Class Based QoS

[hs08]

hello,

When we config QoS with bandwidth percent 30 for example, this mean the QoS is dedicated for 30% of interface bandwidth?

How if there are no any traffic, the CM-FS class still only can consume 30% and can't reach to 100%? It's possible for example the CM-FS can consume 100% if no other traffic but limited to 30% only if the traffic is full.

[ammahend · Accepted solution]

the bandwidth assigned under class is minimum guarantee not maximum. If other classes do not use their allocated bandwidth, the remaining bandwidth can be utilized by the active class, so in this case CM-FS can consume 100% if no other traffic but will be guaranteed 30% if there is other traffic

[hs08]

Hello,

So if in the interface i set bandwidth 30Mbps due we use OSPF, then CM-FS can consume minimum 9Mbps if the traffic is fully utilize and can consume 30Mbps if there no any traffic. Is my understanding correct?

[ammahend]

if your service-policy is applied to an interface where your interface bandwidth is set to 30mbps then yes CM-FS is gauranteed 30% of 30mbps which is 9mbp and all 30 when there is no traffic.

On a separate note keep in mind when you define percentage in class you typically want to use 75%-80% total allocation and leave rest (20%) for control plane and management plane traffic, in your case you have used up 100%

[Joseph W. Doherty]

"On a separate note keep in mind when you define percentage in class you typically want to use 75%-80% total allocation and leave rest (20%) for control plane and management plane traffic, in your case you have used up 100%"

Actually, percentage allocation would depend on bandwidth need of control plane and management traffic relative to available bandwidth.  Over allocation would likely not be an issue, but under allocation can very much be.  (Now adays, we usually have sufficient bandwidth for such traffic even on WANs.  [Laugh, I don't fondly look back on 64Kbps WAN links.])

[hs08]

We need another class-map for control plane and management plane traffic?

[ammahend]

Its a good practice, most Cisco devices will mark routing traffic with DCSP value for instance in IOS-XE, its marked with CS6, so you can use this to match class and other traffic types which you think are critical for network operations, if you really suspect congestion in your network

[Joseph W. Doherty]

@ammahend is correct in that Cisco device will tag some "important traffic" with CS6 or IPPrec6 (unsure which it will use - it might depend on the age of the device - the two are not identical - IPPrec is 1st 3 bits of ToS, DSCP is the 1st 6 bits of ToS - the 1st 3 bits, though, would be the same).  It will also take some of the traffic and provide it pak_priority treatment.  The latter is special egress queuing, although not all device CS6/IPPrec6 will also get the pak_priority treatment.

Further Cisco info:

Understanding How Routing Updates and Layer 2 Control Packets Are Queued on an Interface with a QoS Service Policy

Pak Priority

[Joseph W. Doherty]

The policy map's class bandwidth sets minimum, but does NOT limit maximum, which can be all available bandwidth.

Also, the minimum can be more than what the bandwidth statement has because it also depends on how the other policy map classes are defined.  For examples, if you have just two classes, each with the same percentage number, any percentage assignment, from 1 up to 50 will effectively be 50.

[hs08]

This mean if i have class1 is 20% and class2 is 30%, actually each class will have 50%?

[Joseph W. Doherty]

No, if you had just two classes, one 20% and the other 30%, their actual minimums would be 40% and 60%.

What class bandwidth allocations actually do is determine dequeuing ratios.