This command disables bandwidth management for service categories other than CBR.
When infinite bandwidth oversubscription is enabled, the interface allows as much bandwidth as possible with no upper limits (except on digital subscriber line (DSL) ATM interfaces, in which oversubscription is a not infinite, but is a factor of 2). The system determines if the ATM link is already oversubscribed. If so, the command is rejected. Otherwise, the total bandwidth available on the link is recorded, and all future connection setup requests are monitored to ensure that the link is not oversubscribed.
The bandwidth allocated for each service category appears in the output of the show atm interface atm command.
The ATM bandwidth manager tracks the bandwidth used by virtual circuits (VCs) on a per-interface basis. Because many services require guaranteed bandwidth (for example, for variable bit rate-real time (VBR-RT), available bit rate (ABR), and CBR), bandwidth management is required. The purpose of the bandwidth manager is to reserve resources for connections that require guaranteed services. Bandwidth management for CBR is turned on automatically for all interfaces supporting CBR. Bandwidth management for other service categories must be turned on by the user. All service categories outside CBR are monitored only if specifically requested.
Because unspecified bit rate (UBR) does not provide guaranteed bandwidth, the bandwidth specified for a UBR connection is not used in any calculations.
Bandwidth checking for a permanent virtual circuit (PVC) is done when that PVC is configured. Bandwidth checking for a switched virtual circuit (SVC) is done when a signaling call is placed or received.
When you use the atm pvp command, the system checks if the specified bandwidth is available on the interface. If the bandwidth available is greater than or equal to the peak rate specified for the Permanent Virtual Path (PVP), the command is accepted; otherwise, the command is rejected.
Within the VC mode, the available bandwidth check will determine whether the bandwidth is already used by the VC to fulfill the request. If the VC being configured is a PVC and belongs to a PVP, the bandwidth available on the PVP is used for the check; otherwise, the bandwidth available on the interface is used for the check.
When services within a VC class are being configured, the check determines whether the new bandwidth requirement can be fulfilled for all VCs using the class (on a per-interface basis) by comparing it with the bandwidth available on the corresponding interface.
Bandwidth checking for an SVC occurs before a SETUP message is sent for an outbound call. If the bandwidth check fails, the SETUP message is not sent. If the bandwidth check passes, the traffic class from which the service category is inherited is updated with the requirements for the new SVC.
When an SVC setup is requested for remotely initiated calls, a bandwidth check occurs as soon as the SETUP message is received. This bandwidth check has two components:
If the bandwidth check fails, a RELEASE message is sent out, and the call is rejected. If the bandwidth check passes, resources are reserved for the VC, and the call is accepted.
Cisco 10000 Series Router
Oversubscription of the ATM interfaces is off by default. Oversubscription of the tunnels (the number and bandwidth of VCs that can be in a tunnel) is on by default and is not subject to any oversubscription factor. Oversubscription of the tunnels cannot be adjusted or turned off. On the Cisco 10000 Series router, to enable the oversubscription feature for a particular interface or tunnel, use the atm over-subscription-factor command (but not the atm oversubscribe command, which can cause undesirable results). To prevent oversubscription of the interface, use the no atm oversubscribe command.
With variable bit rate-nonreal time (VBR-NRT) oversubscription, because of congestion on the physical interface, the accuracy of priority queueing (PQ) and class-based weighted fair queueing (CBWFQ) on individual VCs degrades. For example, if you configure each of three queues at a distribution of 50, 30, and 20 percent, respectively, the actual distribution might be 45, 40, and 15 percent, respectively. The distribution of bandwidth for each VC might be less than expected based on the speed of the VC. Typically, low-speed VCs are allocated the expected bandwidth, and high-speed VCs share the remaining bandwidth equally. The amount of bandwidth allocated for the PQ or latency might be less than expected.
The following example enables the oversubscription feature:
Router(config)# interface atm 4/0/0
Router(config-if)# atm oversubscribe
The following example displays the available bandwidth in kbps after the router enters VC mode. In this example, the available bandwidth is 139,000 kbps:
show atm interface atm 2/0
AAL enabled: AAL5, Maximum VCs:1024, Current VCCs:5
Maximum Transmit Channels:64
Max. Datagram Size:4496
PLIM Type:SONET - 155Mbps, TX clocking:LINE
877 input, 120843834 output, 0 IN fast, 20 OUT fast
ABR parameters, rif:16 rdf:16, 0 out drop
Bandwidth distribution :CBR :16000 Avail bw = 139000
Config. is ACTIVE