Dialer Watch has a lot of details, and it's my experience that the average CCNP candidate doesn't know DW as well as they should. Since you have no interest in just being average, I'm repeating a section of my BCRAN Study Guide here to get you up to speed on DW. And if you feel you're up to speed on it - read it anyway! : )

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 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

If you're concerned about flapping routes putting Dialer Watch into action, use the dialer watch-disable command to set a delay (in seconds). The watched route will need to be removed from the routing table for x seconds before Dialer Watch brings the ISDN link up.

R2(config-if)#dialer watch-disable ?
<1-2147483> Watch route disable time in seconds

DW works really well once it's configured correctly, but as I noted earlier, there are quite a few details to get right. Pay special attention to the following common DW errors for your exam and real-world configurations:

- Use dialer watch-list to define the routes being watched. If no routes are being watched, DW obviously won't do its job. Be careful with this command - make sure the route listed with dialer watch-list matches the prefix of the watched route exactly.
- Use the dialer watch-group command on the interface - without this command, DW can't work correctly.
- Configure an additional dialer map statement for each and every route being watched.
Always check these three commands and their syntax before checking anything else with a dialer watch configuration. It's my experience that 90 - 95% of DW problems stem from a misconfiguration or omission of one of these three commands.

To your success,

Chris Bryant

CCIE #12933