This guide is for network engineers and architects using their 128T Session Smart router to connect to AT&T’s MPLS VPN (AVPN) service. It will cover:
- Service class definitions for the various COS queues on the AT&T MPLS network
- Strategies for mapping
serviceconfiguration to the COS queues using
- Guidelines for setting your
traffic-engineeringproperties, to match the circuit profile of your AT&T MPLS link
While the techniques described here apply to any MPLS connection, they will be most valuable when engineering branch office (i.e., smaller) MPLS links, due to the higher likelihood of congestion.
This document is intended to be a companion guide to the AT&T Network-Based Class of Service Customer Router Configuration Guide. At the time of this writing, the latest version is Release 4.0, December 2016.
AT&T Service Classes
The AT&T AVPN MPLS network uses six distinct classes of service for carrying customer traffic, as well as a seventh “control” queue for communication between the CE and PE router. This section describes each of the classes of service and provides 128T configuration fragments to illustrate how to configure your 128T Session Smart router to interact with the AVPN network.
This document assumes the use of a 6COS circuit profile. The 4COS circuit does not use the COS2V nor COS5 classes of service. Otherwise, the recommendations made in this document apply.
In each of the
service-class configuration excerpts below, you will see it has been assigned a
traffic-class. The 128T Session Smart router has four traffic classes, in order of priority: high, medium, low, best-effort. This is different from other network equipment, which may order the priorities as high, medium, best-effort, low.
All of the
service-class elements in this section are provided as an appendix to this document, to facilitate their import (or copy/paste) into your existing 128T configuration.
The COS1 class is treated differently than the other classes on AVPN; any packets received that are marked as COS1 exceed the configured bandwidth, they are explicitly dropped ("hard policed"). Packets are identified as COS1 through the use of the "EF" DSCP marking (decimal 46).
COS1 is typically reserved for Voice over IP (VoIP) traffic.
COS2V: Delay-sensitive applications
The COS2V queue, identified through the use of the DSCP value AF41 (decimal 34), is typically reserved for video conferencing traffic.
COS2: Time-critical applications
The COS2 queue, identified through the use of DSCP AF31 (decimal 26), is to be used for time-sensitive, mission-critical, low bandwidth applications. This is the recommended class VoIP signaling (e.g., SIP, H.323, SCCP), as it is low-bandwidth (typically a small number of kilobits per second) but extremely time-sensitive.
COS3: Time-sensitive applications
This class of service should include all mission-critical applications that are interactive in nature; that is, they correspond to applications that are transactional in nature. In enterprises this may be a CRM system, an ERP system, or other important client/server applications.
COS3 is distinguishable by its DSCP value of AF21 (decimal 18).
COS4: Best Effort
The "best effort" class is the one that should be used by the majority of network traffic.
There is no DSCP value associated with best effort traffic (decimal 0).
The last class of service for customer traffic is COS5, or scavenger. This is for all non business-oriented traffic. This typically gets the lowest allocation when assigning traffic engineering percentages, and thus will experience congestion first.
Scavenger traffic is identified as COS5 through the use of the AF11 DSCP value (decimal 10).
The control queue is used for communicating between the CE and PE router, and should typically be limited to BGP and BFD only. This is an extremely low bandwidth queue.
In many deployments the 128T does not BGP peer with the PE router, and in no deployments will the 128T send BFD to the PE router. Do not mark BFD or BGP with DSCP CS6/decimal 48. This is only presented for completeness, or when BGP peering with the PE router.
The 128T will use
service-policy to indicate which sessions need to get marked and treated with the
session-type configurations. Each
service should have a corresponding
service-policy, to ensure that the markings are applied and the correct
traffic-class is used for traffic engineering.
The base class
service-policy configurations presented here are derived from the BCP on Service Policy.
|Base ||AVPN |
Traffic Engineering Strategies
The 128T router uses four traffic engineering queues for prioritizing egress traffic during times of congestion or link contention. The general practice of mapping the
traffic-class assignments (high, medium, low, best-effort) into the various 6COS queues is shown below.
Each AT&T AVPN circuit has a profile associated with it (referred to as a "COS Package"), that maps to bandwidth allocations for the various COS queues. These in turn need to be mapped to the four egress traffic engineering queues on the 128T. The COS Package from AT&T is expressed as a set of six numbers (corresponding to the queues), where the first number is the percentage of the circuit bandwidth allocated for COS1, and the remaining five numbers (which sum to 100%) represent the amount of bandwidth remaining from the bandwidth not used by COS1.
Sizing the Traffic Engineering Policy
|high||COS1 percentage from COS Profile|
|medium||(sum of COS2V + COS2 + COS3) * (100% - COS1 bandwidth)|
|low||COS4 * (100% - COS1 bandwidth)|
|best-effort||COS5 * (100% - COS1 bandwidth)|
Example (simple COS profile for a 6COS model):
In this case, the
high percentage is
medium class gets 60% (20% + 20% + 20%) of the remaining 95%, which is
low class gets 20% of the remaining 95%, which is
best-effort also gets 20%, which is
These values are merely starting points that should line up to the COS Profile of the AT&T AVPN circuit. Further tuning is left to the discretion of the network engineer.
traffic-profile would therefore look like this:
traffic-profile is applied to a
Appendix: Service Class Configuration
The following configuration output is presented here to facilitate copy/paste into your 128T conductor.
dscp value is a key field for the
service-class object, no two
service-class configurations can share the same value. This will likely require you to delete existing
service-class configuration, as the 128T ships with factory default
service-class elements that will conflict with the ones presented below.