QoS Standards and Guidelines with Avaya and Cisco

Definitions

COS: Layer 2 marking comprised of a 3 bit field within the 802.1Q Frame.

DSCP: Layer 3 marking comprised of the first 6 high order bits of the IP TOS field.

Precedence: Layer 3 marking comprised of the first 3 high order bits of the IP TOS field.
Precedence has largely been replaced by DSCP and should not be used when a DSCP alternative
exists. Used mainly for situations requiring backward compatibility

TOS: Layer 3 marking comprised of all 8 TOS bits. Not used much in the Cisco or Avaya
configurations, but some documentation makes reference to TOS. Both IP Precedence and
DSCP are sub-components of the TOS field.

Ingress Queue: Queue used on input switch ports prior to data being written to the switch
backplane.

Egress Queue: Queue used on output switch ports prior to data being written to the TX ring.

Threshold: A percentage of Queue capacity which, when exceeded, will trigger a tail drop action.

CLAN: Avaya module that is installed within a G650 chassis and provides a connection point
through which IP telephones can register with the S8XXX telephone system.

Medpro: Avaya module that is installed within a G650 chassis and contains shared DSP resources
needed to convert TDM voice to packet voice and also to convert between codecs.

Class-Map: A Cisco router or switch configuration element that is used to classify traffic into
classes, which will be acted on by a policy

Policy-Map: A Cisco router or switch configuration element that is used to apply per hop
behaviors (PHB) to traffic which matches a certain class. Policy maps are commonly used to
manage output traffic queues or to perform rate enforcement on incoming packets.

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In general, the QoS standards I use follow the Differentiated Services (DiffServ) model. In this model, traffic is classified and marked, and downstream network devices are configured to
recognize marked packets and modify their behavior in response. The response action could be to drop a packet, prioritize it, or re-mark it with another value. The Per-Hop Behavior (PHB) depends on the administratively defined policy. My standard is to give preference to packets which are marked with a DSCP value of 46.

It’s my policy to utilize industry best-practices wherever possible. The practices outlined in this document are a combination of those best practices combined with what I have found to work best through numerous successful installations.

Class of Service (COS) and Differentiated Services Code Point (DSCP)

It’s my standard to not use COS values unless specifically requested by the customer or if there is a limitation of the device(s) in the customer network. All Avaya hardware, including their phones, has the ability to produce a valid DSCP value and therefore DSCP (with a value of 46) will be used as the standard method of packet marking. This creates a great amount of simplicity in
data switch configuration, as there is no need to map COS and DSCP values to each other. Because all Avaya equipment will be marking voice related traffic with a DSCP value of 46, all switch ports connected to Avaya equipment should be set to trust DSCP. All switch uplinks and router ports should also be set to trust DSCP so that tags are preserved through the network. In the event that COS must be used, COS will be trusted on all phone ports and a COS value of 5 will be mapped to DSCP 46.

VLANS

At each location where an Avaya VoIP enabled cabinet, switch, gateway, or server resides, I expect one VLAN be created for voice traffic. All IPSI, CLAN, MedPro, and IP phones will reside in this VLAN. It may be necessary to create additional VLANs in some instances and this is determined on a case by case basis.

Queuing

Layer 3: My standard is to utilize Low Latency Queuing (LLQ) at all WAN interfaces or other links that could potentially be points of congestion. At installation of QoS I will utilize a strict priority of 60% of the gross bandwidth of the link. This may be increased to 75% in certain cases as needed. I will also utilize RTP Header Compression wherever possible.

Layer 2: Primarily the defaults are applied here with the priority outbound queue being applied to any switch uplinks. This should suffice in most implementations.

Putting it Together

I expect the following settings be configured on the Avaya Voice over IP equipment and the corresponding switch ports.

CLAN

Avaya Specific
• Configure as a NON TRUNK port
• Configure DSCP 46 for signaling
• COS will have no bearing on a NON TRUNK port
• Speed locked at 100Mbps
• Duplex locked at “Full”

Data Switch
• Configure for STATIC ACCESS
• Configure to trust the DSCP value of the received traffic
• Disable CDP
• Enable Portfast
• Speed locked at 100Mbps
• Duplex locked at “Full”

MEDPRO

Avaya Specific
• Configure as a NON TRUNK port
• Configure DSCP 46 for signaling
• Configure DSCP 46 for Bearer traffic
• COS will have no bearing on a NON TRUNK port
• Speed locked at 100Mbps
• Duplex locked at “Full”

VAL Announcement Board

S8XXX

IP Phones

Avaya Specific
• Configure as an 802.1Q trunk
• Configure DSCP 46 for signaling
• Configure DSCP 46 for Bearer traffic
• Speed locked at 100Mbps
• Duplex locked at “Full”

Data Switch
• Set as STATIC ACCESS on the “data” VLAN
• Configure so the voice VLAN will be recognized on the port
• Configure to trust the DSCP value of the received traffic
• Disable CDP
• Enable Portfast
• Speed locked at 100Mbps
• Duplex locked at “Full”

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