Cisco CCNP Certification Exam Training:

Dial-on-Demand Backup Strategies

By Chris Bryant, CCIE #12933

To earn your CCNP certification, you've got to take your CCNA-based knowledge of ISDN and Frame Relay to the next level. Knowing about ISDN backup doesn't just help on exam day, it's a practical skill for real-world networks much like the one we're going to use as an example today.

One of the first things you learned about Frame Relay and ISDN is that Frame is a lot cheaper!  We're charged by the minute for ISDN connections, so it's imperative to keep that line down whenever possible.  The cost issue is the reason that Frame is the primary L2 transport of choice for most of today's networks.

We always need a backup, though, and ISDN makes an excellent backup.  There are several different methods of using ISDN as a backup when Frame becomes unavailable, and we're going to see each one in action now.  We'll start with the basic dial backup strategy and configuration, often called DDR backup.

R1 and R2 are on the 172.12.123.0 /24 subnet running over the Frame, and the 172.12.12.0 /24 subnet running over ISDN.  The BRI configs of both routers are shown below the illustration.

R1:

interface BRI0
 ip address 172.12.12.1 255.255.255.0
 encapsulation ppp
 dialer map ip 172.12.12.2 name R2 broadcast 8358662
 dialer-group 1
 isdn switch-type basic-ni
 isdn spid1 0835866101
 isdn spid2 0835866301
 ppp authentication chap

R2:

interface BRI0
 ip address 172.12.12.2 255.255.255.0
 encapsulation ppp
 dialer map ip 172.12.12.1 name R1 broadcast 8358661
 dialer-group 1
 isdn switch-type basic-ni
 isdn spid1 0835866201
 isdn spid2 0835866401
 ppp authentication chap

Right now, only interesting traffic is going to bring the ISDN link up.  The Frame link going down doesn't generate any such traffic, as you can see below.  R1's Serial0 interface is shut down, but the ISDN link stays down.

R1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#int s0
R1(config-if)#shut

00:11:15: %LINK-5-CHANGED: Interface Serial0, changed state to administratively down
00:11:16: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, changed state to down

< ISDN link does not come up >

A basic dial backup strategy will have the ISDN link come up if the Frame goes down, and by "down" I mean the line protocol goes down.  We'll configure this and look at our options with the backup delay and backup interface commands.  The backup interface command will be configured on the primary interface, which in this case is Serial0.

R1(config-if)#int s0
R1(config-if)#no shut

00:17:32: %LINK-3-UPDOWN: Interface Serial0, changed state to up
00:17:33: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, changed state to up
R1(config-if)#backup interface ?
  Async              Async interface
  BRI                ISDN Basic Rate Interface
  BVI                Bridge-Group Virtual Interface
  Dialer             Dialer interface
  Group-Async        Async Group interface
  Lex                Lex interface
  Loopback           Loopback interface
  Multilink          Multilink-group interface
  Null               Null interface
  Serial             Serial
  TokenRing          IEEE 802.5
  Tunnel             Tunnel interface
  Virtual-Template   Virtual Template interface
  Virtual-TokenRing  Virtual TokenRing
  Vlan               Catalyst Vlans

R1(config-if)#backup interface bri0
00:17:48: %ISDN-6-LAYER2DOWN: Layer 2 for Interface BRI0, TEI 97 changed to down
00:17:48: %ISDN-6-LAYER2DOWN: Layer 2 for Interface BRI0, TEI 98 changed to down
00:17:48: %ISDN-6-LAYER2DOWN: Layer 2 for Interface BR0, TEI 97 changed to down
00:17:48: %ISDN-6-LAYER2DOWN: Layer 2 for Interface BR0, TEI 98 changed to down
00:17:48: %LINK-5-CHANGED: Interface BRI0, changed state to standby mode

The backup interface command isn't limited to naming the BRI interface as the backup, as you can see.  Also note that the TEI numbers are showing as down and the interface went to "standby mode".  Let's make sure we can still ping from R2 from R1.

R1#ping 172.12.12.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.12.12.2, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
R1#

We can't send pings from R1 to R2, but there's a good reason.  When the BRI interface goes into standby mode as a result of the backup interface command, the ISDN link becomes unavailable for other calls.  show interface bri0 also reflects this new state.  Taking it one step further, the routing table on R1 doesn't even show this directly connected ISDN network!

R1#show int bri0
BRI0 is standby mode, line protocol is down

R1#show ip route

     172.12.0.0/24 is subnetted, 1 subnets
C       172.12.123.0 is directly connected, Serial0

We now need to define how long the primary interface's line protocol should be down before the ISDN link will be brought up.  A common problem with some Frame links is flapping, where the Frame link goes down for a few seconds, then back up, then back down, then back up.  You may choose to wait for 10 - 30 seconds in order to avoid the phone line coming up if the Frame is only unavailable for a few seconds.  We'll split the difference here and have the ISDN link come up after the Frame is down for 20 seconds.

R1(config)#int s0
R1(config-if)#backup delay ?
  <0-4294967294>  Seconds
  never           Never activate the backup line

R1(config-if)#backup delay 20 ?
  <0-4294967294>  Seconds
  never           Never deactivate the backup line

R1(config-if)#backup delay 20 60

IOS Help lets us know that this command uses seconds as the unit measurement, but there's one little detail left out.. make that two.  What does the first number mean, and what does the second number mean?

The first value indicates how long the primary interface's line protocol should be down before the secondary interface is enabled, and the second value indicates how long the primary should be back up before the secondary is put back into standby.   The above configuration will bring the BRI interface out of standby after the Frame is down for 20 seconds, and will wait until the Frame is back up for 60 seconds before putting the BRI interface back into standby.  These values can be seen with show interface serial0. 

R1#show interface serial0
Serial0 is up, line protocol is up
  Hardware is HD64570
  Internet address is 172.12.123.1/24
  Backup interface BRI0, failure delay 0 sec, secondary disable delay 0 sec,
  kickin load not set, kickout load not set

To test the configuration, we'll change the Serial0 LMI type to ANSI.  When the line protocol goes down due to this LMI mismatch, the ISDN link will come up in 20 seconds.

R1(config)#int s0
R1(config-if)#frame lmi-type ansi
R1(config-if)#
01:14:54: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, changed state
 to down

01:15:14: %LINK-3-UPDOWN: Interface BRI0:1, changed state to down
01:15:14: BR0:1 DDR: disconnecting call
01:15:14: %LINK-3-UPDOWN: Interface BRI0:2, changed state to down
01:15:14: BR0:2 DDR: disconnecting call
01:15:14: %LINK-3-UPDOWN: Interface BRI0, changed state to up

01:15:14: %ISDN-6-LAYER2UP: Layer 2 for Interface BR0, TEI 105 changed to up
01:15:15: %ISDN-6-LAYER2UP: Layer 2 for Interface BR0, TEI 106 changed to up

Checking the timestamps, we see that the line protocol went down at 1:14:54 and the ISDN link came out of standby and was dialed at 1:15:14 - exactly 20 seconds later.  The ISDN link now appears in the routing table.

R1#show ip route
     172.12.0.0/24 is subnetted, 1 subnets
C       172.12.12.0 is directly connected, BRI0

By setting the LMI type back to Cisco, the line protocol will come back up.  Sixty seconds later, the BRI interface will be placed back into standby mode.

R1(config)#interface serial0
R1(config-if)#frame lmi-type cisco

01:36:18: %SYS-5-CONFIG_I: Configured from console by console
01:36:44: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, changed state
 to up

01:37:44: BR0:1 DDR: disconnecting call
01:37:44: BR0:2 DDR: disconnecting call
01:37:44: %ISDN-6-LAYER2DOWN: Layer 2 for Interface BRI0, TEI 105 changed to down
01:37:44: %ISDN-6-LAYER2DOWN: Layer 2 for Interface BRI0, TEI 106 changed to down
01:37:44: %ISDN-6-LAYER2DOWN: Layer 2 for Interface BR0, TEI 105 changed to down

01:37:44: %ISDN-6-LAYER2DOWN: Layer 2 for Interface BR0, TEI 106 changed to down
01:37:44: %LINK-5-CHANGED: Interface BRI0, changed state to standby mode
01:37:44: %LINK-3-UPDOWN: Interface BRI0:1, changed state to down
01:37:44: BR0:1 DDR: disconnecting call
01:37:44: %LINK-3-UPDOWN: Interface BRI0:2, changed state to down
01:37:44: BR0:2 DDR: disconnecting call

The line protocol came up at 1:36:44 and the BRI interface is put back into standby mode at 1:37:44 - exactly 60 seconds later.

ISDN backup is just one of the many topics you've got to master to earn your CCNP certification - so keep studying, and I'll see you soon with another free CCNP exam training tutorial!

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com