TRACING NETWORK PACKETS AT A MANAGED FORWARDING ELEMENT

A process is performed by controller cluster 2035 which receives trace requests from a user and generates trace packets. A command is received (1) to insert a test packet marked for a trace operation with specified source and destination addresses on a set of logical forwarding elements 2090, 2092 into the physical network implementing the logical forwarding elements. The request specifies a source logical port that maps to a VM connected to the MFE 2090 and a destination logical port that maps to a VM connected to another MFE (e.g., MFE 2092). In response to the request, the controller cluster 2035 generates a packet 2002 and sends (2) this to the MFE 2090. When the MFE 2090 receives the packet 2002, the MFE 2090 stores a register bit indicating that the packet is marked for a trace operation. The MFE 2090 then performs an ingress context mapping 2005 on the packet. In the second stage 2010 of processing pipeline 2000, a set of logical operations are performed on the packets by the flow entries that implement logical switch 2045 at the MFE 2090. In the third stage 2015, a set of logical forwarding operations are performed on the packets by the flow entries that implement logical router 2055 at the MFE 2090. The MFE 2090 sends (4) an observation for each of the L3 operations of the logical switch 2045. In the fourth stage 2020, an additional set of logical forwarding operations are performed on the packets by the flow entries that implement logical switch 2050 at the MFE 2090. The MFE 2090 sends (shown by the encircled 5) an observation for each of the L2 operations of the logical switch 2050. At the sixth stage 2025, a set of operations to implement physical forwarding of the packet is performed. The MFE 2090 sends (6) one or more observation messages to the controller cluster 2035 for the set of physical operations performed at the fourth stage 2040 when the register bit indicates that the packet is marked for the trace operation.