Cisco CCNP Certification Training:

Dialer Watch, Frame Relay, And ISDN

By Chris Bryant, CCIE #12933

You might have thought you were done with your ISDN studies when you earned your CCNA - but think again! ISDN's not as popular as it once was, but it still comes in handy from time to time. That's why your CCNP certification exams will contain questions on topics such as Dialer Watch.

Dialer Watch allows you to configure a route or routes as "watched"; when the watched route leaves the routing table and there is no other valid route to that specific destination, the ISDN link will come up.  In the following example, the Ethernet segment 10.1.1.0 /24 has been added to R1's OSPF configuration.  We want R2 to place a call to R1 if either the loopback or Ethernet networks leave R2's routing table, but we don't want to have to depend on interesting traffic.  That dictates the use of Dialer Watch.

Dialer Watch configuration is a four-step process:

1.  Make sure the routes to be watched are in the routing table.  Yes, I know that's common sense.  I'm telling you anyway.  ;)  Below, we see both the route to the loopback and Ethernet segment on R1 in R2's OSPF routing table.

R2#show ip route ospf
     1.0.0.0/32 is subnetted, 1 subnets
O       1.1.1.1 [110/65] via 172.12.123.1, 00:00:07, Serial0
     10.0.0.0/24 is subnetted, 1 subnets
O       10.1.1.0 [110/128] via 172.12.123.1, 00:00:08, Serial0

2.  Configure the list of watched routes with dialer watch-list.  Only one of the watched routes needs to leave the routing table for the ISDN link to come up.  In this example, R2 will watch both routes from its OSPF routing table.

Be careful with this command.  The entries here need to match exactly the routes and masks being watched.  Dialer watch-lists use subnet masks, not wildcard masks.

R2(config)#dialer watch-list 5 ip 10.1.1.0 255.255.255.0
R2(config)#dialer watch-list 5 ip 1.1.1.1 255.255.255.255

3.  Configure the dialer watch-group command on the BRI interface, AND frame map statements for the watched routes. As with dialer-list and dialer-group, the group number referenced in the dialer watch-group command must match the number assigned to the dialer watch-list.

The Dialer Watch configuration will not work without frame map statements for each watched route.  I repeat this because this is the step a lot of people leave out.

R2(config)#interface bri0
R2(config-if)#dialer watch-group 5

R2(config-if)# dialer map ip 1.1.1.1 255.255.255.255. name R1 5557777 broadcast
R2(config-if)# dialer map ip 10.1.1.0 255.255.255.0 name R1 5557777 broadcast

To test Dialer Watch, the Serial0 interface on R2 will be shut down.  Since we're running OSPF, the route table will be updated almost immediately and the ISDN link should come up right after that.

R2(config)#int s0
R2(config-if)#shut

01:12:47: %OSPF-5-ADJCHG: Process 1, Nbr 1.1.1.1 on Serial0 from FULL to DOWN, N
eighbor Down: Interface down or detached

01:12:47: %LINK-3-UPDOWN: Interface BRI0:1, changed state to up
01:12:48: %SYS-5-CONFIG_I: Configured from console by console
01:12:48: %LINEPROTO-5-UPDOWN: Line protocol on Interface BRI0:1, changed state to up
01:12:49: %LINK-5-CHANGED: Interface Serial0, changed state to administratively down
01:12:50: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, changed state to down
01:12:53: %ISDN-6-CONNECT: Interface BRI0:1 is now connected to 5557777 R1

Within five seconds, the ISDN link is up.  show dialer verifies that Dialer Watch is the reason the line was brought up.

R2#show dialer

BRI0 - dialer type = ISDN

Dial String      Successes   Failures    Last DNIS   Last status
5557777                  2          0    00:00:11       successful
0 incoming call(s) have been screened.
0 incoming call(s) rejected for callback.

BRI0:1 - dialer type = ISDN
Idle timer (120 secs), Fast idle timer (20 secs)
Wait for carrier (30 secs), Re-enable (15 secs)
Dialer state is data link layer up
Dial reason: Dialing on watched route loss
Time until disconnect 108 secs
Connected to 5557777 (R1)

A final note regarding Dialer Watch ... it will not work with RIP, but will with all our other dynamic IGPs (IGRP, EIGRP, OSPF). Keep these details in mind when you're taking your CCNP certification exams, and you're well on your way to success!

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com