I have been brushing up on my QOS and just come across something that i cant get my head round it....bascially i setup MQC for PING class (marked as AF33 and part of access-group 1 ) ....i have policed the traffic to 8000bps (1000bytes) but when i generate ping of size 1000 from acces-group 1 class it drop every second packet, when i generate ping size of 300 it on average drops a packet after every 5 packets and few times after 6 packets....if i set ping packet size to 500 then after every three packets it drops one packets....i need lil clearification since i set CIR to be 8000 bps and policed it i was expacting 1000 Byte or 8000 bit to pass per second why i see more than 1000bytes per second , current traffic rate passing through is 1500bytes which is same as in Bc value....can someone explain what BC referes to in my configs when i use police and if i use police rate it shows burst not bc....
I need clarification on when to use police command and when to use Police rate ...with police it show bc value and with police rate burst...
My understanding is to use polic for data plane traffic and police rate is for control plane traffic needed lil clarification
i need to know what bc is (its not frame relay just normal link ) and why is it passing more traffic than its configured for ....
Actualy i figured it out but would still like to validate .....
bc value is same as CIR But only difference is its in bytes not bits and time is one fourth of second ....so in belwo example bc is 9375*8and then the value is multiple to 4 which gives us CIR 300000 , in my previous post bc is higher than CIR becuase i chose 8000bps minimum possible vlaue and for that cisco router chooses 1500bytes which means minimum CIR for a class is 48000bps anything lower then that will still have 1500byte assigned as bc....
Also police CIR is normal syntax and police rate is used for control plane traffic ....we can only use police too without CIR keyword.....
R1#sh policy-map interface fastEthernet 0/0 output class WEB
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One fourth of a second, 250ms, or 25ms? The latter is, I recall, is a common default on Cisco policers.
What you may be bumping into, especially with a policer, policers and shapers "count" transmitted bytes during some time period. When the maximum allowed count is reached, any packets that exceed the count for the current interval are subject to policing (or shaping). Since policer's don't queue, even if a packet's size, during the time interval, only exceeds the count by 1 byte, it will be dropped. Larger packets are more likely to "overflow" a count, especially during small time intervals. Also when using pings, I believe consective ping packets are not generated as quickly as the interface bandwidth would allow. If this is true, larger ping packets are more likely to exceed the "count" during a measured time interval.
Because policers don't queue, it's quite common that traffic at some policed rate doesn't actually obtain that rate, due to drops during packet bursts. The way to mitigate this is to insure your Bc equals the buffer space of an interface. For example, often T1/E1 will default to an egress queue of 40 packets. Assuming each would be about 1500 bytes, means we need to insure a 1.5 Mbps policer needs a Bc of about (40*1500) 60,000 bytes for similar behavior.
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