Your first project is setting up dial backup on the routers. You’ll keep this fairly basic. R2
will call R3 when serial 0.202 goes down. The interesting traffic you’ll designate is all IP.
You will not use a routing protocol, so you’ll have to use a floating static route. Typically,
floating static routes are used with DDR because they can be set to a higher administrative
distance than the routing protocol being used. This enables the router to automatically
bring up the BRI line if the main serial line were to drop.
In the following configuration, you’ll issue a show isdn status command on Router 2 to
verify that the interface configuration is working correctly:
r2#show isdn status
The current ISDN Switchtype = basic-ni
ISDN BRI0 interface
Layer 1 Status:
ACTIVE
Layer 2 Status:
TEI = 100, State = MULTIPLE_FRAME_ESTABLISHED
TEI = 101, State = MULTIPLE_FRAME_ESTABLISHED
Spid Status:
TEI 100, ces = 1, state = 5(init)
spid1 configured, spid1 sent, spid1 valid
Endpoint ID Info: epsf = 0, usid = 1, tid = 1
TEI 101, ces = 2, state = 5(init)
spid2 configured, spid2 sent, spid2 valid
Endpoint ID Info: epsf = 0, usid = 3, tid = 1
Layer 3 Status:
0 Active Layer 3 Call(s)
Activated dsl 0 CCBs = 1
CCB: callid=0x0, sapi=0, ces=1, B-chan=0
Total Allocated ISDN CCBs = 1
As you can see, layers 1 and 2 are up, you are using TEI 100 and 101, and the SPIDs and dialed
numbers (DNs) are valid. This is one of the most important commands you can use. If the SPIDs
are invalid or the configuration is wrong, you will see it in the show isdn status command.
Now you’ll issue the backup interface bri0 command under serial 0.202. This tells the
interface s0.202 to use interface BRI0 if the serial interface loses DCD (data carrier detect),
which means the link is down:
r2(config)#interface serial0.202
r2(config-subif)#backup interface bri0
r2(config-subif)#
%ISDN-6-LAYER2DOWN: Layer 2 for Interface BRI0, TEI 100 changed to down
%ISDN-6-LAYER2DOWN: Layer 2 for Interface BRI0, TEI 101 changed to down
%LINK-5-CHANGED: Interface BRI0, changed state to standby mode
%LINK-3-UPDOWN: Interface BRI0:1, changed state to down
%LINK-3-UPDOWN: Interface BRI0:2, changed state to down
As you can see, this command places the main interface in Standby mode, effectively turning
the interface down. This deactivates layer 1 on the BRI0 interface. This can be verified by issuing
a show ISDN status command at the router prompt:
r2#show ISDN status
The current ISDN Switchtype = basic-ni
ISDN BRI0 interface
Layer 1 Status:
DEACTIVATED
Layer 2 Status:
Layer 2 NOT Activated
Spid Status:
TEI Not Assigned,ces = 1, state = 1(terminal
down)
spid1 configured,spid1 NOT sent,spid1 NOT
valid
TEI Not Assigned,ces = 2, state = 1(terminal
down)
spid2 configured,spid2 NOT sent,spid2 NOT
valid
Layer 3 Status:
0 Active Layer 3 Call(s)
Activated dsl 0 CCBs = 0
Total Allocated ISDN CCBs = 0
Using the physical BRI interface as a backup can cause problems because the BRI interface
appears to be disconnected to the service provider. There is no way to verify that the ISDN BRI
circuit is in proper working order unless you remove it as a backup interface. This is why it’s
best to use a dialer interface as the backup and not the physical ISDN BRI interface, which is
illustrated later in this chapter.