Problem: RIP Updates Are Not Going Across the Dialer Interface-Cause: Missing broadcast Keyword in a dialer map Statement

Problem: RIP Updates Are Not Going Across the Dialer Interface ‚ Cause: Missing broadcast Keyword in a dialer map Statement

When a dialer interface (ISDN, for example) comes up, you might want to run a routing protocol over this link. Static routes might do the job, but in networks with a large number of routes, static routes might not scale. Therefore, running a dynamic routing protocol such as RIP is necessary. In some situations, the ISDN link might be up, but no routing informa-tion is going across. Without a routing protocol, no destination addresses can be learned and no traffic can be sent to those destinations. This problem must be fixed because the ISDN interface is of no use when it is not carrying any traffic.

Figure 3-46 shows the flowchart to follow to solve this problem based on this cause.

Figure 3-46. Flowchart to Solve the RIP Updates Not Going Across the Dialer Interface Problem

Debugs and Verification

Example 3-125 shows the configuration on R1 that produces this problem.

Example 3-125 Configuring R1 When No Routing Updates Will Go on the ISDN Link

 R1#  interface BRI3/0   ip address 192.168.254.13 255.255.255.252   encapsulation ppp    dialer map ip 192.168.254.14 name R2 57654    dialer-group 1   isdn switch-type basic-net3   ppp authentication chap  

Example 3-126 shows that RIP is sending the broadcast update toward R2. You can see that it's failing because of the encapsulation failed message. Also in Example 3-126, R1 is running a debug ip packet command with access-list 100 to display only the UDP port 520 output. RIP-1 and RIP-2 use UDP port 520 to exchange updates with other RIP running routers.

Example 3-126 Discovering Why RIP Routes Are Not Going Across an ISDN Interface

 R1#  access-list 100 permit udp any any eq 520   access-list 100 deny ip any any  R1#  debug ip packet 100 detail  IP: s=192.168.254.13 (local), d=255.255.255.255 (BRI3/0), len 46, sending         broad/multicast UDP src=520, dst=520 IP: s=192.168.254.13 (local), d=255.255.255.255 (BRI3/0), len 72,  encapsulation   failed  UDP src=520, dst=520 

Solution

The root of the issue is RIP's use of broadcasts to send its routing updates. In DDR, dialer map statements are necessary to associate the next -hop protocol address to the phone number dialed to get to the destination. The broadcast keyword must be used in the dialer map statements; otherwise , the broadcast will encounter the encapsulation failure message demonstrated by Example 3-126. To correct this problem, add the broadcast keyword in the dialer map statement, as demonstrated in Example 3-127 for Router R1.

Example 3-127 Corrected Configuration of R1 to Enable RIP Updates to Go Across the ISDN Interface

  interface BRI3/0   ip address 192.168.254.13 255.255.255.252   encapsulation ppp   dialer map ip 192.168.254.14 name R2   broadcast   57654   dialer-group 1   isdn switch-type basic-net3   ppp authentication chap