R0106-HP MSR Router Series Layer 2 - WAN Configuration Guide(V7)

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Overview
The callback feature has the following benefits:
• Enhanced security—The server initiates a return call to the number configured locally, which
prevents caller number spoofing.
• Reduced charges—The charge payer can be changed when the charge rates are different in the
two directions.
ISDN caller number callback uses the following process:
1. When a server receives a call, it determines the configured dialer call-in command relevant to the
dial-in number.
{ For traditional DDR, the server queries the relevant dialer call-in command from the dialer
call-in commands configured on the physical interface or the corresponding dialer interface
receiving the call.
{ For bundle DDR, the server queries the relevant dialer call-in command from the dialer call-in
commands configured on the dialer interface receiving the call.
2. After determining the relevant dialer call-in command, the system matches the dial-in number
against the number configured in the dialer call-in command, by using rightmost matching.
Rightmost matching compares the call-in number with the configured numbers digit by digit,
starting with the rightmost character. Asterisks (*) are used as wildcards to match any character.
The number of digits to be compared is the number of digits of the shorter number. For example,
compare the call-in number 12345 with the configured number 345, and the two numbers match
because the three rightmost characters match. If a dial-in number matches multiple dialer call-in
commands, the best match is selected in the following order:
{ The one with the fewest asterisks (*).
{ The one that is found first.
3. The server can process an incoming call in one of the following ways, depending on the result of
matching the dial-in number against numbers configured in dialer call-in commands at the local
end:
{ Deny the incoming call—One or multiple dialer call-in commands exist, but no match is found.
{ Accept the incoming call—The dial-in number matches a dialer call-in command without the
callback keyword or if no dialer call-in command exists.
{ Callback—The dial-in number matches a dialer call-in command with the callback keyword.
Configuration restrictions and guidelines
When you configure ISDN caller number callback, follow these restrictions and guidelines:
• Configure the dialer route or dialer number command on a dialup interface where the dialer call-in
command is configured with the callback keyword. To ensure correct callback, you must use the
same value for dial-number in the dialer route or dialer number command and remote-number in
the dialer call-in command.
• To leave enough time for a server to call back, set the holddown timer (configured by using the
dialer timer enable command) on the client at least 10 seconds longer than the holddown timer on
the server. HP recommends that you set this timer to 5 seconds (the default) on the server and to 15
seconds on the client.