Cisco 2600 SERIES 8MB COMPRESSION MODULE Specification Guide
A. This is known problem Cisco bug ID CSCdk39968 (
registered customers only
) . The solution
is to upgrade to Cisco IOS Software Release 11.3(7) or later code.
Q. Why does the show compress command show software compression
when hardware compression is enabled?
when hardware compression is enabled?
A. This can happen for a number of reasons:
If a link is in a shutdown state, issue the show compress command in order to show
that it runs software compression. When the link comes up, it shows hardware
compression. The command shows this because of the necessity to negotiate
hardware compression, either through CCP for PPP, or through the FRF.9 process for
Frame Relay. In order to run this negotiation, the link must not be shut down.
that it runs software compression. When the link comes up, it shows hardware
compression. The command shows this because of the necessity to negotiate
hardware compression, either through CCP for PPP, or through the FRF.9 process for
Frame Relay. In order to run this negotiation, the link must not be shut down.
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When you run hardware compression over PPP with some earlier versions of Cisco
IOS software, do not type compress stac in order to issue the command, it is
necessary to type ppp compress stac in order to issue the command. This is a
holdover from an earlier command syntax.
IOS software, do not type compress stac in order to issue the command, it is
necessary to type ppp compress stac in order to issue the command. This is a
holdover from an earlier command syntax.
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In order to run hardware compression in a 7500 series router, the compression service
adapter must be in the same VIP2 as the interface that is to be compressed. Interfaces
on other VIP2s and on interface processor cards cannot communicate with the
compression service adapters.
adapter must be in the same VIP2 as the interface that is to be compressed. Interfaces
on other VIP2s and on interface processor cards cannot communicate with the
compression service adapters.
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Q. What does it mean if show compress indicates that you have a
compression ratio of less than one? What can cause it?
compression ratio of less than one? What can cause it?
A. A compression ratio of less than one means that the compression algorithm increases the
size of the data. It does not decrease the size of the data. This is caused by one of these
reasons:
size of the data. It does not decrease the size of the data. This is caused by one of these
reasons:
If you try to compress data that has already gone through a compression algorithm at
a higher layer. Compression algorithms are designed with the assumption that there
exists redundancy to be removed, and the algorithm performs its calculations
accordingly. If data has already been compressed, the redundancy has already been
removed, and if you apply another compression algorithm to the same data, it can
result in the expansion of the data. Such a result occurs if you try to compress at
Layer 2 large data packets that contain zipped data. The only previously
uncompressed portion of the payload are the TCP/IP headers. A large data packet
(such as FTP) can expand such that the total compression ratio is less than one.
a higher layer. Compression algorithms are designed with the assumption that there
exists redundancy to be removed, and the algorithm performs its calculations
accordingly. If data has already been compressed, the redundancy has already been
removed, and if you apply another compression algorithm to the same data, it can
result in the expansion of the data. Such a result occurs if you try to compress at
Layer 2 large data packets that contain zipped data. The only previously
uncompressed portion of the payload are the TCP/IP headers. A large data packet
(such as FTP) can expand such that the total compression ratio is less than one.
♦
Compression ratios of less than one can result from an overly taxed CPU. If you run
software compression on a router that does not have the cycles in order to perform the
necessary calculations, the process stops. One symptom of this is compression ratios
of less than one. The only solutions are to remove compression from some links, or
to install a hardware compression module.
software compression on a router that does not have the cycles in order to perform the
necessary calculations, the process stops. One symptom of this is compression ratios
of less than one. The only solutions are to remove compression from some links, or
to install a hardware compression module.
♦
Related Information
Cisco IOS Data Compression
•
Data Compression AIM for the Cisco 2600 Series
•
Data Compression AIM for the Cisco 3660 Series
•
Routing IP, chapter on Compressing TCP Headers
•
Configuring the SA−COMP/1 and SA−COMP/4 Data Compression Service Adapters
•
Cisco − WAN Compression FAQs