qos and packet reception on the int

Answered Question
Aug 1st, 2010

Hi every body

.


Based on the last discussion based on the somewhat similar topic( it was how the routers pick picket to transmit out of int), i learnt following.


Congestion does not necessary mean that link is congested,  for example if router receives a lot of packet supposed to be transmitted out of same int,  that router  might drop packet based on if the tx ring is full and no qos is configured  though link is not running at full capacity and  service time is a lot higher than transmission delay/serial delay.


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But  now i am wondering  how router behave  when receiving  packets,


1)  Let say we have router  with no qos configured, ,  router is receiving a lot of packets ,consequently rx ring gets completely full , will router drop packet as there is no qos configured and rx ring is full?


2)  Let say  a router is receiving a lot of packets  with marking on an int.  Rx ring gets full  , Will router put the packets in queues based on qos value?


3)  Am i  correct  in assuming   when a packet is received on the int of router,  packet is placed on rx ring first?


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So when we configured qos on a router,  does it mean  router have qos queues both in bound and out bound on an int?  Wil those queues only get filled if rxring or tx ring is full?



thanks and have a good day.

Correct Answer by Collin Clark about 6 years 6 months ago

1)  Let say we have router  with no qos configured, ,  router is  receiving a lot of packets ,consequently rx ring gets completely full ,  will router drop packet as there is no qos configured and rx ring is  full?


It will drop packets, but not because QoS is not configured. The router must drop packets if the interface is congested.


2)   Let say  a router is receiving a lot of packets  with marking on an  int.  Rx ring gets full  , Will router put the packets in queues based  on qos value?


Yes assuming the router switch supports it. There are not a whole lot of devices that support ingress queueing. Also if your downstream router is queuing outbound, the high prioirty packets are already first out the door and hence first inthe door on your upstream router. You rarely need to apply QoS inbound.


3)  Am i  correct  in assuming   when a packet is received on the int of router,  packet is placed on rx ring first?


If there is no QoS, I beleive that is correct.


So when we configured qos on a router,  does it mean  router have qos  queues both in bound and out bound on an int? 


As long as your downstream router is applying QoS you shouldn't need to inbound on your upstream routers.


Wil those queues only get  filled if rxring or tx ring is full?


Yes


Hope it helps.

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Correct Answer
Collin Clark Mon, 08/02/2010 - 06:24

1)  Let say we have router  with no qos configured, ,  router is  receiving a lot of packets ,consequently rx ring gets completely full ,  will router drop packet as there is no qos configured and rx ring is  full?


It will drop packets, but not because QoS is not configured. The router must drop packets if the interface is congested.


2)   Let say  a router is receiving a lot of packets  with marking on an  int.  Rx ring gets full  , Will router put the packets in queues based  on qos value?


Yes assuming the router switch supports it. There are not a whole lot of devices that support ingress queueing. Also if your downstream router is queuing outbound, the high prioirty packets are already first out the door and hence first inthe door on your upstream router. You rarely need to apply QoS inbound.


3)  Am i  correct  in assuming   when a packet is received on the int of router,  packet is placed on rx ring first?


If there is no QoS, I beleive that is correct.


So when we configured qos on a router,  does it mean  router have qos  queues both in bound and out bound on an int? 


As long as your downstream router is applying QoS you shouldn't need to inbound on your upstream routers.


Wil those queues only get  filled if rxring or tx ring is full?


Yes


Hope it helps.

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