QoS: Priority & Bandwidth commands

[haji_akhundov]

Hello everyone!

I have the following:

policy-map subnets

class VLAN20

priority percent 20

class VLAN40

priority percent 20

class VLAN60

bandwidth percent 40

class VLAN80

  bandwidth percent 10

interface Serial0/0:0

bandwidth 2000

ip address 172.23.0.1 255.255.255.252

max-reserved-bandwidth 90

service-policy output subnets

VLAN20 and VLAN40 classes are needed to be low-latency because it will be used for VoIP, that's why I used priority commands for LLQ.

I have noticed that during no congestion, the classes that use priority command do not utilize the whole bandwidth, seems like there is a policer. Is that a normal behavior? Can I make it utilize the entire bandwidth when the link is free?

I used iperf to measure. See the attachment for more details. Note that in this case there is no activity in VLAN20 and VLAN80.

Hoping to get some help here.

Thanks. =)

[smitesh kharecha]

Hi Haji,

You have applied two classes to the priority queue. Inside the priority queue it is basically FIFO.

Moreover, you have reserved only 20% on interface bandwidth to be used by Priority Queue, so even if there is bandwidth available in other class, it won't be using that bandwidth.

As per Cisco's recommendation, they suggest to assign atleast 33% of interface bandwidth to the Priority Queue.

Kindly let us know if you have more concerns.

Regards,

Smitesh

PS: Please rate helpful posts.

[Joseph W. Doherty · Accepted solution]

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Posting

LLQ classes have an implicit policer that normally only engages when there's link congestion.

If your LLQ classes already don't push to saturate the link, changing their class parameters shoudln't result in their using more bandwidth.