Under extreme rxload (see below) – why would AF31 not be dropped over AF21 traffic without the use of WRED, shaping or policers on the inbound?

[James Clifford]

Kit: Cisco 3845 - Advanced Security

IOS: 151-4.M10

Under extreme rxload (see below) – why would AF31 not be dropped over AF21 traffic without the use of WRED, shaping or policers on the inbound MQC? i.e. AF31 is not been dropped but AF21 is.

MTU 1500 bytes, BW 30000 Kbit/sec, DLY 100 usec,

     reliability 255/255, txload 9/255, rxload 212/255

Is some measure of priority and proportional fairness defined between traffic in different AF classes as standard? Should congestion occur between classes, is the traffic in the higher class been given priority? If not, the only other thing I can think of is the Service Provider is prioritizing. 

Please note - The type of the traffic (ssh, http etc) doesn't matter as we have tried these in either classes and af31 is always persevered over af21.

Thanks

James

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

class-map match-any mission_crit_af31

match ip dscp af31

class-map match-any transactional

 match ip dscp af21

policy-map qos_in

class voice

class video

class mission_crit_af31

  set ip dscp af31

class mission_crit_af32

  set ip dscp af21

class transactional

  set ip dscp af11

class general

  set ip dscp 1

interface FastEthernet0/0/0

Description *****WAN interface *****

 bandwidth 30000

ip address x.x.x.x 255.255.255.252

ip access-group inbound-filter in

ip nbar protocol-discovery

ip flow ingress

ip flow egress

load-interval 30

duplex full

speed 100

max-reserved-bandwidth 100

service-policy input qos_in

service-policy output qos_out

[Joseph W. Doherty]

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Posting

WRED and shapers shouldn't be applicable to ingress.  Policers can be, but drops would depend on the volume of different kinds of traffic relative to your ingress policy.

In other words, usually an ingress interface (again without policers) shouldn't be dropping any traffic, based on ToS.

Some devices do support SPD (http://www.cisco.com/c/en/us/support/docs/routers/12000-series-routers/29920-spd.html), but I don't think it generally selects between AF31 vs. AF21.

What might happen though, if the packets vary greatly in size between your two markings and/or there are more bursts of one marked packet vs. the other marking, that might reflect in dropping.  I.e. whatever packets might be causing resource exhaustion.