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This feature module describes the Segmentation and Reassembly Buffer Management Enhancements feature. It includes information on the benefits of the enhancements, supported platforms, related documents, and configuration.
This document includes the following sections:
This feature module describes the following enhancements to segmentation and reassembly (SAR) buffer management:
Prior to this release, the default size of the PVC segmentation buffer was 256 packets. This meant that each PVC could queue up to 256 packets to be segmented and sent. Now the default size is 32 packets, and a new command allows you to manually change the segmentation buffer size.
Prior to this release, the default input/output (I/O) memory size was 16 MB for NRPs with 64 MB or 128 MB DRAM. Now the default I/O memory size is 18 MB for an NRP with 64 MB DRAM, and 36 MB for an NRP with 128 MB DRAM. You can also manually set the I/O memory size with an environment variable under ROMMON.
For each PVC, a segmentation buffer slot is reserved for high-priority packets.
Improved Distribution of Memory Resources
The SAR buffer management enhancements reduce the amount of memory resources that can be held by congested PVCs. This prevents a small group of congested PVCs from using all available memory resources and adversely affecting the performance of other PVCs.
Improved High-Prioity Packet Transmission
With a segmentation buffer slot reserved for high-priority packets, each PVC accommodates high-priority packets even when the segementation buffer is full.
Because of process memory usage, setting the I/O memory size to a larger value might reduce the number of sessions that your NRP can handle.
The Segmentation and Reassembly Buffer Management Enhancements are supported on the node route processor (NRP) of the Cisco 6400 UAC.
Standards
None
MIBs
None
RFCs
None
Cisco recommends that you simultaneously run Cisco IOS Release 12.1(1) DB on the node switch processor (NSP) of the Cisco 6400 UAC.
See the following sections for configuration tasks for the Segmentation and Reassembly Buffer Management Enhancements feature. Each task in the list is optional.
To manually set the size of all PVC segmentation buffers, enter the following commands beginning in global configuration mode:
| Command | Purpose | |
|---|---|---|
Step 1 | Router(config)#service internal | Enables modification of PVC segmentation buffers. |
Step 2 | Router(config)#interface atm 0/0/0 | Specifies the ATM interface. |
Step 3 | Router(config-if-atm-vc)# | Sets the maximum number of packets in the PVC segmentation buffers. |
To verify successful configuration of the segmentation buffer size, use the show running-config EXEC command.
To manually set the size of I/O memory, enter the following command in ROMMON mode:
| Command | Purpose | |
|---|---|---|
Step 1 | Rommon>IOMEM=size | Sets the size, in MB, of I/O memory. Allowed values depend on the amount of DRAM on your NRP, and they are listed in Table 1. |
| Main Memory on NRP | Allowed I/O Memory Range | Default IOMEM Setting |
|---|---|---|
64 MB DRAM | 18 MB to 24 MB | 18 MB |
128 MB DRAM | 18 MB to 60 MB | 36 MB |
 |
Note IOMEM entries must be an even number. If you enter an odd number, the NRP will round it down to an even number. If you enter a number outside of the allowed I/O memory range, the NRP will use the default IOMEM setting. You can also enter unset IOMEM in ROMMON to return to the default setting. |
To verify that you successfully set the I/O memory size, use the show memory EXEC command. The following example shows an NRP with an I/O memeory size of 16 MB:
router# show memory
Head Total(b) Used(b) Free(b) Lowest(b) Largest(b)
Processor 60E27540 35490496 5517076 29973420 14919296 29838876
I/O 3000000 16777216 6006460 10770756 5385388 10770108
This section provides the following configuration examples:
In the following example, the PVC segmentation buffer size is set to 64 packets.
! service internal interface atm 0/0/0 atm vc tx 64 !
In the following example, the I/O memory size is set to 20 MB.
Rommon>IOMEM=20
This section documents new commands. All other commands used with this feature are documented in the Cisco IOS Release 12.1 command reference publications.
To set the PVC segmentation buffer size, use the atm vc tx interface configuration command. To revert to the default value of 32, use the no form of this command.
atm vc tx number
Syntax Description
number Maximum number of packets in the buffer queue.
Possible values: 32, 64, 128, 256.
Defaults
32
Command Modes
ATM VC
Command History
12.1(1) DC1 This command was introduced on the node route processor (NRP) of the Cisco 6400.
Release
Modification
Usage Guidelines
For each PVC, a segmentation buffer slot is reserved for high-priority packets.
Examples
The following example sets the maximum number of packets in the segmentation buffer of each PVC to 64:
! service internal interface atm 0/0/0 atm vc tx 64 !
Related Commands
service internal Enables modification of PVC segmentation buffers. interface atm Configures an ATM interface type and enters interface configuration mode.
Command
Description
buffer--Storage area used for handling data in transit. Buffers are used in internetworking to compensate for differences in processing speed between network devices. Bursts of data can be stored in buffers until they can be handled by slower processing devices. Sometimes referred to as a packet buffer.
congested PVC--A PVC is congested if traffic enters the PVC faster than the traffic can be sent, and the segmentation buffer is full.
I/O--Input/output.
PVC--Permanent virtual circuit. Virtual circuit that is permanently established. PVCs save bandwidth associated with circuit establishment and tear down in situations where certain virtual circuits must exist all the time.
VC--Virtual channel. Logical circuit created to ensure reliable communication between two network devices. A VC is defined by a VPI/VCI pair, and can be either permanent (PVC) or switched (SVC).
Posted: Fri Aug 4 17:36:38 PDT 2000
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