vlib_trace_buffer (vm, node, next_index, b, /* follow_chain */ 0);
tr = vlib_add_trace (vm, node, b, sizeof (*tr));
tr->next_index = next_index;
vlib_trace_buffer (vm, node, next_index, b, /* follow_chain */ 0);
tr = vlib_add_trace (vm, node, b, sizeof (*tr));
tr->next_index = next_index;