After the configuration, it’s important to test your backup link. You don’t want to wait for an
actual outage before discovering you have made a configuration mistake. You’ll test the backup
by disabling the connected serial interface on R2.
When the test is performed, it takes 11 seconds for the backup line to come out of Standby
mode and another four seconds for layers 1 and 2 to come up. The following router output
shows this. Why would using a dialer interface save you four seconds in this scenario?
00:46:22: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, _
➥changed state to down
00:46:23: %LINK-3-UPDOWN: Interface Serial0, changed state to down
00:46:23: %FR-5-DLCICHANGE: Interface Serial0 - DLCI 202 state changed_
➥to DELETED
00:46:23: %FR-5-DLCICHANGE: Interface Serial0 - DLCI 100 state changed
➥_to DELETED
00:46:23: %FR-5-DLCICHANGE: Interface Serial0 - DLCI 200 state changed
➥_to DELETED
00:46:23: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0.202,
➥_ changed state to down
00:46:34: %LINK-3-UPDOWN: Interface BRI0:1, changed state to down
00:46:34: %LINK-3-UPDOWN: Interface BRI0:2, changed state to down
00:46:34: %LINK-3-UPDOWN: Interface BRI0, changed state to up
00:46:38: %ISDN-6-LAYER2UP: Layer 2 for Interface BR0, TEI 107 changed to up
00:46:38: %ISDN-6-LAYER2UP: Layer 2 for Interface BR0, TEI 108 changed to up
00:46:59: %LINK-3-UPDOWN: Interface BRI0:1, changed state to up
00:47:00: %LINEPROTO-5-UPDOWN: Line protocol on Interface BRI0:1, _
➥changed state to up
00:47:06: %ISDN-6-CONNECT: Interface BRI0:1 is now connected to 8358662
00:47:23: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, _
➥changed state to up
00:47:24: %LINK-3-UPDOWN: Interface Serial0, changed state to up
00:47:24: %FR-5-DLCICHANGE: Interface Serial0 - DLCI 202 state changed_
➥to ACTIVE
00:47:24: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0.202, _
➥changed state to up
00:48:24: %LINK-3-UPDOWN: Interface BRI0:1, changed state to down
00:48:24: %ISDN-6-DISCONNECT: Interface BRI0:1 disconnected from_
➥unknown, call lasted 85 seconds
00:48:24: %ISDN-6-LAYER2DOWN: Layer 2 for Interface BRI0, TEI 107_
➥changed to down
00:48:24: %ISDN-6-LAYER2DOWN: Layer 2 for Interface BRI0, TEI 108_
➥changed to down
00:48:24: %LINK-5-CHANGED: Interface BRI0, changed state to standby mode
00:48:24: %LINK-3-UPDOWN: Interface BRI0:1, changed state to down
00:48:24: %LINK-3-UPDOWN: Interface BRI0:2, changed state to down
00:48:25: %LINEPROTO-5-UPDOWN: Line protocol on Interface BRI0:1, _
➥changed state to down
You should also note in the preceding router output that the backup line dropped one minute
after the primary link came up. Changing the delay between primary failure and activation of
the backup line plus delay between primary recovery and deactivation of the backup line can be
modified by using the backup delay 10 60 command. The first number (10) is how many seconds
to wait before activating the backup interface, and the second number (60) is how many
seconds to stay up once the primary line recovers.
As we stated earlier, it is best to use a dialer profile, or dialer interface, as the backup interface,
so we will show you how this is done. Setting up a dialer profile requires two steps: configuring
the primary interface and configuring the dialer interface. The primary interface needs
only some basic information; for example, take a look at this configuration:
interface BRI0
no ip address
encapsulation ppp
isdn spid1 0835866101 8358661
isdn spid2 0835866301 8358663
dialer pool-member 1
!
Basically, all we did was set up ISDN layers 1 and 2, enable PPP encapsulation, and assign
this interface to dialer pool 1—pretty simple so far.
The next step involves the dialer interface. A dialer interface is virtual, meaning it is not
a physical interface, and you add it by using the global command interface dialer 1.
The connection-specific configuration commands are placed under this interface, including
creation of the dialer pool, phone number to dial, remote device name, interesting traffic,
authentication, and IP address information. Again, it’s not that difficult. Take a look at this
configuration:
interface Dialer1
ip address 192.168.254.2 255.255.255.0
encapsulation ppp
dialer remote-name r3
dialer string 8358662
dialer pool 1
dialer-group 1
ppp authentication chap callin
Note that the callin option on the ppp authentication command indicates authentication
on incoming (received) calls only.
You will notice that the dialer interface goes into Standby but the BRI interface doesn’t. You
can verify this by using the show ISDN status command:
r2#show isdn status
The current ISDN Switchtype = basic-ni
ISDN BRI0 interface
Layer 1 Status:
ACTIVE
Layer 2 Status:
TEI = 109, State = MULTIPLE_FRAME_ESTABLISHED
TEI = 110, State = MULTIPLE_FRAME_ESTABLISHED
Spid Status:
TEI 109, ces = 1, state = 5(init)
spid1 configured, spid1 sent, spid1 valid
Endpoint ID Info: epsf = 0, usid = 1, tid = 1
TEI 110, 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
service timestamps log uptime
no service password-encryption
no service udp-small-servers
no service tcp-small-servers
!
hostname r2
!
enable password cisco
!
username r3 password 0 cisco
isdn switch-type basic-ni
!
interface Serial0
no ip address
encapsulation frame-relay
no fair-queue
!
interface Serial0.202 point-to-point
backup delay 10 60
backup interface Dialer1
ip address 172.16.34.2 255.255.255.0
frame-relay interface-dlci 202
!
interface BRI0
no ip address
encapsulation ppp
isdn switch-type basic-ni
isdn spid1 0835866101 8358661
isdn spid2 0835866301 8358663
dialer pool-member 1
!
interface Dialer1
ip address 192.168.254.2 255.255.255.0
encapsulation ppp
dialer remote-name r3
dialer string 8358662
dialer pool 1
dialer-group 1
ppp authentication chap
!
ip classless
ip route 0.0.0.0 0.0.0.0 172.16.34.3
ip route 0.0.0.0 0.0.0.0 192.168.254.3 210
!
dialer-list 1 protocol ip permit
!
end
r2#
r3#show run
Building configuration...
Current configuration:
!
version 12.0
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname r3
!
enable password cisco
!
username r2 password 0 cisco
ip subnet-zero
!
isdn switch-type basic-ni
!
interface FastEthernet0/0
ip address 192.168.252.3 255.255.255.255
no ip directed-broadcast
!
interface Serial0/0
no ip address
no ip directed-broadcast
encapsulation frame-relay
no ip mroute-cache
frame-relay lmi-type cisco
!
interface Serial0/0.203 point-to-point
ip address 172.16.34.3 255.255.255.0
no ip directed-broadcast
frame-relay interface-dlci 203
!
interface BRI0/0
ip address 192.168.254.3 255.255.255.0
no ip directed-broadcast
encapsulation ppp
dialer map ip 192.168.254.2 8358661
dialer-group 1
isdn switch-type basic-ni
isdn spid1 0835866201 8358662
isdn spid2 0835866401 8358664
ppp authentication chap
dialer hold-queue 75
!
ip classless
ip route 172.16.2.0 255.255.255.0 172.16.34.2
ip route 172.16.2.0 255.255.255.0 192.168.254.2 210
!
dialer-list 1 protocol ip permit
!
end
As you can see, the configuration is not that complex. Having a good working knowledge of
this will help you solve many dial backup scenarios. Of course, you can make this as complex
as you’d like; we kept this example fairly simple as an illustration.
The command dialer-list creates the interesting traffic. The command dialer-group
assigns the dialer list to an interface. The numbers must match. In the previous example, both
the dialer list and the dialer group are 1. The dialer hold-queue command creates a buffer
for incoming interesting traffic that is waiting for the BRI to be dialed. The 75 means is that if
75 interesting packets arrive on queue before the interface comes up, the 76th and subsequent
will be dropped until the line comes up and the queue gets some relief.