在我的其中一台路由器上，我看到以下切换模式：

#show fabric switching-mode 
Global switching mode is Truncated
dCEF mode is not enforced for system to operate
Fabric module is not  required for system to operate
Modules are allowed to operate in bus mode
Truncated mode is allowed, due to presence of DFC, CEF720, Standby supervisor module

Module Slot     Switching Mode
1                      Bus
2                 Crossbar
3                     dCEF
4                      Bus
5                      Bus
6                 Crossbar
7                      Bus
8                      Bus
9                 Crossbar

Crossbar、Bus 和 dCEF 切换模式有什么区别？