Why are there negative values in the %Bytes Saved column in the output of "compression show"?


Article ID: 167492


Updated On:




The %Bytes Saved column in the output of compression show shows negative values for some traffic classes:

PacketShaper# comp show
Tunnel Interface: main
Tunnel Partner:
Tunnel Status: Normal operation (Up: 1h 10m 10s, Idle: 1s)
Last Shutdown (tunnelProcessShutdownRequest)
Tunnel Quality: 89
Tunnel Savings: 583 Bpm

Compressors Type %Bytes Saved
GROUP DICTIONARY cna-1M 2% ( 1 secs old)
ICMP cna-1M 9% ( 1 secs old)
Telnet-Clear cna-1M -5% ( 3 secs old)
LotusNotes cna-1M -18% ( 12 secs old)
pcANYWHERE-D cna-1M 75% ( 15 secs old)
SNMP-Mon cna-1M ----% (196 secs old)
Observed cna-1M ----% (234 secs old)
NetBIOS-IP-NS cna-1M ----% (2757 secs old)
SAP cna-1M ----% (5314 secs old)

Decompressors Type %Bytes Saved
DICTIONARY cna-1M 0% ( 1 secs old)


Negative compression gains can result when the original packets are not compressible. The post compression IPCOMP packet ends up being larger than the pre-compressed packet. In order for positive compression gains to take place, the original packet must be compressed by a greater number of bytes than are added by the additional IPCOMP packet header. 

The compression show output is a snapshot showing the compression gains for traffic types over the last eight packets compressed.  For a more realistic view of compression gains, use the compression reports for those classes to measure compression gains over a longer period of time.

If class compression reports consistently show low or negative compression gains,  compression for a class can be disabled using the command:

class compress <tclass> off