QoS Flashcards
Quality of Service
What is the command to configure a voice VLAN on a switchport
switchport voice vlan vlan
Jitter is defined as:
The variation in one-way delay between packets sent by the same application
Explain Random Early Detection (RED)
To prevent TCP global synchronization, once the amount of traffic in queue reaches a certain threshold, the device will start randomly dropping packets from select TCP flows.
Those TCP flows that dropped packets will reduce the rate at which they are sending traffic, but TCP global synchronization is avoided.
In standard RED, all kinds of traffic are treated the same, but with WRED (Weighted RED), you can control which packets are dropped depending on the traffic class
Which of the following are recommended standards for acceptable interactive video quality?
a) Delay: 30 ms or less
b) Delay: 150 ms or less
c) Jitter: 30 ms or less
d) Jitter: 50 ms or less
e) Loss: 1% or less
f) Loss: 2% or less
B, C, E
Which of the following is a negative effect of tail drop?
a) TCP sliding window
b) RED
c) WRED
d) TCP global synchronization
D
Which of the following is the default manner of forwarding queued packets?
a) FIFO
b) LIFO
c) LILO
d) FILO
A) FIFO
Which of the following is a congestion avoidance method that drops lower-priority packets if network congestion is detected?
a) WFQ
b) RED
c) FIFO
d) WRED
d) WRED
Define classification
Organizing network traffic into traffic classes, in order to be able to prioritize certain types of traffic
Name an L2 header field and an L3 header field that can be used to identify high/low priority traffic
L2: PCP (Priority Code Point) field of dot1q tag header (so only applicable when there is VLAN tagging)
L3: DSCP (Differentiated Services Code Point) field of IP header
IEEE 802.1p specifies that traffic with a PCP == 0 is:
Best-effort traffic. There is no guarantee that data is delivered or that it meets any QoS standard. This is regular traffic
IEEE 802.1p specifies that traffic with PCP == 3 is:
Critical applications traffic.
IP phones mark call signaling traffic (used to establish calls) as PCP-3. They mark the actual voice traffic as PCP-5
IEEE 802.1p specifies that traffic with PCP == 5 is:
Voice traffic. Specifically, the actual voice traffic, excluding the call signaling traffic.
IEEE 802.1p specifies that call signaling traffic is marked as PCP-?
PCP-3, critical applications traffic
IEEE 802.1p specifies that voice traffic is marked as PCP-?
PCP-5, voice traffic
IEEE 802.1p specifies that by default traffic is marked as PCP-?
PCP-0, best effort
T/F: Because the PCP field is only found in the dot1q header, it can only be used in trunk links and access links with a voice VLAN
T
The PCP header field is only found in the dot1q (VLAN) tag.
In addition, traffic going between two routers has no dot1q tagging, so it can’t be marked with PCP fields
T/F: Because the PCP field is found in the L3 IP header, it is more versatile than the DSCP field of the L2 MAC header.
F
The PCP field is found at L2 header, and the DSCP field is found at the L3 header
The IP ToS (Type of Service) byte is composed of these two fields:
DSCP and ECN
(Differentiated Code Services Point) (Explicit Congestion Notification)
DSCP: DF is used for:
Best-effort traffic
DSCP == 000000 , or 0
DSCP: EF is used for:
Traffic that requires low loss/latency/jitter
DSCP == 101110 , or 46
Describe the AF DSCP tag
AF defines four traffic classes. All packets in a class have the same priority. Within each class, there are 3 levels of drop precedence. The higher the drop precedence the more likely the packet will be dropped during connection
How is an AF priority tag written:
AFXY
Where X is the decimal value of the first 3 bits (which class), and Y is the decimal value of the drop precedence (bits 4 and 5)
