Setting the Log Size
Problem
You want to change the size of the router's log.
Solution
You can use the optional size attribute with the logging buffered configuration command to change the size of your router's internal log buffer:
Router#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#logging buffered 16000
Router(config)#end
Router#
Be careful, though, because adjusting the size of the router's logging buffer wipes out all of the current contents of the buffer.
Discussion
The typical default size of a router's logging buffer is 4,096 bytes (although some high-end routers will default to a higher value). A buffer of this size can hold approximately 50 log messages before overwriting occurs. Fifty messages, although better than no logging, is relatively small, and most engineers will want increase their buffer size to store more messages. To check the size of your router's logging buffer, use the show buffer command:
Router>show logging
Syslog logging: enabled (0 messages dropped, 0 messages rate-limited, 0 flushes, 0 overruns)
Console logging: level debugging, 653 messages logged
Monitor logging: level debugging, 65 messages logged
Buffer logging: level debugging, 1 messages logged
Logging Exception size (4096 bytes)
Trap logging: level informational, 657 message lines logged
Log Buffer (16000 bytes):
Router>
As you can see, this router's buffer size is currently set to 16,000 bytes (roughly 16 KB).
The router will theoretically accept a wide range of buffer sizes ranging from 4,096 bytes (nothing smaller) to an astronomical 2,147,483,647 bytes (about 2 GB). Exercise caution when choosing the size of your logging buffer because it comes out of the router's system memory. A good rule is to set your logging buffer to 16 KB for smaller routers. Routers with more than 32 MB of memory can safely dedicate 32 KB, or even 64 KB, without problems. To be safe, always check the amount of free memory on your router with the show memory command before increasing your buffer size.
We note in passing that you can combine the keywords in Recipe 18.1 and 18.2 into a single router command:
Router#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#logging buffered 16000 informational
Router(config)#end
Router#
In this case, we set the buffer size to 16,000 bytes, and the severity level to informational, by using a single configuration command.