QoS Flashcards
What is QoS?
Quality of Service - a collection of technologies and methods that ensures critical application communication is delivered reliably across the network by prioritizing certain types of traffic.
What are the three QoS implementation models?
- Best Effort: no QoS
- Integrated Services (IntServ) - old
- Differentiated Services (DiffServ) - modern
What is the key component of how the Integrated Services Model (IntServ) functions?
End to End bandwidth reservations using RSVP (Resource Reservation Protocol)
What are the key component to how the Differentiated Services Model (DiffServ) functions?
- Classification and Marking of Traffic
- IP Precedence (service levels)
Given the following Layers of the OSI Model, what are the ways that traffic can be marked for classification at each layer? (Layer 1, 2, 3, 4, and 7)
- Layer 1: Interface/Port
- Layer 2: MAC Address, 802.1Q Class of Service
- Layer 3: DSCP, IP Precedence, Source/Destination IP address
- Layer 4: TCP or UDP ports
- Layer 7: NBAR (Network Based Application Recognition)
In practice, what are the most common methods to classify (identify) QoS traffic on a Cisco device?
- Access Lists (ACLs)
- Class-Maps: class maps can match traffic on an array of criteria such as interface, protocol, dscp, etc.
At Layer 2, what 802.1Q header field and sub-fields are responsible for marking traffic?
Tag Control Information (TCI) field which is composed of three sub-fields:
- Priority Code Point (PCP)
- Drop Eligible Indicator (DEI)
- VLAN Identifier (VLAN ID)
What are the possible Class of Service (CoS) marking values found in the PCP field of an 802.1Q header and which ones are more preferred?
- CS0 Background Traffic (Lowest)
- CS1 Best Effort (default)
- CS2 Excellent Effort
- CS3 Critical Applications
- CS4 Video
- CS5 Voice
- CS6 Internetwork control
- CS7 Network Control (Highest)
What is the ToS field and what is the relationship to IP Precedence?
The ToS field is is an 8-bit field that is part of a packet header (Layer 3). Only the first 3 bits of the field are used and they are known as IP Precedence.
What is the ToS field and what is the relationship to the DiffServ QoS Model?
DiffServ redefines the ToS field to use 6 bits for marking traffic at Layer 3 (instead of 3) known as Differentiated Services Code Point (DSCP).
What are the four Per Hop Behavior classes that are defined in DiffServ’s DSCP marking?
- Class Selector (CS)
- Default Forwarding (DF)
- Assured Forwarding (AF)
- Expedited Forwarding (EF)
What are the DSCP marking value equivalents compared to Class of Service values?
- CS7 (No DSCP equivalent)
- CS6 (No DSCP equivalent)
- CS5 (EF)
- CS4 (AF4)
- CS3 (AF3)
- CS2 (AF2)
- CS1 (AF1)
- CS0 (BE)
When looking at DSCP values such as AF41 and AF 43 which one is more likely to be dropped?
AF43
*the second number is the drop probability - higher is more likely to be dropped.
What is the formula to convert DSCP values to decimal?
8x + 2y where x is the class selector and y is drop precedence.
Example: AF31
8(3) + 2(1) = 26
What is special about traffic marked with CS6 and CS7
This is reserved for network control and routing protocol types of packets only.
When marking packets, what is best practice for where in the network the traffic should be marked?
As close to the source as possible.
In some cases endpoints, such as phones, can mark their own traffic. When this is not possible, it should be marked on the switch as it enters the switchport.
What is a Trust Boundary as it relates to DSCP and QoS?
The trust boundary is the concept of where a DSCP value will be trusted and forwarded on.
By default a PC connected to the PC port on a phone will have QoS markings “zeroed out” meaning that the phone will not trust the DSCP markings coming from the PC. In this scenario, the phone is the trust boundary.
In a wireless network, where is the Trust Boundary located by default?
At the wireless LAN controller, before the traffic is placed on the wired network (on the other end of the CAPWAP tunnel).
What is Scavenger Traffic?
Traffic that is classified as being lower than Best Effort. This traffic is marked with CS1.
After traffic has been identified on a network device. What can be done to “treat” or take action on the traffic?
- Marking
- Shaping
- Policing
What is the difference between a Shaper and a Policer?
- Policers will drop or re-mark traffic
- Shapers will attempt to delay, buffer, or queue traffic
What is Markdown?
When a Policer re-marks a packet’s DSCP value to a lower priority due to exceeding a defined bit-rate.
What are Token Bucket Algorithms used by Policers and Shapers?
Different types of algorithms that use a concept of “token” to determine if packets should be buffered, dropped, or marked down when congestion occurs on a network link.
In Token Bucket Algorithms, what is Token Bucket?
A bucket size defines how many tokens it has. Tokens are removed from the bucket when a packet arrives and is checked. New tokens are added into the bucket at a rate that is equal to the Committed Information Rate. If the bucket gets empty, the enforcement mechanism will take affect.
