QoS Policing Configuration Example

[lagapidis (Lazarus Agapidis)]

Hello Hussein

@hussien.samer

1- In your example of Single-Rate-Three-Color how much the the traffic will be exceed so the violate-action drop the packets ?

The police CIR is set to 128000, the BC is set to 4000 and the BE is set to 4000.

So, any packets that are within the 128000 CIR limit plus the 4000 BC limit are conforming. These will be transmitted and their DSCP values will be unchanged.

Any packets arriving above the 128000 + 4000 limit and within the additional BE limit of 4000 fall into the exceed action category. These are transmitted but their DSCP values are modified to 0.

Finally, any packets arriving above the BE limit are violating, so these will be discarded.

The BE and BC values can be seen in the output of the show policy-map interface FastEthernet 0/0 command. These must be configured manually, otherwise the Cisco IOS will select them automatically based on the CIR rate.

@hussien.samer

2- After police keyword we can specify bits per second directly and also we can specify it with cir !! what is the difference ?

The police cir command is always followed by a percent value. The value indicated is the percent of the CIR which will be used to determine conforming. The police bps command is a value in bytes per second.

@hussien.samer

3- You said that bc select a value based on the CIR rate, what the formula it’s used ? and I see 4000 bytes is not enough and give only 1/4 of CIR rate ??? and when we need to configure bc ourselves ?? can you please explain this in detail for me ??

By default the Cisco IOS will choose a BC that is 1/32 of the CIR. In this case, 128000/32 = 4000.

I hope this has been helpful!

Laz