max-reserved-bandwidth

Answered Question
Feb 8th, 2010
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Hi,


If QoS is NOT configured, is there still 75% max-reserved-bandwith on the interface?


Thank you,

Correct Answer by Jon Marshall about 7 years 3 months ago

vanessawen wrote:


Hi,


If QoS is NOT configured, is there still 75% max-reserved-bandwith on the interface?


Thank you,


If QOS is not configured then max-reserved-bandwidth makes no sense. So no QOS and the interface has 100% useable bandwidth with no reservation. With QOS interface has 100% bandwidth (assuming no policing/shaping)  but you can only actually allocate 75% of it in your QOS policies unless you change the max-reserved-bandwidth.


Jon

Correct Answer by francisco_1 about 7 years 3 months ago

75% limitation manifest itself when applying the policy to an interface so yes i believe it only applies when Qos is enable

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francisco_1 Mon, 02/08/2010 - 08:24
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75% limitation manifest itself when applying the policy to an interface so yes i believe it only applies when Qos is enable

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Jon Marshall Mon, 02/08/2010 - 09:17
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vanessawen wrote:


Hi,


If QoS is NOT configured, is there still 75% max-reserved-bandwith on the interface?


Thank you,


If QOS is not configured then max-reserved-bandwidth makes no sense. So no QOS and the interface has 100% useable bandwidth with no reservation. With QOS interface has 100% bandwidth (assuming no policing/shaping)  but you can only actually allocate 75% of it in your QOS policies unless you change the max-reserved-bandwidth.


Jon

vanessawen Mon, 02/08/2010 - 11:13
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i thought by default the router interface has 75% reserved bandwidth so that routing packet or important layer 2 traffic can go through.  if this is not the case, then the data traffic could take over all the bandwidth and cause route flapping if using dynamic routing protocol, i.e. ospf.

Jon Marshall Mon, 02/08/2010 - 11:24
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vanessawen wrote:


i thought by default the router interface has 75% reserved bandwidth so that routing packet or important layer 2 traffic can go through.  if this is not the case, then the data traffic could take over all the bandwidth and cause route flapping if using dynamic routing protocol, i.e. ospf.


Well yes that can happen and indeed does with an overutilised link and if it does happen thats when you look to implement QOS. Bear in mind routing protocols do get marked with network priority by Cisco but you can still get route flapping on an overutilised link without QOS.


Jon

AJAZ NAWAZ Thu, 05/13/2010 - 04:13
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In terms of verifying the situation, you can issue the command ' show queueing interface '.

If the interface in question is not subject to congestion management through queueing such as fair or CBWFQ then you know it's 100% available.


The output in this case looks like this:


RTR-01#show queueing interface gigabitEthernet 0/0
Interface GigabitEthernet0/0 queueing strategy: none
RTR-01#


However, as soon as you enable any type of congestion management on the interface then by default 25% will be reserved for RP updates etc. Take a look here:


RTR-01#show queueing interface fastEthernet 0/1
Interface FastEthernet0/1 queueing strategy: none
RTR-01#c
Enter configuration commands, one per line.  End with CNTL/Z.
RTR-01(config)#if 0/1
RTR-01(config-if)#fair-queue
RTR-01(config-if)#do show queueing interface fastEthernet 0/1
Interface FastEthernet0/1 queueing strategy: fair
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: weighted fair
  Output queue: 0/1000/64/0 (size/max total/threshold/drops)
     Conversations  0/0/256 (active/max active/max total)
     Reserved Conversations 0/0 (allocated/max allocated)
     Available Bandwidth 75000 kilobits/sec


RTR-01#


------------------------

hth,

Ajaz

Giuseppe Larosa Thu, 05/13/2010 - 06:45
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Hello Vanessa,

the max-reserved-bandwidth provides 25% of bandwidth for the hidden system queue used for routing protocol messages by allowing only 75% of bandwidth to be usable by user traffic classes.

The parameter can be tuned increasing it on high speed links where 25% can be too much.


To be noted not all platforms support the hidden system queue, so there can be a need for QoS tools to provide resources to routing protocol messages (example on GSR, C7500, may be C7200) explicitly.


the hidden system queue is supported on ISR, C2600, C3600 routers


Edit:

as noted by Francisco and Jon if you don't attempt to configure a QoS scheduler the parameter has no effect.

There is no built in shaper or policer limiting user traffic to 75% of bandwidth so you don't need to worry about this.

note also that even bandwidth is an administrative parameter that may not reflect the real speed of the link.


Hope to help

Giuseppe

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