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Re: QoS and Class-map

Hi,

I would like to confirm if I have a policy-map as shown below and if the voice traffic has reached 19 percent of the bandwidth and there is additional voice traffic coming in, these access voice traffic would be dropped isn't it ?

policy-map TEST

class VOIP_Signalling

set dscp af31

bandwidth percent 6

class VOIP_RTP

priority percent 19

set dscp ef

class DEFAULT

set dscp default

Pls advice,

Regards,

InternetB.

1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

Re: Re: QoS and Class-map

Hello,

Yes - your VOIP_RTP class is both guaranteed to receive 19% of the bandwidth, and at the same time, it is policed to the same amount. No more than 19% of the interface bandwidth may be used by the VOIP_RTP class and the exceeding traffic will be dropped.

Best regards,

Peter

5 REPLIES
Cisco Employee

Re: Re: QoS and Class-map

Hello,

Yes - your VOIP_RTP class is both guaranteed to receive 19% of the bandwidth, and at the same time, it is policed to the same amount. No more than 19% of the interface bandwidth may be used by the VOIP_RTP class and the exceeding traffic will be dropped.

Best regards,

Peter

Super Bronze

Re: Re: QoS and Class-map

"I would like to confirm if I have a policy-map as shown below and if the voice traffic has reached 19 percent of the bandwidth and there is additional voice traffic coming in, these access voice traffic would be dropped isn't it ? "

Not always. The implicit policer only activates when the LLQ contains packets. So it's possible to have LLQ traffic beyond the implicit policed value if there's no congestion. For example, you could have 50% LLQ traffic with another 20% other traffic even with your policy. If you need to guarantee that the LLQ traffic won't ever exceed a certain bandwidth %, you need to add an explicit policer.

Cisco Employee

Re: Re: QoS and Class-map

Joseph,

Thanks again! I'm not sure if these things are written in books. You've obviously got large experiences with the QoS on Cisco boxes. Hat off.

Best regards,

Peter

Super Bronze

Re: Re: QoS and Class-map

Peter,

This information, I believe, is Cisco documented, but you really have to dig to find it. Actually, if I remember correctly, I thought the same as you until someone else documented this on these forums (where I also find other interesting tidbits from time to time). I then confirmed it by searching Cisco documentation, although I've never actually tested it in a lab.

You're correct, I do have lots of experience with QoS on Cisco boxes and QoS knowledge in general, as I'm often involved in obtaining best network performance across WANs.

Cisco documentation is probably the best, by far, put out by any network vendor. Although, you sometimes have to wade through more than just their primary QoS docs, such as their whitepapers, technotes, and troubleshooting papers. If you do that, you do learn much about what Cisco platforms support, although even then there are some gaps. (I've thought the additional QoS features available on the old 7500s or the FlexWANs or the SIP-200/400 were not really highlighted well. Pretty nice features, as is the new HQF starting with 12.4.20T.)

Again, though, even with all the above, I believe I too discovered this tidbit on these forums.

Cisco Employee

Re: Re: QoS and Class-map

Joseph,

It's great to have you here.

Best regards,

Peter

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