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

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

When a dialer interface ‚ say, ISDN ‚ comes up, it could be desirable 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 well. Therefore, running a dynamic routing protocol is necessary. In some situations, the ISDN link is up but no routing information is going across. Without a routing protocol, no destination addresses can be learned and no traffic can be sent to those destinations. This problem needs to be fixed because ISDN interfaces are of no use when not carrying any traffic.

Figure 5-36 shows the flowchart to follow to solve this problem.

Figure 5-36. Problem-Resolution Flowchart

Debugs and Verification

Example 5-90 shows the configuration on R1 that produces this problem. The dialer map is used to map the neighbor IP address with a string, which is normally an ISDN number. This is called a static mapping for dialer. When using static mapping, the keyword broadcast must be included at the end; otherwise , it will not propagate the broadcast traffic across the link.

Example 5-90 R1 Configuration Preventing IGRP Updates Across Dialer Interface

 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 5-91 shows that IGRP is sending the broadcast update toward R2, but because of an encapsulation failure, it is not getting on the other side.

Example 5-91 Confirming an Encapsulation Failure

 R1#  show access-list 100  access-list 100 permit ip any host 255.255.255.255 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, proto=9 IP: s=192.168.254.13 (local), d=255.255.255.255 (BRI3/0), len 72,  encapsulation   failed  , proto=9 

Solution

This problem occurs because IGRP uses broadcasts to send its routing updates. When using dialer map statements, you must include the broadcast keyword; otherwise, the broadcast will not be allowed to cross the circuit and the encapsulation failures occur. To correct this problem, add the broadcast keyword in the dialer map statement.

Example 5-92 shows the new configuration change on Router R1.

Example 5-92 Configuring R1 to Allow Broadcasts Across the Dialer 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