Interface without max bandwidth reservation command

Rupesh Kashyap · ‎12-19-2009

Hi,

I am using 7200 VXR cisco router. I have NOT configured any max bandwidth reservation command on the serial interface, which has 20Mbs DS3 link.

I have applied LLQ on the interface and seeing bandwith percentage is calculating on the basis of 100%. Please confirm as it should be 75 % default.

Router#sh policy-map int s2/1
Serial2/1

Service-policy output: QOS-OUT

    queue stats for all priority classes:
      Queueing
      queue limit 64 packets
      (queue depth/total drops/no-buffer drops) 0/7545/0
      (pkts output/bytes output) 765979175/50375100643

    Class-map: voip-traffic (match-any)
      765986730 packets, 50378894507 bytes
      30 second offered rate 32000 bps, drop rate 0 bps
      Match: access-group name VOIP
        732317198 packets, 47625068097 bytes
        30 second rate 0 bps
            Priority: 45% (9045 kbps), burst bytes 226100, b/w exceed drops: 7545

QoS Set
dscp ef
--More--

prasad.gsmc · ‎12-19-2009

Hi Rupesh,

the interface will be refereing to the bandwidth comand entered or the default bandwidth value of the interface. the 75% limit comes into play when you attempt to use bandwidth more than 75% for your Qos purpose.

For example if you try to put priority 80% the system might give an error since you are no allowed to use the last 25% under normal condition... the default behaviour could be modified with bandwidth allocation command....

Giuseppe Larosa · ‎12-20-2009

Hello Rupesh,

not all Cisco router platforms have the system hidden queue for routing protocols and so they miss the concept of max-reserved bandwidth.

This happens on high end routers like C7500, C12000 and may be also on C7200 VXR.

if so it is your responsability to provide a class-map to protect routing traffic when the link is full.

I remember a test I did with one C7500 on one side and a C3640 on the other side: the C7500 needed a class to protect the BGP or OSPF traffic the C3640 has the hidden system queue. Using a traffic generator to fill the link, the routing adjacencies were falling down on the C3640 side (it couldn't receive C7500's hello messages). With appropriate QoS the BGP or OSPF session could be stable also under heavy load.

Hope to help

Giuseppe