Help! Need quick QoS Sanity Check

Unanswered Question

Since implementing QoS on a T1 link it seems that the utilization has dropped ... a lot, and I need a quick review to make sure I didn't fat-finger or do something dumb (it happens).


The interface is a full T1 (private point-to-point), which should be 1.544Mbps = 1544Kbps so for the interface I have specified:

!

interface Serial0/1/0:0

bandwidth 1544

max-reserved-bandwidth 100

ip address 192.168.16.1 255.255.255.252

ip directed-broadcast

ip nbar protocol-discovery

encapsulation ppp

ip tcp header-compression iphc-format

service-policy output PtP-Edge

ip rtp header-compression iphc-format

!


!

a 'sh int' results in:

MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,


My policy map is:

class-map match-any Database

match access-group name Database_List

match dscp af21

class-map match-any Voice

match protocol rtp audio

match access-group name VoIP_List

match dscp ef

match access-group name Video_Conference

class-map match-any Signaling

match dscp cs3

match protocol rtp

match dscp af31

match access-group name VoIP_Control

!

!

policy-map PtP-Edge

class Voice

priority percent 33

class Database

bandwidth percent 37

random-detect

class Signaling

bandwidth percent 5

class class-default

bandwidth percent 25

random-detect


Did I miss something major in here? my tx/rx load usually was pegged before, and now is only:

reliability 255/255, txload 2/255, rxload 6/255


  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Loading.
Joseph W. Doherty Fri, 09/12/2008 - 11:40
User Badges:
  • Super Bronze, 10000 points or more

Why max-reserved-bandwidth 100 ?


Were you also using the header compression options before application of the policy?

max-reserved-bandwidth 100 so that I can explicitly define and control the 'class-default' setting. I tried to follow the QoS SRND pretty closely on regards to the setup.


The IP header compression was applied before the policy was implemented ... I added the RTP header compression afterwords since it was previously unavailable on the old IOS version the routers were running before being upgraded. It is a private link, and we control the routers on each side.


Joseph W. Doherty Fri, 09/12/2008 - 16:08
User Badges:
  • Super Bronze, 10000 points or more

Can you provide an explicit reference where setting max-reserved-bandwidth is recommended (not just possible). Reason I ask, about all the Cisco references I recall having seen often seem to advise caution, such as "If you want to override the fixed amount of bandwidth, exercise caution and ensure that you allow enough remaining bandwidth to support best-effort and control traffic, and Layer 2 overhead."


I wouldn't think max-reserved-bandwidth is the problem, although I'm a bit concerned about setting it to 100%. You might try setting it back to the default and see if the behavior changes.


With regard to the two header compression options, I have a hazy recollection they might not be recommended for full T-1 bandwidth or better. These too, I would be surprised if the cause of your issue.


Other than the two prior concerns, again both of which I don't know are causing your issue, yet both are unusual, I don't see anything in your policy or policy stats that should cause such a reduction in performance.


You might try a policy like this:


policy-map PtP-Edge


class Voice

priority percent 33


class class-default

fair-queue


If you really feel the need to treat Signaling or Database special, for either, increase their IP precedence value (I believe FQ within CBWFQ class-default is WFQ). Or, for Signaling, add it to the LLQ class.


PS:

Was the interface using WFQ or FIFO before you applied the policy?

Actions

This Discussion