Thursday, March 09, 2006


Configuring data compression on PPP link. (Question #33)

Consider two routers connected back-to-back via serial links with PPP encapsulation. The links are already up. You decide to configure STAC data compression on the link and so you enter configuration mode on the routers at either end and type in compress stac under the interface configuration.

However, when you then issue a show compress command you see that compression is not enabled. Here is the output from one router

R1#sh runn int s 2
Building configuration...

Current configuration : 112 bytes
interface Serial2
ip address
encapsulation ppp
no keepalive
compress stac

R1# show compress
Compression not active
uncompressed bytes xmt/rcv 0/0
compressed bytes xmt/rcv 0/0
1 min avg ratio xmt/rcv 0.000/0.000
5 min avg ratio xmt/rcv 0.000/0.000
10 min avg ratio xmt/rcv 0.000/0.000
no bufs xmt 0 no bufs rcv 0
resyncs 0

What is the problem here? Why isn't compression getting enabled?


I am going to answer this, because I just had this problem and couldn't find the answer. Probably everyone learns right off the bat, when you make a change to an interface, it is a good idea to reset the interface to force the changes through. One does this via the "no shutdown" command in the global configuration of that interface.

For me, it was that simple to go from "Compression not active" to "Software compression enabled."

Cisco also says it is a good idea to check your CPU utilization and make sure it is < 65%. You do this with a: show process cpu
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Hope this helps!

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