You may run into situations where a router in a remote location needs to dial in to a central router, but the toll charges are much higher if the remote router makes the call. This scenario is perfect for PPP Callback, where the callback client places a call to a callback server, authentication takes place, and the server then hangs up on the client! This ensures that the client isn't charged for the call. The server then calls the client back.
In the following example, R2 has been configured as the client and R1 is the callback server. Let's look at both configurations and the unique commands PPP Callback requires.
Client:
username R1 password CCIE
interface BRI0
ip address 172.12.12.2 255.255.255.0
encapsulation ppp
dialer map ip 172.12.12.1 name R1 broadcast 5557777
dialer-group 1
isdn switch-type basic-ni
ppp callback request
ppp authentication chap
Most of that configuration will look familiar to you, but the ppp callback request command might not. This command enables the BRI interface to request the callback.
Simple enough, right? The PPP Callback Server config requires more configuration and an additional map-class as well.
Server:
username R2 password CCIE
interface BRI0
ip address 172.12.12.1 255.255.255.0
encapsulation ppp
dialer callback-secure
dialer map ip 172.12.12.2 name R2 class CALL_R2_BACK broadcast 5558888
dialer-group 1
isdn switch-type basic-ni
ppp callback accept
ppp authentication chap
map-class dialer CALL_R2_BACK
dialer callback-server username
Examining the PPP Callback Server command from the top down...
dialer callback-secure enables security on the callback. If the remote router cannot be authenticated for callback, the incoming call will be disconnected.
The dialer map statement now calls the class CALL_R2_BACK, shown at the bottom of the config excerpt.
ppp callback accept enables PPP callback on this router.
dialer callback-server username tells the callback server that the device referenced in the dialer map statement is a callback client.
The only way to find out if the config works is to test it, so let's send a ping from R2 to R1 and see if the callback takes place.
R2#ping 172.12.12.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.12.12.1, timeout is 2 seconds:
02:45:42: BR0 DDR: Dialing cause ip (s=172.12.12.2, d=172.12.12.1)
02:45:42: BR0 DDR: Attempting to dial 5557777
02:45:42: %LINK-3-UPDOWN: Interface BRI0:1, changed state to up
02:45:42: BR0:1 DDR: Callback negotiated - Disconnecting now
02:45:42: BR0:1 DDR: disconnecting call
02:45:42: %ISDN-6-CONNECT: Interface BRI0:1 is now connected to 5557777 R1
02:45:42: %LINK-3-UPDOWN: Interface BRI0:1, changed state to down
02:45:42: DDR: Callback client for R1 5557777 created
02:45:42: BR0:1 DDR: disconnecting call.....
Success rate is 0 percent (0/5)
R2#
02:45:57: %LINK-3-UPDOWN: Interface BRI0:1, changed state to up
R2#
02:45:57: BR0:1 DDR: Callback received from R1 5557777
02:45:57: DDR: Freeing callback to R1 5557777
02:45:57: BR0:1 DDR: dialer protocol up
02:45:58: %LINEPROTO-5-UPDOWN: Line protocol on Interface BRI0:1, changed state to up
The callback was successfully negotiated, and the call then disconnected. R1 then called R2 back, and show dialer on R1 confirms the purpose of the call.
R1#show dialer
BRI0 - dialer type = ISDN
Dial String Successes Failures Last DNIS Last status
5558888 2 4 00:00:20 successful
0 incoming call(s) have been screened.
0 incoming call(s) rejected for callback.
BRI0:1 - dialer type = ISDN
Idle timer (120 secs), Fast idle timer (20 secs)
Wait for carrier (30 secs), Re-enable (15 secs)
Dialer state is data link layer up
Dial reason: Callback return call
Time until disconnect 99 secs
Connected to 5558888 (R2)
Pretty cool! PPP Callback isn’t just important for passing your CCNA and CCNP exams – in circumstances such as shown in this example, it can save your organization quite a bit of money!
Showing posts with label caller. Show all posts
Showing posts with label caller. Show all posts
Tuesday, December 23, 2008
Cisco CCNA / CCNP Certification Exam: Caller ID Screening And Callback
As a CCNA and/or CCNP candidate, you've got to be able to spot situations where Cisco router features can save your client money and time. For example, if a spoke router is calling a hub router and the toll charges at the spoke site are higher than that of the hub router, having the hub router hang up initially and then call the spoke router back can save the client money (and make you look good!)
A popular method of doing this is using PPP callback, but as we all know, it's a good idea to know more than one way to do things in Cisco World! A lesser-known but still effective method of callback is Caller ID Screening & Callback. Before we look at the callback feature, though, we need to know what Caller ID Screening is in the first place!
This feature is often referred to simply as "Caller ID", which can be a little misleading if you've never seen this service in operation before. To most of us, Caller ID is a phone service that displays the source phone number of an incoming call. Caller ID Screening has a different meaning, though. Caller ID Screening on a Cisco router is really another kind of password - it defines the phone numbers that are allowed to call the router.
The list of acceptable source phone numbers is created with the isdn caller command. Luckily for us, this command allows the use of x to specify a wildcard number. The command isdn caller 555xxxx results in calls being accepted from any 7-digit phone number beginning with 555, and rejected in all other cases. We'll configure R2 to do just that and then send a ping from R1 to R2. To see the results of the Caller ID Screening, debug dialer will be run on R1 before sending the ping. I’ve edited this output, since the output you see here will be repeated fire times – once for each ping packet.
R2(config-if)#isdn caller 555xxxx
R1#debug dialer
Dial on demand events debugging is on
R1#ping 172.12.12.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.12.12.2, timeout is 2 seconds:
03:30:25: BR0 DDR: Dialing cause ip (s=172.12.12.1, d=172.12.12.2)
03:30:25: BR0 DDR: Attempting to dial 8358662.
Success rate is 0 percent (0/5)
R1 doesn't give us any hints as to what the problem is, but we can see that the pings definitely aren't going through. On R2, show dialer displays the number of screened calls.
R2#show dialer
BRI0 - dialer type = ISDN
Dial String Successes Failures Last DNIS Last status
8358661 1 0 00:03:16 successful
7 incoming call(s) have been screened.
0 incoming call(s) rejected for callback.
The callback option mentioned in the last line shown above enables the router to reject a phone call, and then call that router back seconds later.
R2 will now be configured to initially hang up on R1, and then call R1 back.
R2(config-if)#isdn caller 8358661 callback
R1 will now ping R2. The pings aren't returned, but seconds later R2 calls R1 back.
R1#ping 172.12.12.2
Success rate is 0 percent (0/5)
R1#
03:48:12: BRI0: wait for isdn carrier timeout, call id=0x8023
R1#
03:48:18: %LINK-3-UPDOWN: Interface BRI0:1, changed state to up
R1#
03:48:18: BR0:1 DDR: dialer protocol up
R1#
03:48:19: %LINEPROTO-5-UPDOWN: Line protocol on Interface BRI0:1, changed state to up
R1#
03:48:24: %ISDN-6-CONNECT: Interface BRI0:1 is now connected to 8358662 R2
show dialer on R2 shows the reason for the call to R1 is a callback return call.
R2#show dialer
BRI0 - dialer type = ISDN
Dial String Successes Failures Last DNIS Last status
8358661 3 0 00:00:48 successful
7 incoming call(s) have been screened.
10 incoming call(s) rejected for callback.
BRI0:1 - dialer type = ISDN
Idle timer (120 secs), Fast idle timer (20 secs)
Wait for carrier (30 secs), Re-enable (15 secs)
Dialer state is data link layer up
Dial reason: Callback return call
Time until disconnect 71 secs
Connected to 8358661 (R1)
The drawback to Caller ID Callback is that not all telco switches support it, so if you have the choice between this and PPP Callback, you're probably better off with PPP Callback. However, it's always a good idea to know more than one way to get things done with Cisco!
A popular method of doing this is using PPP callback, but as we all know, it's a good idea to know more than one way to do things in Cisco World! A lesser-known but still effective method of callback is Caller ID Screening & Callback. Before we look at the callback feature, though, we need to know what Caller ID Screening is in the first place!
This feature is often referred to simply as "Caller ID", which can be a little misleading if you've never seen this service in operation before. To most of us, Caller ID is a phone service that displays the source phone number of an incoming call. Caller ID Screening has a different meaning, though. Caller ID Screening on a Cisco router is really another kind of password - it defines the phone numbers that are allowed to call the router.
The list of acceptable source phone numbers is created with the isdn caller command. Luckily for us, this command allows the use of x to specify a wildcard number. The command isdn caller 555xxxx results in calls being accepted from any 7-digit phone number beginning with 555, and rejected in all other cases. We'll configure R2 to do just that and then send a ping from R1 to R2. To see the results of the Caller ID Screening, debug dialer will be run on R1 before sending the ping. I’ve edited this output, since the output you see here will be repeated fire times – once for each ping packet.
R2(config-if)#isdn caller 555xxxx
R1#debug dialer
Dial on demand events debugging is on
R1#ping 172.12.12.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.12.12.2, timeout is 2 seconds:
03:30:25: BR0 DDR: Dialing cause ip (s=172.12.12.1, d=172.12.12.2)
03:30:25: BR0 DDR: Attempting to dial 8358662.
Success rate is 0 percent (0/5)
R1 doesn't give us any hints as to what the problem is, but we can see that the pings definitely aren't going through. On R2, show dialer displays the number of screened calls.
R2#show dialer
BRI0 - dialer type = ISDN
Dial String Successes Failures Last DNIS Last status
8358661 1 0 00:03:16 successful
7 incoming call(s) have been screened.
0 incoming call(s) rejected for callback.
The callback option mentioned in the last line shown above enables the router to reject a phone call, and then call that router back seconds later.
R2 will now be configured to initially hang up on R1, and then call R1 back.
R2(config-if)#isdn caller 8358661 callback
R1 will now ping R2. The pings aren't returned, but seconds later R2 calls R1 back.
R1#ping 172.12.12.2
Success rate is 0 percent (0/5)
R1#
03:48:12: BRI0: wait for isdn carrier timeout, call id=0x8023
R1#
03:48:18: %LINK-3-UPDOWN: Interface BRI0:1, changed state to up
R1#
03:48:18: BR0:1 DDR: dialer protocol up
R1#
03:48:19: %LINEPROTO-5-UPDOWN: Line protocol on Interface BRI0:1, changed state to up
R1#
03:48:24: %ISDN-6-CONNECT: Interface BRI0:1 is now connected to 8358662 R2
show dialer on R2 shows the reason for the call to R1 is a callback return call.
R2#show dialer
BRI0 - dialer type = ISDN
Dial String Successes Failures Last DNIS Last status
8358661 3 0 00:00:48 successful
7 incoming call(s) have been screened.
10 incoming call(s) rejected for callback.
BRI0:1 - dialer type = ISDN
Idle timer (120 secs), Fast idle timer (20 secs)
Wait for carrier (30 secs), Re-enable (15 secs)
Dialer state is data link layer up
Dial reason: Callback return call
Time until disconnect 71 secs
Connected to 8358661 (R1)
The drawback to Caller ID Callback is that not all telco switches support it, so if you have the choice between this and PPP Callback, you're probably better off with PPP Callback. However, it's always a good idea to know more than one way to get things done with Cisco!
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