QoS Concepts
- Delay
- Fixed Delay
- Variable Delay
| Delay | Description |
|---|---|
| Code Delay | Fixed Delay, amount of time it takes to compress data at source before transmitting to the first internetworking device usually a switch |
| Packetization Delay | Fixed Delay, the time it tasks to encapsulate a packet with all the necessary header information |
| Queuing Delay | Variable Delay, the time a frame or packet waiting to be transmitted on the link |
| Serialization Delay | Fixed Delay, the time it takes to transmit a frame onto the wire |
| Propagation Delay | Variable Delay, the time it takes for the frame to travel between the source and destination |
| De-jitter Delay | Fixed Delay, the time it takes to buffer a flow of packets and then send them out in evenly spaced intervals |
Voice
- Predictable
- 200 bytes every 20 ms
- RTP port range 16384 to 32767 to prioritize the voice traffic
- Latency should be no more than 150 milliseconds(ms)
- Jitter should be no more than 30 ms
- voice packet loss should be no more than 1%
- Voice traffic requires at least 30 Kbps of bandwidth
Video
- Not predictable
- every 33 ms
- UDP port 554 Real-Time Streaming Protocol (RTSP)
- Latency should be no more than 400 milliseconds
- Jitter should be no more than 50 ms
- video packet loss should be no more than 1%
- Video traffic requires at least 384 Kbps of bandwidth
Queuing algorithm
FIFO (First in First out)
- when no other queuing strategies are configured, all interfaces use FIFO by default
- except serial interface at E1 (2.048 Mbps) and below
- Serial interface at E1 (2.048 Mbps) and below use WFQ by default
Weighted Fair Queuing (WFQ)
- automated scheduling method
- provides fair bandwidth allocation to all network traffic
- classifies traffic into different flows based on packet header addressing
- not supported with tunneling and encryption
Class-Based Weighted Fair Queuing (CBWFQ)
- define traffic classes based on match criteria including protocols, access control lists (ACLs), and input interfaces
- A FIFO queue is reserved for each class
Low Latency Queuing (LLQ)
- brings strict priority queuing (PQ) to CBWFQ
- Strict PQ allows delay-sensitive packets such as voice to be sent before packets in other queues
QoS Policy Model
-
Best-effort model
- use this when QoS is not required
-
Integrated Services (IntServ)
- Provides very high QoS to IP packets with guaranteed delivery
- It can severely limit the scalability of a network
-
Differentiated service (DiffServ)
- provides high scalability and flexibility in implementing QoS
- Network devices recognize traffic classes and provide different levels of QoS to different traffic classes
QoS tools
- Classification and marking tools
- Congestion and avoidance tools
- Congestion management tools
DSCP Values
- Best Effort (BE)
- Value is 0
- Default for all IP packets
- when experience congestion, packets will be dropped
- Expedited Forwarding (EF)
- Value 46
- First 3 bits map to layer 2 CoS value 5
- used for voice traffic
- Assured Forwarding (AF)
- use the 5 most significant DSCP bits to indicate queues and drop preference
- class 4 is the best queue, class 1 is the worst queue
- first 3 digits indicate the class
- Low drop, medium drop, high drop
Class Selector Bits
- the first 3 bits of DSCP that indicate the class
- These 3 bits map directly to the 3 bits of the CoS field and the IPP field
QoS Policy Guidelines
- Enable queuing at every device in the path between source and destination.
- Classify and mark traffic as close the source as possible.
- Shape and police traffic flows as close to their sources as possible.
CDP (Cisco Discovery Protocol)
LLDP (Link Layer Discovery Protocol)
NTP (Network Time Protocol)
- use UDP port 123
- max hop count is 15
SNMP (Simple Network Management Protocol)
- application layer protocol
- 3 elements
- SNMP manager
- SNMP clients
- Management Information Base(MIB)
- UDP port 161 and 162
- SNMP manager Actions
- Get
- Set
- versions
- SNMPv1
- SNMPv2c
- SNMPv3
Syslog
- UDP port 514
- gather logging information for monitoring and troubleshooting
- select the type of logging information that is captured
- specify the destinations of captured syslog messages
- Desnations
- logging buffer
- console line
- terminal line
- syslog server
| Severity Name | Severity Level | Explanation |
|---|---|---|
| Emergency | Level 0 | System Unsuable |
| Alert | Level 1 | Critical Condition |
| Critical | Level 2 | Critical Condition |
| Error | Level 3 | Error Condition |
| Warning | Level 4 | Wanring Condition |
| Notification | Level 5 | Normal, but significant condition |
| Informational | Level 6 | Informational Message |
| Debug | Level 7 | Debugging Messages |
- Level 0-4 error messages
- Level 5 normal but significant
- Level 6 normal and does not affect device functionality
Every Awesome Cisco Engineer Will Need Icecream Daily
- add timestamp on the log messages
- issue the command : service timestamps log datetime
Password Recovery Procedures
- Enter the ROMMON mode
- press ctrl+c in the process of booting
- change the configuration register
- confreg 0x2142
- reset
- copy the startup-config to the running-config
- copy start run
- change the password
- enable secret password
- save the running-config as the new startup-config
- config-register 0x2102
- copy run start
- reload the device
- reload