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The SA-Comp/1 and SA-Comp/4 data compression service adapters (CSAs) are available on Cisco 7200 series routers, on second-generation Versatile Interface Processors (VIP2s) in Cisco 7500 series routers, and on Cisco 7000 series routers with the 7000 Series Route Switch Processor (RSP7000) and 7000 Series Chassis Interface (RSP7000CI). (CSAs require VIP2 model VIP2-40.)
These service adapters provide high-performance, hardware-based data compression capabilities via simultaneous Stacker compression data compression algorithms with independent full-duplex compression and decompression capabilities on point-to-point (PPP) encapsulated packets.
The SA-Comp/1 supports up to 64 compression contexts and the SA-Comp/4 supports up to 256 compression contexts. There is one compression context per PPP link.
The CSA supports the compression and decompression of data passing through synchronous serial interfaces that are configured for a peak line rate of 16 Mbps or lower. You can use the CSA to compress and decompress data passing through any synchronous serial interface; however, the serial interface must be configured to send and receive data at a rate no greater than 16 Mbps. For example, the CSA supports high-speed serial interfaces (such as HSSI interfaces) if the interfaces are configured for a peak line rate of 16 Mbps or lower.
On the Cisco 7200 series routers you can optionally specify which CSA the interface uses to perform hardware compression.
This feature is supported on these platforms:
You can configure point-to-point compression on serial interfaces that use PPP encapsulation. Compression reduces the size of a PPP frame via lossless data compression. PPP encapsulations support both predictor and Stacker compression algorithms.
If the majority of your traffic is already compressed files, do not use compression.
When you configure Stacker compression on Cisco 7000 series routers with RSP7000, on Cisco 7200 series routers, and on Cisco 7500 series routers, there are three methods of compression: hardware compression, distributed compression, and software compression. Specifying the compress stac command with no options causes the router to use the fastest available compression method:
Using hardware compression in the CSA frees the router's main processor for other tasks. You can also configure the router to use the VIP2 to perform compression by using the distributed option, or to use the router's main processor by using the software option. If the VIP2 is not available, compression is performed in the router's main processor.
When compression is performed in software installed in the router's main processor, it might significantly affect system performance. We recommend that you disable compression in the router's main processor if the router CPU load exceeds 40 percent. To display the CPU load, use the show process cpu EXEC command.
To configure compression over PPP, perform the following tasks in interface configuration mode:
The following example enables hardware compression and PPP encapsulation on serial interface 3/1/0:
router(config)#interface serial 3/1/0router(config-if)#encapsulate ppprouter(config-if)#compress stacrouter(config-if)#exitrouter(config)#
This section documents new or modified commands. All other commands used with this feature are documented in the Cisco IOS Release 11.2 command references.
To configure compression for Link Access Procedure, Balanced (LAPB), Point-to-Point Protocol (PPP), and High-Level Data Link Control (HDLC) encapsulations, use the compress interface configuration command. On Cisco 7000 series routers with RSP7000, Cisco 7200 series routers, and Cisco 7500 series routers, hardware compression on the compression service adapter (CSA) is supported for PPP links. To disable compression, use the no form of this command.
compress {predictor | stac}Compression is disabled.
Interface configuration
This command first appeared in Cisco IOS Release 10.0 (as compress predictor). The command compress {predictor | stac} first appeared in Cisco IOS Release 10.3.
This command was modified in Cisco IOS Release 11.2 P and 11.1 CA to include the distributed, software, and csa keywords.
Using CSA hardware compression on Cisco 7000 series routers with RSP7000, Cisco 7200 series routers, and Cisco 7500 series routers removes the compression and decompression responsibilities from the VIP2 or the main processor installed in the router. By using the compress stac command, the router determines the fastest compression method available on the router.
When using hardware compression on Cisco 7200 series routers with multiple CSAs, you can optionally specify which CSA is used by the interface to perform compression. If no CSA is specified, the router determines which CSA is used. On Cisco 7000 series routers with RSP700 and on Cisco 7500 series routers, the router uses the CSA on the same VIP2 as the interface.
You can configure point-to-point software compression for all LAPB, PPP, and HDLC encapsulations. Compression reduces the size of frames via lossless data compression. HDLC encapsulations supports the Stacker compression algorithm. PPP and LAPB encapsulations support both predictor and Stacker compression algorithms.
When compression is performed in software installed in the router's main processor, it might significantly affect system performance. We recommend that you disable compression if the CPU load exceeds 40 percent. To display the CPU load, use the show process cpu EXEC command.
Compression requires that both ends of the serial link be configured to use compression.
If the majority of your traffic is already compressed files, we recommend that you not use compression. If the files are already compressed, the additional processing time spent in attempting unsuccessfully to compress them again will slow system performance.
Table 55 provides general guidelines for deciding which compression type to select.
| Situation | Compression Type to Use |
|---|---|
| The bottleneck is caused by the load on the router. | Predictor |
| The bottleneck is the result of line bandwidth or hardware compression on the CSA is available. | Stacker |
| Most files are already compressed. | None |
Software compression makes heavy demands on the router's processor. The maximum compressed serial line rate depends on the type of Cisco router you are using and which compression algorithm you specify. Table 56 shows a summary of the compressed serial line rates for software compression. The maximums shown in Table 56 apply to the "combined" serial compressed load on the router. For example, a Cisco 4000 series router could handle four 64-kbps lines using Stacker or one 256-kbps line. These maximums also assume there is very little processor load on the router aside from compression. Lower these numbers when the router is required to do other processor-intensive tasks.
| Compression Method | Cisco 1000 Series | Cisco 3000 Series | Cisco 4000 Series | Cisco 4500 | Cisco 4700 | Cisco 7000 Family |
|---|---|---|---|---|---|---|
| Stacker (kbps) | 128 | 128 | 256 | 500 | T1 | 256 |
| Predictor (kbps) | 256 | 256 | 500 | T1 | 2xT1 | 500 |
We recommend that you do not adjust the maximum transmission unit (MTU) for the serial interface and the LAPB maximum bits per frame (N1) parameter.
The following example enables hardware compression and PPP encapsulation on serial interface 3/1/0:
interface serial 3/1/0 encapsulate ppp compress stac
The following example enables predictor compression on serial interface 0 for a LAPB link:
interface serial 0 encapsulation lapb compress predictor
encapsulation lapb
encapsulation ppp
encapsulation x25
ppp compress
show compress
show processes
To display compression statistics, use the show compress EXEC command.
show compressThis command has no arguments or keywords.
EXEC
This command first appeared in Cisco IOS Release 10.0.
This information was modified in Cisco IOS Release 11.2 P and 11.1 CA to include sample output for hardware compression (implemented in the CSA hardware).
The following is sample output from the show compress command when software compression is used on the router:
Router# show compress
Serial0
uncompressed bytes xmt/rcv 10710562/11376835
1 min avg ratio xmt/rcv 2.773/2.474
5 min avg ratio xmt/rcv 4.084/3.793
10 min avg ratio xmt/rcv 4.125/3.873
no bufs xmt 0 no bufs rcv 0
resets 0
Table 57 describes the fields shown in the display.
| Field | Description |
|---|---|
| Serial0 | Name and number of the interface. |
| uncompressed bytes xmt/rcv | Total number of uncompressed bytes sent and received. |
| 1 min avg ratio xmt/rcv
5 min avg ratio xmt/rcv 10 min avg ratio xmt/rcv | Static compression ratio for bytes sent and received, averaged over 1, 5, and 10 minutes. |
| no bufs xmt | Number of times buffers were not available to compress data being sent. |
| no bufs rcv | Number of times buffers were not available to uncompress data being received. |
| resets | Number of resets (for example, line errors could cause resets). |
The following is sample output from the show compress command when hardware compression is enabled (that is, compression is implemented in the CSA hardware):
Router# show compress
Serial6/1
Hardware compression enabled
Compressed bytes sent: 402 bytes 0 Kbits/sec ratio: 4.092
Compressed bytes recv: 390 bytes 0 Kbits/sec ratio: 3.476
restarts:1
last clearing of counters: 1278 seconds
Table 58 describes the fields shown in the display. The information displayed by the show compress command is the same for hardware and distributed compression. For Cisco 7200 series routers with multiple CSAs, an additional line is displayed indicating the CSA in use.
| Field | Description |
|---|---|
| Serial6/1 | Name and number of the interface. |
| Compressed bytes sent | Total number of compressed bytes sent including the kilobits per second. |
| Compressed bytes recv | Total number of compressed bytes received including the kilobits per second. |
| ratio | Compression ratio for bytes sent and received since the link last came up or since the counters were last cleared. |
| restarts | Number of times the compression process restarted or reset. |
| last clearing of counters | Duration since the last time the counters were cleared with the clear counters command. |
The compression service adapters can be monitored with the Cisco Compression Service Adapter (CSA) MIB (CISCO-COMPRESSION-SERVICE-ADAPTER-MIB.my). For information on accessing Cisco MIB files, refer to the Cisco MIB User Quick Reference.
For more information on the CSA service adapters, refer to the SA-Comp/1 and SA-Comp/4 Data Compression Service Adapter Installation and Configuration publication.
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