CCNA exam success depends partially on knowing the details of ISDN, and there are plenty of them! To help you review for your CCNA exam, here are a few ISDN details that you must know on exam day. (They help in the real world, too – and there are still plenty of ISDN networks out there!
The Cisco-proprietary version of HDLC is the default encapsulation type for serial and ISDN interfaces.
R2#show interface serial0
Serial0 is up, line protocol is up
Hardware is HD64570
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255
Encapsulation HDLC, loopback not set, keepalive set (10 sec)
While there’s only one D-channel in BRI, PRI (US) and PRI (EU), the bandwidth of that D-channel does vary from BRI to PRI. It’s 16 kbps in BRI and 64 kbps in both PRI versions.
The global command isdn switch-type must be configured before you can even begin to have ISDN work. show isdn status will tell you whether or not you’ve done this correctly.
R2#show isdn status
**** No Global ISDN Switchtype currently defined ****
ISDN BRI0 interface
dsl 0, interface ISDN Switchtype = none
Layer 1 Status:
DEACTIVATED
Layer 2 Status:
Layer 2 NOT Activated
Layer 3 Status:
0 Active Layer 3 Call(s)
PAP allows passwords to be different; CHAP requires that they be the same.
PAP requires the “ppp pap sent-username” interface-level command. CHAP has no equivalent command.
Define interesting traffic with dialer-list and link that list to the interface with dialer-group.
R2#conf t
R2(config)#dialer-list 1 proto ip permit
R2(config)#int bri0
R2(config-if)#dialer-group 1
The dialer idle-timeout value is expressed in seconds, not minutes. (Even IOS Help isn’t totally clear on this.)
R2(config)#int bri0
R2(config-if)#dialer-group 1
R2(config-if)#dialer idle-timeout ?
<1-2147483> Idle timeout before disconnecting a call
R2(config-if)#dialer idle-timeout 120
Dialer map maps a remote IP address to a remote phone number. You never dial the local router’s phone number.
dialer load-threshold requires the ppp multilink command to be configured, and the value of dialer load-threshold is expressed as a ratio of 255, NOT 100. For example, if you want the second b-channel to come up when the first reaches 50% of capacity, the value to express with dialer load-threshold would be 50% of 255 – which equals 127.
R2(config)#int bri0
R2(config-if)#encap ppp
R2(config-if)#ppp multilink
R2(config-if)#dialer load-threshold ?
<1-255> Load threshold to place another call
Success on the CCNA exam depends on knowing the details. Keep studying, keep practicing on real Cisco routers and switches, keep a positive attitude, and you're on your way to CCNA exam success!
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