On some networks when you do traceroute, some of
the intermediate routers will not appears,
becuase they wont send the ICMP Time Exceeded
packets back at you, these I beleive are referred
to as "black hole". They still route these packets
As to grey holes, it is the first time I encounter this term.
So I did a search on Google, and here is what I found (different
meaniong of black-hole):
MTU via PMTU Discovery
Most commonly deployed method
Defined in RFC 1191
'Black Hole Router' is a device in a path that does not send a "packet too
big" ICMP message when required
'Grey Hole Router' is a device in a path that send the wrong MTU size in the
"packet too big" ICMP message
Negotiation failure causes some applications to run slowly, or not at all.
Most applications continue to run.
For more information on MTU download the MTU whitepaper at
> Some Jalaam marketing material (www.jaalam.com) sez that their
> appareNet (tm) NMS product can detect "Black-Hole / Gray-Hole Hops".
> I've never heard this terminology before. I assume a black-hole hop is a
> packet that get's dropped and instead of being forwarded by a router?
> But what would a gray-hole hop be?