Cisco WAN Switching Modules

Field Notice 980818: UXM UBU Allocation Using cnfbusbw

August 26, 1998

Products Affected


Problem Description

It is possible to oversubscribe the UXM's allocated bus bandwidth. After the initial Universal Bus Unit (UBU) allocation, switch software does not automatically allot additional UBUs to accomodate increasing load on the UXM. After some point, the UXM bus bandwidth is oversubscribed and traffic-affecting cell discards are observed.


A likely situation that will exhibit the discarded cells symptom is when a high capacity card, such as an IGX-UFM-U card, that does not use all four cell lanes and has a high aggregate of connections routed through a single UXM card equipped with an IMATM trunk. In such a situation, the UFM may be outputting a large amount of traffic on the single Fastpacket Cell Lane towards a UXM with an IMATM trunk without an adequate number of UBUs assigned. The symptoms observed may include cell discards at the UXM or Cell or Frame Discards at the far end of the affected connections.

Problem Symptoms

The primary symptoms include cell discards on trunks seen with the dsptrkerrs slot.port command. Additionally, Frame Relay connections may exhibit frame discards observable using the dspchstats slot.port.dlci command. In severe situations, network users may observe slow response times or time-outs.



There are two preferred workarounds:

  1. Use the cnfbusbw slot# command to increase the allocated UBUs. Consult Example Display for cnfbusbw for information on capacity.

  2. Execute the on3 6 command before adding connections. The on3 6 command requires service level access and is recommended for debugging over a short term period. An additional limitation to using this method is that it only applies to UXM cards in port mode. Since a future release of switch software will allow the UXM to operate with both port and trunks simultaneously, this limitation to the on3 6 command method may no longer apply. The on3 6 command enables the auto allocate UXM UBU feature but is very processor intensive. Executing the on3 6 command on an IGX 8400 with a very active processor may result in software aborts. To disable the auto allocate feature, execute the off3 6 command.

A third, less desirable method of adding UBUs to the UXM is with the getubus command. This method is not preferred because the added UBUs are not kept after a switchcc command, a node rebuild or a node power outage.


A longer term solution is to regularly monitor each UXM's UBU allocation by using the dspbusbw slot# command. This activity should be supplemented by statistics gathered with StrataView Plus. Pro-active planning, in conjunction with the information gathered, helps network managers allocate bus resources before traffic is severly affected. Future releases of switch software will allow automatic allocation of UBUs.

Other Considerations

When allocating UBUs, consider trunk failures and routing over UXM trunks that have not been allocated additional UBUs. It is possible for connections to exhibit no problems until rerouted over a trunk without sufficient bus bandwidth allocated as may happen during a trunk failure. The rerouted connections, as well as other connections already travering the trunk may discard cells until additional UBUs are allocated to the appropriate UXM.

For More Information

If you require further assistance, or if you have any further questions regarding this field notice, please contact the Cisco Systems Technical Assistance Center (TAC) by one of the following methods:

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