ONT and the Bandwidth command

TheBlueRaja

Hi All,
I need to clear something up in my head as the cisco Student material is quite frankly shocking sometimes and leaves a lot of questions unanswered / unclear.

Im currently studying for the ONT (Passed the BSCI (Brutal), BCMSN (easy IMO) and ISCW (which was bloody harder than i thought it was going to be) so im on the home run - YAY!

Im looking at the section on CBWFQ and LLQ and talking about bandwidth on the interface and in Policy-maps.

Now i may be going a bit beyond ONT here but the question popped in my mind while studying and now i got to know.

Where i work we use VoIP, lots of it, some site have 2000+ users and some are really small.

On the smaller sites, there is a router (which we don't manage) with a Layer 3 switch behind it (dont ask) that connects to the router via a Gig or Fast Ethernet connection.

Now the router (crappy small Nortel thing) may only have a 2 or a 4mbps connection behind it to the WAN, so say we wanted to tightly control the flow of traffic from the switch to the router to use the L3 switch as a method of limiting the flow of traffic to the WAN rather than the router. That way im letting the Cisco L3 switch dictate the flow to the WAN link rather than the router and the router will never need to use the software queue.

So (if your still with me), say i use the bandwidth command on the outgoing interface, which for the sake of argument is F0/1 and is 100/Full.

So

int f0/1
bandwidth 4000000
exit

Now all im doing here is instructing the interface that it only has 4Mbps available but this does not shape or limit any traffic its only used by routing protocols during weighting.

So using this command to manage the actual link speed is useless.

So, classification, (now for the ONT stuff). Lets say i do the following:-

class-map VOICE
match DSCP 46
match DSCP 26
exit

#Match DSCP values 46 (or EF for voice packets) and 26 (or AF31 for signalling)

Policing

policy-map VOIP
class VOICE
priority percent 33
exit

#Match class voice packets and allocate 33 percent of available bandwidth (so 33% of 75% of total bandwidth - (75% being the total bandwidth configurable on a link)) in the LLQ.

Implementing

int fa0/1
service-policy out VOIP
exit

#use the policy-map on the "WAN" interface.

Now from what i understand this will allocate 33% of the available bandwidth to packets matching this policy-map BUT.....

What is the total bandwidth that the interface thinks it has? How do i configure this? How do i tell it that even though its a 100/Full link that in fact there is only a 4Mbps connection behind it?

Would in this case using the bandwidth command on the interface allow the software queue to recognise that it has less than 100/Full available?

Or is there something im missing?

Thanks

networker050184

You would need to use a parent shaper to 4 mbps and then put your policy map as the child.

One thing to keep in mind is the default behavior of a class-map. The way you have your VOICE class now its not going to match anything.