Linux Print System at Cisco Systems, Inc.

Cisco runs a redundant system of 50 print servers using Linux, Samba and Netatalk. It prints to approximately 1,600 printers worldwide, serving 10,000 UNIX and Windows 95 users, some of whom are in mission-critical environments.
Initial Cisco Configuration

When I started at Cisco, the printing was becoming difficult to manage (see Figure 1). While Cisco was no worse in this respect than most companies, they depended on printing for their manufacturing process. They knew it was very important for the system to work efficiently. Thus, I was taken on to maintain and improve on the existing UNIX printing, which used two small SunOS-based print servers.

Although these two servers by no means controlled the entire printing at Cisco, since I was a dedicated print system administrator, it was widely assumed that I did.

Figure 1. Original Printing Configuration at Cisco

The company's big UNIX servers were sometimes printing to one print server, sometimes to the other. The UNIX workstations were printing either via the print servers or directly to the printer. The Apple Macintoshes (used extensively on the desktop) always printed directly. Most of the printers recognized only AppleTalk, so Gatorboxes (from Caiman Systems) were used to translate UNIX print jobs into AppleTalk.

Caiman Systems had gone out of business and the Gatorboxes were intermittently crashing. My predecessors had started to enable TCP/IP protocols on some of the Hewlett-Packard (HP) printers so that the UNIX print servers could talk to them directly. Doing this required either plugging an IP address into each printer (via the front panel) or setting up a bootptab entry in each print server so that a printer could find its IP address using the BOOTP protocol (see Glossary).

In theory, one print server was the main one and the other was a backup. However, these two servers were substantially different in configuration. Duplication of the setups was manual, i.e., one had to configure the print queue on both machines. Some central UNIX servers were queueing to the “primary” and others to the “backup” print server. A few printers were set up for printing on some of the central UNIX servers and others were not. I spent much of my time tracking down print problems, only to find they usually came down to an incorrect configuration.

Cisco had never directly instructed me to design a new print system. They just asked me to make sure printing worked. They trusted me to do whatever I felt necessary. My motivation for improving it was simply that I find repetitive tasks boring and unfulfilling. I find nothing more frustrating than treating the symptom, while ignoring the disease. I never decided to throw out the old system entirely, I just slowly improved on it—tackling the biggest problem of the moment.

Remove Duplication in the Client System

Each printer needed to be individually set up on each UNIX server (the LPR client). This meant a lot of manual work, either when setting up a new UNIX server or creating a new printer. I looked at the client LPR system and realized it had a very simple function: just forward the print job to the print server.

Here is a typical /etc/printcap entry for the printer “foo”, which sends the job straight on to the print server “prntsrv”:

foo:\
    :mx#0:\
    :sh:\
    :sd=/var/spool/lpd/foo:\
    :lf=/var/spool/lpd/foo/log:\
    :lp=/var/spool/lpd/foo/.null:\
    :rm=prntsrv:\
    :rp=foo:

The only item which changed when using a different printer is the word foo.

I took the LPR source and replaced the routines that look for the entry for a particular printer in /etc/printcap with routines that faked the entry. If LPR asked for the printer “bar”, my routines would return a printcap entry much like the one above, but with bar in place of foo. The only other variable was the name of the print server which was looked up in a master configuration file I created for the whole system. There were a few other things to do, such as creating a spool directory, but essentially this is all the work the routines did.

The remainder of the LPR code proceeded as before, not realizing anything had changed. Since I hadn't touched the remainder of the code, I had very few bugs. I had removed a large source of information duplication, and I could now be sure that all the company's printers were available on all the central UNIX servers, with print jobs being sent to the correct print server.

Note that the client will also accept a print job for a non-existent printer (it doesn't know the difference) and send it to the print server. The print server will reject the job, but will not say why (the protocol doesn't allow it). The client keeps retrying for 48 hours before finally rejecting the job and e-mailing the user. This is not an ideal situation but was acceptable at Cisco.

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