCs fire back with another

If I recall well, Cisco GSRs impose low priority and/or limits for all
ICMP traffic flowing thru the box, not just packets to/from router
itself, and there's not a knob to adjust that.

Also of notice is that packets that expire TTL needs some kind of
low-path processing, and will be subject to increased latency or loss
compared to normal ones, and this affects every tool to trace packets
thru the network I've seen.

More information about the NANOG mailing list