QoS Newbie

Answered Question
May 28th, 2008
User Badges:

All,


Can someone put this into plain english for me?


mls qos map policed-dscp 0 24 to 8

mls qos map cos-dscp 0 8 16 26 32 46 48 56


Thanks


Correct Answer by Chris Deren about 8 years 11 months ago

This is the untrusted scavanger model as discussed in the QoS SRND. Basically it allows only 128 kbps for voice traffic (RTP) per switchport, which is perfect unless you are using Barge feature, as G711 codec will never use this much badnwidth anyway, so the idea here is to protect the network from possible worms, etc simulating voice traffic. Anything above 128 K will be dropped, similarly with the signaling traffic, IPCC desktop, etc where exceeding traffic will be re-marked to the DSCP 8 value. Then on your WAN interfaces you probably have a policy that (LLQ) that matches traffic with dscp of 8 and gives it very small bucket, so that it does not starve out other traffic crossing the link.


HTH, please rate all useful posts!


Chris

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (3 ratings)
Loading.
Chris Deren Wed, 05/28/2008 - 09:43
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,
  • Cisco Designated VIP,

    2017 IP Telephony, Contact Center, Unified Communications

This is just a part of full QoS configuration.

The first line re-marks all packets with DSCP value of 0 or 24 to 8, you still need to apply a policer to a port in order for this to take affect, so if based on your policy a packet needs to be remarked it will be remarked from 0 or 24 to 8, this is to create a scavanger class for further limiting.


The second line maps L2 COS markings to L3 DSCP vlaues, this will map the following:

COS DSCP

0 --> 0

1 --> 8

2 --> 16

3 --> 26

4 --> 32

5 --> 46

6 --> 48

7 --> 56


Check out QoS SRND at www.cisco.com/go/srnd for more details


HTH,


Chris

jgorman1977 Wed, 05/28/2008 - 09:58
User Badges:

Ok,


Here is our policy/class:


service-policy input IPPHONE+PC-BASIC

srr-queue bandwidth share 1 25 70 5

srr-queue bandwidth shape 3 0 0 0

priority-queue out


policy-map IPPHONE+PC-BASIC

class VVLAN-VOICE

set dscp ef

police 128000 8000 exceed-action drop

class VVLAN-CALL-SIGNALING

set dscp cs3

police 32000 8000 exceed-action policed-dscp-transmit

class VVLAN-ANY

set dscp default

police 32000 8000 exceed-action policed-dscp-transmit

class AGENT-DESKTOP-TRAFFIC

set dscp cs3

police 256000 8000 exceed-action policed-dscp-transmit

class class-default

set dscp default

police 5000000 8000 exceed-action policed-dscp-transmit


I'm not sure how the qos map works with the policy?


Thanks again.

Correct Answer
Chris Deren Wed, 05/28/2008 - 10:03
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,
  • Cisco Designated VIP,

    2017 IP Telephony, Contact Center, Unified Communications

This is the untrusted scavanger model as discussed in the QoS SRND. Basically it allows only 128 kbps for voice traffic (RTP) per switchport, which is perfect unless you are using Barge feature, as G711 codec will never use this much badnwidth anyway, so the idea here is to protect the network from possible worms, etc simulating voice traffic. Anything above 128 K will be dropped, similarly with the signaling traffic, IPCC desktop, etc where exceeding traffic will be re-marked to the DSCP 8 value. Then on your WAN interfaces you probably have a policy that (LLQ) that matches traffic with dscp of 8 and gives it very small bucket, so that it does not starve out other traffic crossing the link.


HTH, please rate all useful posts!


Chris

jgorman1977 Wed, 05/28/2008 - 10:07
User Badges:

One last question: what happens to a PC/server that is plugged into the switch port with that policy? Does it drop packets over 128k, or since that traffic is on a different vlan, it is safe?


Thanks

Chris Deren Wed, 05/28/2008 - 10:17
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,
  • Cisco Designated VIP,

    2017 IP Telephony, Contact Center, Unified Communications

It will drop the traffic if it arrives marked with DSCP 46 (ef), for regular non marked data traffic it will obey the defaul map, which mean that everything over 5 Mb gets remarked down to scavanger, so if you are transmitting a lot of files on a PC this may cause a bottleneck.


Chris

Actions

This Discussion