Cisco Certification Exam Review:
ISDN
By Chris Bryant, CCIE #12933
BRI: 2 b-channels, both with 64 kbps capacity. 1 d-channel, 16 kbps.
PRI (North America & Japan): 23 b-channels, 1 d-channel, all 64 kbps.
PRI (Europe, Australia): 30 b-channels, 1 d-channel, all 64 kbps.
Authentication via PAP and CHAP, callback, compression, and multilink PPP are all LCP features.
Dialer maps always refer to the remote device's IP address, the remote device's name, and the remote device's phone number.
PPP callback clients are configured with ppp callback request, while callback servers are configured with ppp callback accept.
When dynamic TEI assignment is in effect, the range is 64 - 126.
The PRI linecodes are AMI, B8ZS, and HDB3.
The PRI framing types are SF, ESF, and CRC4.
In the command backup load 50 20, the backup interface will come up when the primary interface hits 50% of its capacity, and the backup interface will drop when the combined load of the two interfaces drops below their combined capacity.
The interface dialer command is used to create a rotary group as well as a dialer profile. The dialer rotary-group command is required on physical interfaces that will be part of a rotary group.
SPIDs are entered with the isdn spid1 and isdn spid2 command IF they're required by the service provider. Always run show isdn status to double-check their validity, and if they look good in the config but aren't showing as valid, try closing and reopening the physical interface.
The pri-group command is used to logically group timeslots in PRI.
Dialer Watch brings up the ISDN link only if a watched route disappears from the routing table AND there's no other valid path to that destination. The dialer watch-group command defines the watched routes.
The command backup delay 10 20 will bring up the standby interface 10 seconds after the line protocol drops on the primary, and brings the standby link down 20 seconds after the primary interface's line protocol comes back up.
The command backup interface defines the backup interface, and puts the backup interface into standby mode, making it unusable for other purposes.
The commands ppp multilink and dialer load-threshold enable PPP multilink. Remember, the dialer load-threshold has a value range of 1 - 255, not 1 - 100.
The command dialer fast-idle defines the amount of time that an "up" but inactive ISDN link will remain up when there's a second call waiting to use that link.
To troubleshoot a PPP PAP or CHAP issue, you can now use either debug ppp authentication or debug ppp negotiation.
To debug the D-channel at L2, run debug isdn q921. To debug the D-channel at L3, run debug isdn q931.
To find the source and destination IP addresses of a DDR call, run either show dialer or debug dialer. (show debug is short for show dialer interface)
To view only one B-channel on a BRI interface, run show interface bri0 1 or 2.
To view the number and status of active calls, run show isdn status.
To view the kickin load statistics, run show interface x.
To avoid packet drop while the ISDN link is being constructed, build a queue for packets to be held in with the dialer hold-queue command.
To avoid having a BRI interface being put into standby mode by using it as a backup interface, create dialer profiles and use one of the dialer profiles as the backup interface. The physical interface will not be put into standby mode and can still be used to place other calls defined in other dialer profiles.
There are two L3 specifications for ISDN signaling - ITU-T I.450 and ITU-T I.451.
To view the current load on the CPU, run show process cpu.
The ip unnumbered command allows the interface this command is configured upon to use an IP address already assigned to another interface.
The dialer callback-server command enables the dial interface to return a call once callback has been successfully negotiated.
The U reference point is a 2-wire circuit that is found only in North American ISDN networks.
The backup interface command will not allow you to specify a subinterface as the secondary interface.
To your success,
Chris Bryant
CCIE #12933
chris@thebryantadvantage.com
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