• Status: Solved
  • Priority: Medium
  • Security: Public
  • Views: 540
  • Last Modified:

QoS Cisco Routers

Lots of basic questions on QoS...
Does a router only queue and prioritze packets during times of congestion or does applying the service policy to an interface ensure that all data filters through the queue (even if it isn't congested) to make sure all priority traffic is sent first?

What situations would you specify an input policy vs an output policy? Does this apply to packets that switch from one interface to another inside of the router? Example - A packet coming from LAN that is routed out through another interface.

 If you are trying to apply a qos policy on an ethernet interface that is connected at 100Mb but is attached to say Metro Ethernet running at 10Mb. In addition to applying the service policy, Do you need to set the bandwidth lower on the interface (since the router is always going to try to send it 100Mb) for it to effectively prioritze the traffic?

Do routing protocols with precedence 6 and 7 need to be classified or does the router just automatically identify them and send them first?

If I specify to match dcsp cs5 in my class-map is it necessary to also match dscp ef? I'm guessing that cs5 will match anything by the first 3 bits rather than all 6.

  • 5
  • 5
1 Solution
Queuing and prioritizing happens during congestion, policing will happen regardless.

Input policies are for downloading, and output policies are for uploading. Service policies are applied on one interface in one direction. You can have two per interface, one out and one in.

You don't necessarily need to use the bandwidth command for a MetroE 10mb circuit, you could use a parent policy to police/shape to your CIR, then the child policy would contain your queuing and classification.

The router will classify its own routing protocol traffic.

If need clarification or have more questions, ask away!
ntssupportAuthor Commented:
Can you use policing to ensure that all packets that are tagged priority are always sent before any other packets? If so, how?
Well your router will only "prioritize" packets when link is congested. However, if you have some sort of CAR setup on your circuit through your ISP they may have dedicated bandwidth reserved for certain classifications used to guarantee Voice or Video extra priority through their network. Like if it were a P2P or MPLS.
Cloud Class® Course: Certified Penetration Testing

This CPTE Certified Penetration Testing Engineer course covers everything you need to know about becoming a Certified Penetration Testing Engineer. Career Path: Professional roles include Ethical Hackers, Security Consultants, System Administrators, and Chief Security Officers.

Policing won't prioritize, it CAN mark packets to be prioritized by an upstream device, but I don't think that's what you want.

If you want to prioritize, you can use LLQ. But it only works during congestion because that's the only time it's needed. If nothing is congested the packets will be sent as soon as they're received so you don't need to prioritize.
ntssupportAuthor Commented:
Well here is the policy I tried to put in place but I'm still having call quality issues at times. I guess what probably needs to be done is that I just need to reserve bandwidth for my voip calls. Can you reserve bandwidth by matching dscp markings or does it have to be done via an ACL?
class-map match-any voip
 match  dscp ef
 match  dscp cs5
 match  precedence 5
 match  dscp cs3
 match  dscp af31
 match  precedence 3
policy-map qospolicy
 class voip
  priority 2000
 class class-default
! Heres the 10mb 
interface FastEthernet0/1/1
bandwidth 10400
service-policy output qospolicy

Open in new window

Give us the output of:

show policy-map int fa0/1/1 out

We need to see if it is matching properly.
ntssupportAuthor Commented:
here is the output for show policy-map and show queue...If I have the bandwidth set to 10400kb and I am reserving 2000kb where is the rest of my bandwidth going? Available bandwidth shows 5800kb.
Service-policy output: qospolicy

    Class-map: voip (match-any)
      247456 packets, 53050903 bytes
      5 minute offered rate 386000 bps, drop rate 0 bps
      Match:  dscp ef
        234151 packets, 50090374 bytes
        5 minute rate 377000 bps
      Match:  dscp cs5
        219 packets, 111892 bytes
        5 minute rate 0 bps
      Match:  precedence 5
        0 packets, 0 bytes
        5 minute rate 0 bps
      Match:  dscp cs3
        12713 packets, 2590342 bytes
        5 minute rate 2000 bps
      Match:  dscp af31
        373 packets, 258295 bytes
        5 minute rate 1000 bps
      Match:  precedence 3
        0 packets, 0 bytes
        5 minute rate 0 bps
        Strict Priority
        Output Queue: Conversation 264
        Bandwidth 2000 (kbps) Burst 50000 (Bytes)
        (pkts matched/bytes matched) 1/404
        (total drops/bytes drops) 0/0

    Class-map: class-default (match-any)
      693368 packets, 210938567 bytes
      5 minute offered rate 1368000 bps, drop rate 0 bps
      Match: any
        Flow Based Fair Queueing
        Maximum Number of Hashed Queues 256
        (total queued/total drops/no-buffer drops) 0/0/0

Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: Class-based queueing
  Output queue: 0/1000/64/0 (size/max total/threshold/drops)
     Conversations  0/1/256 (active/max active/max total)
     Reserved Conversations 0/0 (allocated/max allocated)
     Available Bandwidth 5800 kilobits/sec

Open in new window

The rest will go to the class-default. Your packets are being matched and forwarded appropriately. So this is for voice traffic? You should be able to use less matching, do you know what the packets are actually marked with?

What issues are you having exactly?
ntssupportAuthor Commented:
That is why I asked the last question on there...
If I specify to match dcsp cs5 in my class-map is it necessary to also match dscp ef or precedence 5? Or is cs5 an all inclusive kind of thing?

I have audible packet loss on calls but I will get it figured out in time as I am just now getting the QoS policy put in place on our network and until I get all of my equipment tagging the packets its hard to say where its coming from.
What kind of router are you running this policy on? You could also match based on protocol, like SIP or H323, or even RTP. This might be beneficial, instead of trying to mark everything.

You should only need to match ef, that should be the marking your phones are using. Here's my QoS policy:

class-map match-any PRIORITY
 description This is all the traffic that gets priority.
 match protocol sip
 match protocol rtcp
 match protocol rtp audio
 match protocol skinny
 match protocol skype
 match protocol dns
policy-map QoS_OUT
    priority 200

ntssupportAuthor Commented:
thanks for the help
Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.

Join & Write a Comment

Featured Post

Free Tool: Subnet Calculator

The subnet calculator helps you design networks by taking an IP address and network mask and returning information such as network, broadcast address, and host range.

One of a set of tools we're offering as a way of saying thank you for being a part of the community.

  • 5
  • 5
Tackle projects and never again get stuck behind a technical roadblock.
Join Now