Vaibhav Singh

The Datacenter NOC where I work requires the network team working in night shifts to test the working status of ISDN circuits as they are very prone to failure due to multiple reasons.

Let’s find the WAN IP address of a remote branch by logging into the Regional Office (RO) router.

RO#sh int desc
Interface                      Status         Protocol Description
Gi0/1                          up             up       *** Connected to Local LAN-Switch ***
Fa0/2                          up             up       *** Connected to Primary Router ***
Gi0/2                          admin down     down     *** Connected to Primary Router ***
Gi0/3                          admin down     down
Se1/1:0                        up             up       *** ROUTER11 ***
Se1/1:1                        up             up       *** ROUTER12 ***
Se1/1:2                        up             down     *** ROUTER13 ***

Login to the remote branch office (BO) router

RO#sh int Se1/1:1
Serial1/1:1 is up, line protocol is up
  Hardware is PA-MC-8TE1 Plus
  Description: *** ROUTER12 ***
  Internet address is 172.18.18.111/30
  MTU 1500 bytes, BW 64 Kbit, DLY 20000 usec,
     reliability 255/255, txload 7/255, rxload 1/255

RO#172.18.18.111
Username:
Password:

ROUTER12>enable
ROUTER12#

Check if there are already any ISDN connection active

ROUTER12#sh isdn active
------------------------------------------------------------------
                                ISDN ACTIVE CALLS
------------------------------------------------------------------
Call    Calling      Called       Remote  Seconds Seconds Seconds Charges
Type    Number       Number       Name    Used    Left    Idle    Units/Currency
------------------------------------------------------------------
------------------------------------------------------------------

ROUTER12#sh isdn history
------------------------------------------------------------------
                                ISDN CALL HISTORY
------------------------------------------------------------------
Call History contains all active calls, and a maximum of 100 inactive calls.
Inactive call data will be retained for a maximum of 15 minutes.
------------------------------------------------------------------
Call    Calling      Called       Remote  Seconds Seconds Seconds Charges
Type    Number       Number       Name    Used    Left    Idle    Units/Currency
------------------------------------------------------------------
-------------------------------------------------------------------

We don’t see any ISDN being dialed at the moment. Now we should test if ISDN does actually dial when our leased line circuit goes down.

 ROUTER12#sh run int dialer10

interface Dialer10
 description *** dialing to RO 011555555 ***
 ip address 172.21.2.14 255.255.255.252
 encapsulation ppp
 dialer pool 1
 dialer remote-name RO
 dialer string 01155555
 dialer-group 1
 ppp authentication chap
 crypto map vpn
end

ROUTER12#sh backup
Primary Interface   Secondary Interface   Status
-----------------   -------------------   ------
Serial0/1/0         Dialer10             normal operation

ROUTER12#conf t
ROUTER12(config)#int Serial0/1/0
ROUTER12(config-if)#no backup int dialer100
ROUTER12(config-if)#do isdn test call int BRI0/0/0 01155555
ROUTER12(config-if)#do sh isdn active
------------------------------------------------------------------ 
Call    Calling      Called       Remote  Seconds Seconds Seconds Charges
Type    Number       Number       Name    Used    Left    Idle    Units/Currency
------------------------------------------------------------------- 
In    01155555    01155555       RO      30      119      0
-------------------------------------------------------------------

We see that the ISDN is working well and is dialing properly to the RO router. If the output was blank, then, run ‘sh isdn history‘ a few times to check if there are any entries in the output.

If it is working, then, reconfigure the ‘backup’ interface to restore the pre-test config state.

ROUTER12(config-if)#backup int dialer100
ROUTER12(config-if)#backup delay 5 60

5 = fire ISDN when Leased Lines goes down.
60 = shut ISDN after 60 of Leased Lines coming up.
ROUTER12(config-if)#exit
ROUTER12#sh backup
Primary Interface   Secondary Interface   Status
-----------------   -------------------   ------
Serial0/1/0         Dialer10              normal operation

Conclusion

This way we can test an ISDN circuit that’s been configured as a backup to a primary leased line circuit.