QoS Flashcards
What are the quality requirements for voice and video?
Latency <150ms
Jitter < 30ms
Loss < 1%
What is delay in reference to QoS?
Latency
What is jitter in reference to QoS?
Variation in Delay
What is loss in reference to QoS?
Packet loss
By default, whenever congestion is experienced on a router or switch, packets are sent out in a ____________ manner
First In First Out (FIFO)
_____________ can be experienced whenever it is possible for packets to come in quicker than they can be sent out
Congestion
When traffic is being processed as First In First Out and with no QoS in place what will the router do?
Buffer traffic and queue it up
What are a couple of ways to mitigate congestion?
Add more bandwidth
Use QoS techniques
QoS queuing can reduce what for particular traffic?
Latency, jitter and loss
What are some of the markings for QoS?
COS (Class of Service)
DSCP (Differentiated Service Code Point)
ACLs
NBAR (Network Based Application Recognition)
What Layer does COS (Class of Service) reside at?
Layer 2
What layer does DSCP (Differentiated Service Code Point) reside at?
Layer 3
What carries the COS QoS marking?
A 3 bit field in the Layer 2 802.1Q Frame Header
What is the value range for the COS 3 bit field in the Layer 2 Frame Header?
0-7
What COS values are reserved for network use like routing protocols?
6 and 7
True or False: With COS, the higher the value the more important the traffic is
True
With COS, what value do IP Phones mark their call signaling traffic as?
3
With COS, what value do IP Phones mark their voice payload traffic as?
5
What is used to carry DSCP QoS Marking?
The TOS (Type of Service) byte in the Layer 3 IP Header
What is the value range with DSCP?
0-63
What is the default DSCP value?
0
What is the default COS value?
0 for best effort traffic
How many bits are used with DSCP marking?
6
With DSCP, IP Phones mark their call signaling with what value?
24 (CS3)
With DSCP, what do IP Phones mark their voice payload with what value?
46 (EF)
True or False: Only DSCP or COS can be in a packet but not both
False. Both DSCP and COS are in the packet. COS for Layer 2 and DSCP for Layer 3
What does NBAR stand for?
Network Based Application Recognition
What layers does NBAR inspect a packet up to?
3-7
What are the 3 main sections of MQC (Modular QoS CLI)
Class Maps
Policy Maps
Service Policies
____________ can be used to manage congestion on routers and switches
Queuing
This type of queuing gives bandwidth guarantees to specified traffic type
CBWFQ (Class Based Weight Fair Queuing)
This type of queuing is just CBWFQ but with a priority value
LLQ (Low Latency Queuing)
___________ and ____________ can be used to control traffic rate
Traffic shaping and policing
Traffic shaping _________ any excess traffic so the overall traffic stays within the desired rate limit
Buffers
Traffic policing ___________ or re-marks excess traffic to enforce the specified rate limit
Drops
This is another term for worm or junk traffic
Scavenger traffic
What is the recommended value for DSCP scavenger traffic?
DSCP 8 (CS1)
