Traffic shaping on demand

Unanswered Question
Aug 28th, 2010

Hello to all,

I am trying to configure a QoS for my WAN.  I have a cisco 881 router with advanced ip services.  My problem is my wan comes from a cable modem which the router connects via a fast ethernet port.  Also, the ISP provider guarantees a minimum upstream bandwidth of 64kbs when congested and up to 512kbps when network is available.

I am interested in enforcing traffic shaping to 96kbps only when an rtp stream is active.  If there are no rtp packets flowing then I'd like the cable modem to take care of the upstream queues.

I am trying with this configuration but it is locking my upstream to 96kbps at all times..

class-map match-any voice-traffic
match  dscp ef
class-map match-any voice-signal
match  dscp af31
policy-map voice-policy
class voice-traffic
  priority 36
class voice-signal
  bandwidth 8
class class-default
policy-map shaper
class class-default
  shape average 96000 9600 0
  service-policy voice-policy
interface FastEthernet4
service-policy output shaper

I am a CCNA student so your help would be highly appreciated.



I have this problem too.
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kyukim Sat, 08/28/2010 - 11:37

Hi, Ricardo.

Currently, no "traffic shaping on demand" is possible.

With your current configuration, it will shape traffic to 96kbps always.


ricardo.viteri Sat, 08/28/2010 - 12:22

Is there a way to achieve this with another configuration?

My problem is that if I enforce 96kbps at all times, I start getting high latency on other services such as web pages and such and my WAN download speed (4mbit) degrades to 1 - 2.5 mbit/sec.

gatlin007 Sat, 08/28/2010 - 12:54


What is your purchased rate (CIR) from the provider in regard to upload; transmit capability?  Has the provider guaranteed you a rate for DSCP EF traffic within the purchased rate?  Some would refer to this a gold CAR.



ricardo.viteri Sat, 08/28/2010 - 13:07

Hello Christopher,

I am setting this up for a SOHO, they have a regular Cable Modem connection.  The ISP provider advertises a 4000kbps/768kbps 8:1 ratio (basically 8 customers on the same line).

This means that at peak hours of congestion the ISP WAN connection can do no less than 500kbps/96kbps.

Thus, I'd like to find a way to only enable shaping when they have a VoIP call (codec g729) is on so I can use whatever bandwidth is available from the ISP but guarantee quality for the VoIP call.

I don't know if it is possible to shape the upload rate to 96kbps at all times without the download speed suffering too much (i.e. not having 2000ms+ of latency when surfing the web, ssh conncetions, vnc, etc.) with another QoS configuration.


gatlin007 Sat, 08/28/2010 - 13:50


frame-realy has something called FECN (forward explicit congestion notification) that informs the router of congestion up ahead and a MINCIR value can then be used by the router to properly shape and queue.  Kinda like when the department of transportation sign tells us about the wreck 10 miles ahead.

Unfortunately Ethernet between a cable modem and a router doesn’t have this mechanism.

You appear to be stuck with the approach you are taking.  Trying to find a shaper value that doesn’t leave to much bandwidth on the table yet ensures some kind of VoIP reliablity.

As I suspect you already know, If you want reliable egress queuing you'll need to purchase a circuit with a service provider guaranteed transmit rate (CIR).

In regard to oversubscribed cable modem circuits, they have an attractive price point but are always troublesome for priority data traffic or any kind of VoIP reliability.  It only takes an upstream neighbor to download a movie and a downstream or peer VoIP call suffers.  Something else to consider in regard to commodity internet circuits for priority business data applications is the issue of 'net neutrality'


ricardo.viteri Sat, 08/28/2010 - 14:06

Hi Chris,

I am aware about frame relay mechanisms to handle congestion.  It seems that the only solution could be to pursue a dual wan set up with a line for VoIP.  Unfortunately, dedicated circuits in this location are about $500.00 for 1mbit/1mbit, the amount of voip traffic that will pass through is minimal and will not justify the expense for now.

Do you know any guides to perform a Dual Wan setup?

I am stuck with a cisco 881, it has 1 FA4 port for WAN and 4 (FA 0 - 3) ports for a managed switch.

I tried to create virtual interface say "vlan 5" with "ip address dhcp" and on port FA0 "switchport access vlan 5" but as soon as I connect to the second provider and get an ip address I lose connectivity to the internet.

Any ideas?




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