Show Policy-Map

[leepeiwai]

Hi,

I have applied QoS on the ATM interface hoping to improve the VoIP calls quality.

interface ATM0.1

service-policy input QoS-PM-Voice

!

ip access-list extended QoS-ACL-Voice

permit ip 0.0.0.0 0.0.0.0 123.123.123.0 255.255.255.0

!

class-map match-all QoS-CM-Voice

match access-group name QoS-ACL-Voice

!

policy-map QoS-PM-Voice

class QoS-CM-Voice

  set dscp ef

123.123.123.0 is the PBX hosted externally.

After few days, I get some counters from show Policy-map:

Router887W#sho policy-map int atm0.1

ATM0.1

  Service-policy input: QoS-PM-Voice

    Class-map: QoS-CM-Voice (match-all)

      0 packets, 0 bytes

      5 minute offered rate 0 bps, drop rate 0 bps

      Match: access-group name QoS-ACL-Voice

      QoS Set

        dscp ef

          Packets marked 0

     Class-map: class-default (match-any)

      15623489 packets, 15318282698 bytes

      5 minute offered rate 506000 bps, drop rate 0 bps

      Match: any

  Service-policy output: QoS-PM-Voice

    Class-map: QoS-CM-Voice (match-all)

      126696 packets, 37368476 bytes

      5 minute offered rate 0 bps, drop rate 0 bps

      Match: access-group name QoS-ACL-Voice

      QoS Set

        dscp ef

          Packets marked 126696

     Class-map: class-default (match-any)

      12328457 packets, 2569388096 bytes

      5 minute offered rate 87000 bps, drop rate 0 bps

      Match: any

Questions:

1. Why there is no counter on for the Service-policy Input?

2. Class-map: QoS-CM-Voice: The counter for total matched packes is equal to the Packets marked (126696).

Does it mean the interface is heavily congested? What can be done to optimise?

Cheers,

Pei Wai

.

[John Blakley · Accepted solution]

Pei Wai,

Since the PBX is hosted externally, the acl that you've applied seems to be marking anything going to that subnet. I'm assuming that 123.123.123.0/24 isn't your subnet, but instead the subnet that the phone switch resides on. With that in mind, the policy that's applied inbound is traffic coming from the outside into your router. A policy that's applied outbound is applied to traffic leaving your router. With that in mind, the acl that you could apply for inbound traffic could be:

access-list 123 permit ip 123.123.123.0 0.0.0.255 any

Then you should start matching packets inbound. For your second question, the packets will always be marked as I don't believe this is a function of congestion by simply marking. The router doesn't know if the packet may be dropped later down the path so it's going to mark in times of non-congestion as well.  From the looks of it, you're not prioritizing anything in your policy which may help. The only problem that you might have is with your phone switch being hosted elsewhere. If you don't control both ends, or they've not set qos on their side, you're going to have a hard time getting great quality but you should be able to get acceptable. Try applying this policy:

class-map match-all QoS-CM-Voice

match access-group name QoS-ACL-Voice

!

policy-map QoS-PM-Voice

class QoS-CM-Voice

priority percent 5 

set dscp ef

class class-default

fair-queue

You may need to play with the percentage for the LLQ, but overall this is a decent start. The policy is now telling the router to give voice packets priority and set the dscp marking to EF. The phone system provider on the other end should be matching EF inbound and giving it priority over FTP, Web, etc.

HTH,

John

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[leepeiwai]

Hi John,

Thanks for your reply.

I have tried to read a bit more about LLQ and need your help to explain.

policy-map QoS-PM-Voice

class QoS-CM-Voice

priority percent 5

set dscp ef

class class-default

fair-queue

1. why apply fair-queue on class-default (instead of QoS-CM-Voice)?

2. Use priority instead of bandwidth on QoS-CM-Voice?

3. If I only apply dscp ef (without fair-queue and priority), would the traffic be prioritised?

Cheers,

Hannah

[John Blakley]

1. why apply fair-queue on class-default (instead of QoS-CM-Voice)?

This is for all of your other traffic. Anything that doesn't match your voice class is going to fall into the class default class (unless you plan on creating other classes later on). Turning on fair-queue allows the router to give fair bandwidth usage to each flow coming through the router.

2. Use priority instead of bandwidth on QoS-CM-Voice?

Yes. Priority also polices the traffic and doesn't allow the voice class to take up all of the bandwidth. The voice class would get 5 percent minimum, but also 5 percent maximum. If you were to use bandwidth command, the class would get a guarantee but it wouldn't be capped (unless you put a police command in the same class).

3. If I only apply dscp ef (without fair-queue and priority), would the traffic be prioritised?

    That's going to depend on what's on the other side. This local router wouldn't prioritize it simply because it's just setting the dscp value. You really need to tell it to set aside some bandwidth, with the priority command, for it to be prioritized. Cisco recommends an LLQ for voice.

HTH,

John

[leepeiwai]

Hi John,

When I apply LLQ as below

policy-map QoS-PM-Voice

class QoS-CM-Voice

priority percent 5

I get error:

Low Latency Queueing feature not supported in input policy.

When I apply:

class class-default

fair-queue

I get error:

Flow Fair Queueing feature not supported in input policy.

Please help.

Cheers,

Pei Wai

[John Blakley]

Yes, you'd apply these as output and not input. Try: service-policy output QoS-PM-Voice on the WAN interface.

HTH,

John

[leepeiwai]

Hi John,

I remove input from ATM interface:

interface ATM0.1 point-to-point

no service-policy input QoS-PM-Voice

service-policy output QoS-PM-Voice

But I still get error:

(config)#policy-map QoS-PM-Voice

(config-pmap)#class QoS-CM-Voice

(config-pmap-c)#priority percent 5

Configuration failed!

(config-pmap-c)#class class-default

(config-pmap-c)#fair-queue

Configuration failed!

Please advise

cheers,

peiwai