If you are playing around with CBWFQ you might have discovered that it’s impossible to attach a policy-map to a sub-interface directly. There is a good reason for this and I’d like to show you why this occurs and how to fix it. This is the topology I will use to demonstrate this:
Just two routers connected to teach other using frame-relay. We will try to configure CBWFQ on the Serial 0/0.1 sub-interface of R1.
Configuration
First i’ll create a simple CBWFQ configuration:
R1(config)#class-map TELNET
R1(config-cmap)#match protocol telnet
R1(config)#class-map HTTP
R1(config-cmap)#match protocol http
R1(config)#policy-map CBWFQ
R1(config-pmap)#class TELNET
R1(config-pmap-c)#bandwidth percent 10
R1(config-pmap-c)#exit
R1(config-pmap)#class HTTP
R1(config-pmap-c)#bandwidth percent 20
R1(config-pmap-c)#exitNothing special here…just a simple CBWFQ configuration that gives 10% of the bandwidth to telnet and 20% to HTTP traffic. Let’s try to apply it to the sub-interface:
R1(config)#interface serial 0/0.1
R1(config-subif)#service-policy output CBWFQ
CBWFQ : Not supported on subinterfacesToo bad, it’s not gonna happen…IOS has a day off. There is a workaround however…we can’t apply it directly, but if we use a hierarchical policy-map it will work. Let me show you what I mean:
R1(config)#policy-map PARENT
R1(config-pmap)#class class-default
R1(config-pmap-c)#service-policy CBWFQI’ll create a policy-map called PARENT that has our service-policy attached to the class-default class. Now let’s try to attach this to the sub-interface:
R1(config)#interface serial 0/0.1
R1(config-subif)#service-policy output PARENT
CBWFQ : Hierarchy supported only if shaping is configured in this classIOS is still complaining, it only allows a hierarchical policy-map when shaping is configured. Let’s give it what it wants: