This can happen if you are trying to ping the link-local address (i.e. those starting with FE80). This address has only link-local scope and therefore might not be unique. Therefore the router is not certain on which interface you want to send the ping packets, therefore it asks.
Yes - the link-local addresses are unique on the link they appear on. But a single link-local address can be reused on various physically distinct links. For example, the serial interface do not have a MAC address so for purposes of link-local address creation, the serial interfaces borrow a MAC address from another interface in the router. I haven't done thorough tests but it seems that often the MAC address of the Fa0/0 interface is used (anyone having better experiences please correct me here!).
So this is the first issue about link-local addresses - while they are unique on a link, they might not be unique among multiple links.
Further on, the link-local addresses do not use any net/subnet semantics. They can't really be found in a routing table. Your router does not know when you ping a particular link-local address which interface should it use. The only way to determine it is to ask you. And so it does :)
We are pleased to announce availability of Beta software for 16.6.3.
16.6.3 will be the second rebuild on the 16.6 release train targeted
towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are
looking for early feedback from customers befor...
Introduction Featured Speakers Luis Espejel is the Telecommunications
Manager of IENova, an Oil & Gas company. Currently he works with Cisco
IOS® and Cisco IOS XE platforms, and NX to some extent. He has also
worked as a Senior Engineer with the Routing P...
In this session you can learn more about Layer 3 multicast and the best
practices to identify possible threats and take security measures. It
provides an overview of basic multicast, the best security practices for
use of this technology, and recommendati...