关于qos带宽限制问题

目前A区地方机房拉了一条专线接入到我们出口路由器，要把我们内网的192.168.14段的带宽出去到A区做qos，使带宽不能少于20M，我做的命令如下：

[H3C]acl basic 2000
[H3C-acl-ipv4-basic-2000]rule 0 permit source 192.168.14.0 0.0.0.255
[H3C-acl-ipv4-basic-2000]quit

[H3C]traffic classifier 1
[H3C-classifier-1]if-match acl 2000
[H3C-classifier-1]quit

[H3C]traffic behavior 1
[H3C-behavior-1]car cir 51200 green pass yellow pass red discard
[H3C-behavior-1]quit

[H3C]qos policy 1
[H3C-qospolicy-1]classifier 1 behavior 1
[H3C-qospolicy-1]quit

[H3C]int gi 1/0/1
[H3C-GigabitEthernet1/0/1]qos apply policy 1 inbound
[H3C-GigabitEthernet1/0/1]qos apply policy 1 outbound
[H3C-GigabitEthernet1/0/1]quit  

我想问一下red当他达到50M时候，做pass和discard有啥区别吗？还有就是只要在出口路由器走qos就行了，还是说从接入-汇聚-核心-路由器这些设备也要做qos ?