Cisco Cisco IOS Software Release 12.4(23)

Delete files in the router flash memory before attempting to download new images. 

Use the delete command, not the erase command, to free up space. Entering the erase command 
removes all files, including the configuration.

Cisco 800 series routers use 4 MB of flash memory for storing internal information such as the ROM 
monitor. Only the remainder of the flash memory is available for storing Cisco IOS images; this 
remaining memory is displayed by using the show flash command. For example, if the router reports 
8 MB of flash memory, the actual amount of onboard flash memory is 12 MB, even though only 8 MB 
are displayed and available for Cisco IOS image storage.

Multilink PPP fragments large data packets to allow small voice packets to be interleaved between them. 
However, apart from FIFO queueing, no other kind of output queueing mechanisms are currently 
supported with PPP over ATM. Consequently, when multilink PPP is configured on the Cisco 800 series 
routers, large packets are fragmented, but small voice packets are not interleaved between them.

Network Address Translation (NAT) does not support alerting H.225 messages; therefore, NAT 
communication cannot be established between router endpoints. NAT support for H.323 signaling is 
limited to the application NetMeeting. 

PPP over AAL5SNAP encapsulation is currently not supported, although the context-sensitive help 
mentions that it can be configured.

To run IP Security (IPSec) successfully, the Cisco 800 series router clock must be set accurately. 
Cisco 800 series router clocks are set and maintained using Simple Network Time Protocol (SNTP). For 
best results, set up a Network Time Protocol (NTP) server to periodically send time information 
messages to Cisco 800 series routers. See the SNTP configuration and command reference 
documentation for configuration instructions. If you do not have an NTP server, you must reset the 
Cisco 800 series router clock using the clock set command each time you restart the router.

The isdn answer1 and isdn answer2 commands determine which called telephone numbers, for 
example 555-0100 and 555-0199, a Cisco 800 series router can answer. Using these commands limits a 
router to using the two dial peers that contain the telephone numbers 555-0100 and 555-0199. (When 
not using these commands, a router can use up to six dial peers.)