When I run show ip flow top-talkers I see null destination interface. What does null interface mean?

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Output interfaces are "Null" in the following cases:

The packets are blocked by an ACL.
The packets are process switcheded. This could be because the feature is not supported
by CEF or fast switching. Do a "show int stats" to distinguish between data being process switched instead of cef/fast switched. Also, check "show cef interface" and "show ip interface" to check if CEF is disabled or packets are being punted/dropped due to an unsupported feature configured.
Multicast traffic.
Packets destined for the router.
Tunnels (IPIP, GRE, IPSEC, L2TP) & WCCP.
Static route to null0 - e.g. ip route 10.1.0.0 255.255.0.0 Null0
Dropped by QoS.

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