Table Of Contents
Segmentation and Reassembly
Buffer Management EnhancementsReduced Segmentation Buffer Size
Increased Input/Output Memory Size
Reserved Segmentation Buffer Slot for High-Priority Packets
Supported Standards, MIBs, and RFCs
Setting the Segmentation Buffer Size
Verifying the PVC Segmentation Buffer Size
PVC Segmentation Buffer Size Configuration
Segmentation and Reassembly
Buffer Management Enhancements
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:
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Supported Standards, MIBs, and RFCs
Feature Overview
This feature module describes the following enhancements to segmentation and reassembly (SAR) buffer management:
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Reduced Segmentation Buffer Size
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.
Increased Input/Output Memory 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.
Reserved Segmentation Buffer Slot for High-Priority Packets
For each PVC, a segmentation buffer slot is reserved for high-priority packets.
Benefits
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.
Restrictions
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.
Related Documents
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Supported Platforms
The Segmentation and Reassembly Buffer Management Enhancements are supported on the node route processor (NRP) of the Cisco 6400 UAC.
Supported Standards, MIBs, and RFCs
Standards
None
MIBs
None
RFCs
None
Prerequisites
Cisco recommends that you simultaneously run Cisco IOS Release 12.1(1) DB on the node switch processor (NSP) of the Cisco 6400 UAC.
Configuration Tasks
See the following sections for configuration tasks for the Segmentation and Reassembly Buffer Management Enhancements feature. Each task in the list is optional.
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Setting the Segmentation Buffer Size
Caution
To manually set the size of all PVC segmentation buffers, enter the following commands beginning in global configuration mode:
Verifying the PVC Segmentation Buffer Size
To verify successful configuration of the segmentation buffer size, use the show running-config EXEC command.
Setting the I/O Memory Size
To manually set the size of I/O memory, enter the following command in ROMMON mode:
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.
Table 1 Allowed Values of I/O Memory on the Cisco 6400 NRP
64 MB DRAM
18 MB to 24 MB
18 MB
128 MB DRAM
18 MB to 60 MB
36 MB
Note
Verifying the I/O Memory Size
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 memoryHead Total(b) Used(b) Free(b) Lowest(b) Largest(b)Processor 60E27540 35490496 5517076 29973420 14919296 29838876I/O 3000000 16777216 6006460 10770756 5385388 10770108Configuration Examples
This section provides the following configuration examples:
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PVC Segmentation Buffer Size Configuration
In the following example, the PVC segmentation buffer size is set to 64 packets.
!service internalinterface atm 0/0/0atm vc tx 64!I/O Memory Size Configuration
In the following example, the I/O memory size is set to 20 MB.
Rommon>IOMEM=20Command Reference
This section documents new commands. All other commands used with this feature are documented in the Cisco IOS Release 12.1 command reference publications.
atm vc tx
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
no atm vc tx number
Syntax Description
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.
Usage Guidelines
For each PVC, a segmentation buffer slot is reserved for high-priority packets.
Caution
Examples
The following example sets the maximum number of packets in the segmentation buffer of each PVC to 64:
!service internalinterface atm 0/0/0atm 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.
Glossary
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).
