Definitions
ingress: arrives/come in/enter
egress: leaving/exit/to go
Its the new words of the day so its going to be used alot
Introduction
Regardless of the speed of individual switches (slower/older vs. faster/newer switches) or links (10/100), speed mismatches (ingress 1000/egress 100), many-to-one switching fabrics(multiple access layer switches into a distribution layer switch), and aggregation (multiple devices communicating through a single connection or to a single device or server) may cause a device to experience congestion, which can result in latency that result in dropped packets.
If and inevitably when congestion occurs (I have heard of enterprise pay-rolls that cause certain amounts of congestion on a network at the end of each month) and congestion management features are not in place (QoS, load balancing on servers, etc) then some packets will be dropped, causing retransmission (TCP) that inevitably increase overall network load and if voice and video are on the network (UDP) the inevitable will be angry employees. QoS can to an extent mitigate latency caused by congestion.
QoS is implemented by classifying and marking traffic at one device while allowing other devices to prioritize or to queue the traffic according to those marks applied to individual frames or packets.
LAN-Based Classification and Marking of Traffic
Classification and marking of traffic is the process of identifying traffic for prioritization as that traffic moves across the network. Traffic is classified by examining information at various layers of the Open Systems Interconnection (OSI) model. IP traffic can be classified according to any values configurable in an access control list (ACL) or any of these layers:
- Layer-2 parameters: MAC Address, Multiprotocol Label Switching (MPLS), ATM Cell Loss Priority (CLP) bit, Frame Relay discard eligible (DE) bit, ingress interface
- Layer-3 parameters: IP precedence, DiffServ Code Point (DSCP), QoS group, IP Address, ingress interface
- Layer-4 parameters: TCP or User Datagram Protocol (UDP) ports, ingress interface
- Layer-7 parameters: Application signature, ingress interface
QoS marks (values) establish priority levels (priority classes of service) for network traffic as it is processed by each switch (Access, Distribution, or Core). Once traffic is marked with a QoS value, then QoS policies on switches and interfaces will handle traffic accordingly at the frame and packet level. As a result of classification and marking, traffic will be prioritized accordingly at each switch to ensure that delay-sensitive traffic receives priority processing (voice, video) while non-delay sensitive data traffic waits it’s turn as each switch manages congestion, delay, and bandwidth allocation.
Layer-2 Qos
QoS layer-2 classification occurs by examining information in the Ethernet or 802.1Q header (trunking), like destination MAC Address, Virtual Local Area Network (VLAN) ID. QoS layer-2 markings occur in the priority field of the 802.1q header (LAN layer-2 headers have no place for this so 802.1Q encapsulation must occur). The priority field is 3 bits long (a.k.a 802.1p User Priority or class of Service (CoS) value).
The 3-bit Priority field can carry a value of 1 to 7; 1 is associated with delay tolerant traffic like TCP/IP traffic. Voice traffic receiving a higher priority for Call Signalling receiving a 3 value and Voice bearer traffic 5 value.
As a result of Layer-2 Classifications and marking, these QoS operations can occur:
- Input queue scheduling: when a frame enters a port, it can be assigned to one of a number of port-based queues before being scheduled for switching to an egress port. Typically, multiple queues are used where traffic requires different levels of service.
- Policing: is the process of inspecting a frame to see if it has exceeded a predefined rate of traffic within a certain time frame that is typically a fixed number internal to a switch. If a frame is determined to be in excess of the predefined rate limit, it can either be dropped, or the CoS value be marked down.
- Output Queue Scheduling: is where the switch will place the frame into an appropriate egress queue for switching. The switch will perform buffer management on this queue by ensuring that the buffer does not overflow.
Layer-3 QoS
QoS layer-3 classification occurs by examining information of the header values such as destination IP address or protocol. Qos Layer-3 markings occurs in the Type of Service (ToS) byte in the IP header. The first three bits of the ToS byte are occupied by IP precedence, which correlates to three CoS bits carried in the Layer-2 header.
The ToS byte can also be used for DSCP marking that allows prioritization hop by hop as packets are processed on each switch and interface.
Trust Boundaries
In QoS campus implementations, trust boundaries are defined/created where existing QoS values that are attached to frames and packets are to be accepted or altered. These “trusts” are established by configuring trust levels on the ports of key peripheral network devices where QoS policies will be enforced (trusted) as traffic makes its way into/onto the network. At this entry point traffic will be allowed or not allowed to retain its original QoS markings or will be ascribed new markings (best practice is to mark traffic as close to the source as possible).
In practice this means that if you have a network with a Desktop/Notebook attached to a Cisco IP Phone attached to a Catalyst Switch attached to a Cisco Router the trust boundary can be set at the Cisco IP Phone. Where the IP Phone attaches priority values which are then trusted.
Otherwise if there is a Desktop/Notebook with Softphone attached to a Catalyst Switch attached to a Router the trust boundary can be set to the Desktop/Notebook. Where the softphone attaches priority values which are then trusted.
Configuration IP Phone Attachment
This goes hand in hand with how to configure VLANs first off we create a VLAN
switch#configure terminal
switch(config)#vlan 10 name 001-WORK-STATION
switch(config)#vlan 100 name 001-IP-PHONE
Now we need to assign the Data and Voice VLAN to a interface
switch(config)#interface gigabitethernet 0/1
switch(config-if)#switchport voice vlan 100
switch(config-if)#switchport access vlan 10
Now we need to setup trust as they arrive at the switch port
switch(config-if)#mls qos trust cos
Finally set the trust conditional to a Cisco IP Phone being attached
switch(config-if)#mls qos trust device cisco-phone
Auto QoS
Cisco AutoQoS gives the ability to deploy QoS features for converged IP Telephony and allow for telephony networks to be deployed quicker and efficiently than if it had to be done manually. Cisco AutoQoS generates traffic classes and policy map command-line (CLI) templates across platforms that are the same where doing things manually might not have the same congruence. Cisco AutoQoS simplifies and automates the QoS CLI (MQC) definition of traffic classes and the creation and configuration of traffic policies.
AutoQos can be beneficial in these scenarios:
- SMB that deploy IP Telephony quickly but lack experience and staffing to deploy IP QoS Services.
- Large enterprises that need to deploy Cisco Systems Telephony solutions on a large scale, while reducing costs, complexity, and time frame for deployment, and ensuring that the appropriate QoS for voice applications is being set in a consistent fashion.
- International enterprises or service providers requiring QoS for VoIP where little expertise exists in different regions of the world and where provisioning QoS remotely and across different time-zones is difficult.
- Service providers requiring a template-driven approach to deliver managed services and QoS for voice traffic of customer premises devices.
Cisco AutoQoS simplifies and shortens the deployment cycle in the following ways:
- Application classification: By leveraging intelligent classification on routers Cisco network-based application recognition (NBAR) provides stateful and deep packet inspection. Cisco AutoQos uses Cisco Discovery Protocol (CDP) for voice packets to ensure that end-device attached to the Local Area Network (LAN) is really an Cisco IP Phones (keep in mind that CDP is Cisco Proprietary).
- Policy Generation: Cisco AutoQos evaluates the network environment and generates the initial policy. This feature automatically generates interface configurations, policy maps, class maps, and Access Control Lists (ACL).
- Configurations: Using one command, Cisco AutoQoS configures the port to prioritize voice traffic without affecting other network traffic, while still offering the flexibility to adjust QoS settings for unique network requirements. Cisco AutoQoS will automatically detect Cisco IP Phones and enable QoS settings, in turn it will also disable QoS settings to prevent malicious activity when a Cisco IP Phone is relocated or moved.
- Monitoring and reporting: Cisco AutoQoS provides visibility into the Class of Service (CoS) deployed via system logging and Simple Network Management Protocol (SNMP) traps, with notification of abnormal events(VoIP packet drops).
- Consistency: Cisco AutoQoS configurations are consistent among router and switch platforms. This level of consistency ensures seamless QoS operation and interoperability within the network.
Cisco Catalyst Switch Configuration – Cat OS
To configure the global QoS settings
Console> (enable) set qos autoqos
.........
All ingress and egress QoS scheduling parameters configured on all ports. CoS to DSCP, DSCP to CoS. Precedence to DSCP and policed dscp maps configured.
Global QoS configured, port specific autoqos recommended:
set port qos <mod/port> autoqos trust <cos/dscp>
set port qos <mod/port> autoqos voip <ciscoipphone/ciscosoftphone>
To configure Cisco AutoQoS settings and the trusted boundary features on/for Cisco IP Phones, CDP V.2 or later needs to be enabled on a port. If the trusted boundary feature is enabled. You will receive a syslog warning message if CDP is not running or CDP V.1 is running.
CDP need not be enabled if you do not use the ciscoipphone QoS configuraiton.
Console> (enable) set port qos 4/1 autoqos voip ciscoipphone
Warning: CDP is disabled or CDP version 1 is in use. Ensure that CDP version 2 is enabled globally, and also ensure that CDP is enabled on the port(s) you wish to configure autoqos on.
Port 4/1 ingress QoS configures for ciscoipphone.
It is recommended to execute the "set qos autoquos" gloval command if not executed previously.
Console> (enable)
To configure the port-specific QoS macro that handles all inbound QoS configurations that are specific to a particular port. This should only be used when the port connects to other known switches or servers because the port tursts all inbound traffic marked.
Console> (enable) set port qos 4/1 autoqos voip code/dscp
Cisco Catalyst Switch Configuration – Cisco IOS
When Cisco AutoQos in enabled on the first interface, QoS is globally enabled. This would be like configuring this command
switch#configure terminal
switch(config)msl qos
To in turn enable QoS on an interface use this command that tells the switch that the interface is connected to a trusted router/switch and that the VoIP classifications in the ingress packet should be trusted:
switch#configure terminal
switch(config)#interface gigabitethernet 0/1
switch(config-if)#auto qos voip trust
OR that the interface is connected to a Cisco IP Phone, the QoS labels of incoming packets are trusted only when the IP Phone is detected; this enabled CDP to detect the IP Phones absence or presence.
switch#configure terminal
switch(config)#interface gigabitethernet 0/1
switch(config-if)#auto qos voip cisco-phone
To check config use the following command
switch#show auto qos interface-id
Cisco AutoQoS Automation
Cisco AutoQoS automates several things when configured. It enforces trust boundaries on Cisco Catalyst switches access ports, uplinks and downlinks. Enables Catalyst strict priority queuing (PQ) (a.k.a expedited queuing) with weighted round-robin (WRR) scheduling for voice and data traffic. It configures queue admission criteria and finally modifies queue sizes and weights as needed.
Notes and Notices:
This is a part of my personal BCMSN notes and research to assist myself in learning and understanding the concepts and theory for the BCMSN exam. I learn by making notes reading and writing things down and wish to file them where I can’t lose them. These notes are not to be seen, judged or mistaken for replacements to Cisco recognized and authorized training which I personally support and attend and suggest you undertake if you are going for the BCMSN Certification.