The ATM Forum publishes multi-vendor recommendations to further the use
of ATM technology. The
Traffic Management Specification Version 4.0
defines five ATM
service categories that describe both the traffic transmitted by users onto a
network and the quality of service that a network needs to provide for that
traffic. The five service categories are:
This document focuses on UBR+.
There are no specific requirements for this document.
This document is not restricted to specific software and hardware
Technical Tips Conventions for more information on document
UBR is typically used for data communications applications such as file
transfer and email. UBR is a best effort service and is the lowest class of
service in the hierarchy. There are no guarantees to the actual bandwidth
allowed. Therefore, UBR virtual circuits (VCs) are susceptible to a large
number of cell drops or a high cell transfer delay as cells move from the
source to the destination. This is because UBR also has no bounds on Cell Delay
Variation Tolerance (CDVT) and is only a best effort service.
The transmit priority for ATM class of services are:
CBR (highest priority)
VBR-rt (real time)
VBR-nrt (non real time)
UBR and UBR+
One can change the transmit priority for these classes of service but
the default is what is listed above.
The only parameter you can specify on a Cisco router for UBR is the
peak cell rate (PCR). Some ATM switches do not enforce the PCR, and the value
of PCR becomes informational only. On switched virtual circuits (SVCs) defined
for UBR, a router communicates to the network that a virtual circuit is UBR by
using the best effort indicator field in the ATM User Cell Rate Information
Element (IE) of a signaling packet.
UBR+ is a special ATM service class developed by Cisco Systems. UBR+
has been available in CLI as of Cisco IOS® Software Release 11.3(T) for SVCs.
The minimum cell rate (MCR) configuration for UBR+ was introduced as of Cisco
IOS Software Release 12.0(T). While UBR defines only (an optional) PCR, UBR+
also defines a MCR and (on the switch) a cell delay variation tolerance (CDVT).
Below are two examples:
router(config-if-vc)# ubr output-pcr
router(config-if-vc)# ubr+ output-pcr output-mcr
What is important to understand about UBR+ is that the MCR is a "soft
guarantee" of minimum bandwidth. A router signals the MCR value at call setup
time when a switched VC is created. The ATM switch is then responsible for the
guarantee of the bandwidth specified in the MCR parameter. A UBR+ VC is a UBR
VC for which the MCR is signaled by the router and guaranteed by the ATM
switch. Therefore, UBR+ affects connection admission control and resource
allocation on ATM switches.
With UBR+, Cisco offers ATM interfaces the ability to communicate both
the minimum and the maximum cell rates to the ATM network. As a result, the
router can have some assurance of a range of bandwidth values necessary for
Quality of Service (QoS).
When you configure SVCs, you can specify
input-pcr and input-mcr
parameters for a UBR+ VC. You typically specify the input parameters if your
output and input parameters are different. If the input parameters on the UBR+
VC are omitted, the router automatically assigns the same values to these
parameters as the output parameters.
ubr+ output-pcr output-mcr [input-pcr] [input-mcr]
In this example, different output and input parameters are specified
for both PCR and MCR.
svc TEST nsap 47.0091.81.000000.0040.0B0A.2501.ABC1.3333.3333.05
ubr+ 10000 3000 9000 1000
Currently, LAN emulation (LANE) QoS supports the creation of UBR+ VCCs.
If the switch cannot guarantee the rate you have specified for the UBR+ VCC,
the LEC reverts to UBR with no MCR guarantee.
The ubr+ command first appeared in Cisco
IOS® Software Release 11.3 T. In Cisco IOS Software Release 12.0(3)T, the
command was enhanced to support the selection of UBR+ QoS and configuration of
output PCR and output MCR for VC bundles.
The ubr+ command was removed from the VC
bundle command line interface (CLI) on the PA-A3 after Cisco IOS Software
Release 12.0(6)T. Refer to Cisco bug ID
(registered customers only)
for additional information.
The ATM Forum allows for a minimum guaranteed cell rate on UBR VCs.
This actually follows Cisco System's implementation of UBR+ on ATM switches and
routers (as in the 7x00 and 2600/3600 series routers). minimum desired cell
rate (MDCR) is how the ATM Forum defines MCR. MDCR is optionally signaled or
configured on either a virtual circuit or virtual path connection.
UBR+ differs from UBR+ MDCR in how the minimum cell rate is signaled to
the ATM network. Cisco's UBR+ uses the MCR information element (IE) of ABR VCs.
The ATM Forum's UBR+ MDCR uses a new MDCR IE. With UBR+ MDCR, ATM switches do
not need to police the ATM cells and determine whether the minimum cell rate
conforms to the signaled value.
The ATM Forum also defines a second service class that implements MCR
called guaranteed frame rate (GFR). GFR is specified by the Forum in the 4.1
update to its Traffic Management Specification. GFR guarantees MCR at the frame
level or at the AAL5, pre-SAR frame level. Only cells with CLP=0 are eligible
for the minimum bandwidth guarantee. An ATM switch can mark the CLP bit on
frames in which the measured cell rate exceeds the signaled MCR.
The ubr+ command is not available on the
PA-A1 and PA-A2.
The PA-A3 ATM port adapter supports UBR+ on SVCs only. It does not
support UBR+ on PVCs. Cisco IOS Software Release 12.0(7)T removed the
ubr+ command from PVC configuration mode. The VC is
now created as a standard UBR VC from a traffic-shaping perspective. The
ubr+ command was also removed from PVC bundles
(refer to Cisco bug ID
(registered customers only)
) and VC classes when these commands were
applied to PVCs. If you apply a VC class with the
ubr+ command, Cisco IOS assigns the UBR class
internally to the PVCs. The router rejects a VC class on a PVC if the PCR and
MCR defined in the ubr+ command are higher than the
line rate of the underlying physical interface (refer to Cisco bug ID
(registered customers only)
The ubr+ command was removed from the CLI
because of how scheduling works on an ATM edge device. The PA-A3 and other edge
devices are designed to rate limit to a value like PCR or available cell rate,
as with the ABR service category. They are not designed to provide a minimum
bandwidth guarantee through active scheduling. In contrast, an ATM switch is
designed to ensure that a VC receives a guaranteed rate. On Cisco campus ATM
switches like the Catalyst 8500 series and the LS1010, the interface scheduler
uses weighted round-robin (WRR) to allocate the remaining bandwidth among VCs
of all ATM service categories other than CBR. (Refer to
the Scheduler and Service Class.) With UBR+ VCs, the PA-A3 is
responsible for limiting the VC to its PCR, and the ATM switch is responsible
for the guarantee of the MCR to the VC.
ABR's use of MCR differs from UBR+'s use. ABR uses MCR as a
"lowest-ever" maximum shaping rate. UBR+ uses MCR as an active
scheduling mechanism for guaranteeing a minimum.
Instead of scheduling a minimum, a router can guarantee a form of Layer
3 QoS to complete packets. It can also ensure that any excess traffic above the
PCR is queued so that the QoS policies can apply to the queued excess. Refer to
IP to ATM Class of Service for further information.
UBR is supported on the PA-A6 OC3 and PA-A6 OC 12. Only UBR and VBR-nrt
CoS is configurable on the PA-A6 OC 12. On the PA-OC12 there is no
transmit priority command under the PVC. This
prevents a user from changing the transmit priority from something other than
If there is no higher priority traffic, each timeslot may be filled by
UBR traffic. This is due to the scheduling on the SAR chip on the PA-A6 where
the cell timeslots are filled per the transmit priority. This may potentially
lead to sending more traffic than the PCR and is why it is recommended that
PVCs on the PA-A6 OC12 be configured with the SCR value rather than the PCR.
Future Cisco IOS Software releases will not display an option for
configuring a PCR parameter at the command line with the
ubr command. All UBR VCs will be forced to use a PCR
of the line rate. Refer to Cisco bug ID
(registered customers only)