qos - priority percent question

dahua.huang · ‎03-30-2012

Hi, all cisco tech

one thing bother me for a while.

we have two point to point routers with a OC3 link and do not know who configured it.

policy-map WAN-EDGE

class AAA

priority percent 65

class class-default

fair-queue

queue-limit 1000

interface POS1/0

ip route-cache flow

service-policy output WAN-EDGE

hold-queue 1000 out!

*************

#show policy-map interface POS1/0
POS1/0

Service-policy output: WAN-EDGE

    Class-map: AAA (match-all)
      60 packets, 3144 bytes
      5 minute offered rate 0 bps, drop rate 0 bps
      Match: access-group name brocade
      Queueing
        Strict Priority
        Output Queue: Conversation 264
        Bandwidth 65 (%)
        Bandwidth 100750 (kbps) Burst 2518750 (Bytes)
        (pkts matched/bytes matched) 5/520
        (total drops/bytes drops) 0/0

    Class-map: class-default (match-any)
      52048631371 packets, 10961955420222 bytes
      5 minute offered rate 3196000 bps, drop rate 0 bps
      Match: any
      Queueing
        Flow Based Fair Queueing
        Maximum Number of Hashed Queues 256
        (total queued/total drops/no-buffer drops) 0/4378877/1115152

Both side with same policy.

Here is my question.

Looks class AAA in LLQ with 65% bandwidth. and others using 35%.

1. Can regular traffic over 35% during high traffic ( class AAA in low traffic and not over 65%)

2. Class AAA with 65% means reserved and no one can using it? ( looks same question with one).

3. does queue-limit 1000 will cause delay becuase of hold too long.

Thanks.

Vasileios Bouloukos MSc, ITIL, CCIE · ‎03-30-2012

Hi dahua,

You use QoS LLQ (configured with priority). The Class A has been configured with the priority command. The bandwidth is not reserved but can be used by the other classes when there is not congestion and Class A needs >65% bandwidth.

However, during congestions the traffic of CLASS A is policed to 65% and do not exceed this value in order to not saturate the link only the CLASS A traffic.

Yes the default class can exceed the 35% if the Class A is not congested and there is not need to use all the 65% of the bandwidth.

Finally, regarding the hold-queue for most other interfaces, queue length must not exceed 100.

The queue length value can be between 0 and 4096. The default value is 75

You can find detailed description to the next link

http://www.cisco.com/en/US/products/hw/routers/ps133/products_tech_note09186a0080094791.shtml

Hope that helps!

Vasilis

dahua.huang · ‎03-30-2012

thank you and it cleared my concerns.

Joseph W. Doherty · ‎03-30-2012

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Posting

"3. does queue-limit 1000 will cause delay becuase of hold too long."

That depends on multiple factors, including what's "too long", bandwidth, and queuing being used.

Since you're using FQ in class-default, the 1000 queue-limit would be of most concern in how long individual flow queues might be. Since queue-limit is in packets, depends on packet sizes.

For example, 1,000 64 byte packets would take only about 3 ms on an OC3; and about 77 ms for 1500 byte packets. Depending on your BDP, and since worst case is under 100 ms, with FQ 1,000 may be a non-issue.