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

Output interfaces are "Null" in the following cases:
  1. The packets are blocked by an ACL.
  2. 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.
  3. Multicast traffic.
  4. Packets destined for the router.
  5. Tunnels (IPIP, GRE, IPSEC, L2TP) & WCCP.
  6. Static route to null0 - e.g. ip route 10.1.0.0 255.255.0.0 Null0
  7. Dropped by QoS.