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Cisco 3600 series routers now support a compression port module that provides high-performance, hardware-based data compression using simultaneous Stacker compression algorithms. Independent full-duplex compression and decompression capabilities are used on point-to-point (PPP) encapsulated packets.
A router's central processing unit is generally reserved for tasks such as creating and maintaining routing tables, not performing compression duties. When a hardware compression port module is used in a router, all compression activity is offloaded from the router's central processing unit. This kind of hardware configuration is needed for routers that require B-channel compression for multiple WAN connections, such as two ISDN PRI interfaces carrying 46 B channels. Signaling over the D channel is not compressed. One compression port module supports up to 128 WAN interfaces.
WAN or serial connections have limited bandwidth and greatly benefit from compressed data. For example, a hardware compression card that achieves 2:1 compression can compress 500 bytes of data down to 250 bytes. Transmission time is reduced by 50 percent. A line that transmits at 56 kbps without compress transmits at 112 kbps with compression. An achieved compression ratio or rate is dependent on the type of file being compressed. Graphics files, sound files, and text files all have different compression requirements and results.
A hardware card can compress and decompress outgoing and incoming data. For negotiated compression configured between two routers, the incoming compressed data sent by the remote router is decompressed by the receiving or local compression card.
This feature is supported on these platforms:
You can configure point-to-point compression on interfaces that use PPP encapsulation. Compression reduces the size of a PPP frame via lossless data compression. PPP encapsulations support Stacker and Predictor compression algorithms, but the compression port module installed in Cisco 3600 series routers support only Stacker compression over PPP encapsulations.
If the majority of your traffic is already compressed files, do not use compression. A hardware compression card should be used if the router's main processor 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. Although the Serial interface in slot 3/1 is configured with the compress stac command, the actual data compression takes place in the hardware compression card inserted in a different slot.
router(config)#interface serial 3/1router(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 Point-to-Point Protocol (PPP) encapsulation, use the compress interface configuration command.To disable compression, use the no form of this command.
compress stacCompression is disabled.
Interface configuration
This command first appeared in Cisco IOS Release 10.0 (as compress predictor). The command compress stac first appeared in Cisco IOS Release 10.3.
The compress stac command determines the fastest compression method available on the router.
PPP encapsulations support Stacker and Predictor compression algorithms, but the compression port module installed in Cisco 3600 series routers support only Stacker compression over PPP encapsulations.
You can configure point-to-point software compression for PPP encapsulations. Compression reduces the size of frames via lossless data compression. PPP encapsulation supports Stacker compression algorithms.
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.
The following example enables hardware compression and PPP encapsulation on serial interface 3/1:
interface serial 3/1 encapsulate ppp compress stac
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.
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 35 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
Serial3/1
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 36 describes the fields shown in the display.
| Field | Description |
|---|---|
| Serial3/1 | 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. |
For more information on the hardware compression card, refer to the publication Compression Network Module Configuration Note.
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