Problem: IGRP Broadcast Is Keeping the ISDN Link Up-Cause: IGRP Broadcasts Have Not Been Denied in the Interesting Traffic Definition

Problem: IGRP Broadcast Is Keeping the ISDN Link Up ‚ Cause: IGRP Broadcasts Have Not Been Denied in the Interesting Traffic Definition

ISDN links typically are used as backup links when primary links go down. Cisco IOS Software requires that routers are instructed on the kind of traffic that can bring up the ISDN link and keep it up. Such traffic is called interesting traffic. Network operators typically want data traffic to be considered as interesting traffic, to bring up and keep up the ISDN link. IGRP or other routing protocol updates should not be defined as interesting traffic. If this is not done, the ISDN link comes up and stays up as long as routing updates (IGRP, in this case) are taking place on a regular basis. That is not the desired behavior because ISDN provides low-speed connectivity and because some data actually could go over the slow link even though the primary faster link is available.

Figure 5-34 shows the network setup susceptible to dial backup issues.

Figure 5-34. Network Setup Conducive to Dial Backup Problems

Figure 5-35 shows the flowchart to follow to fix this problem.

Figure 5-35. Problem-Resolution Flowchart

Debugs and Verification

Example 5-87 shows the configuration on Router R1 that produces this problem. In this configuration, only TCP traffic is denied. In other words, TCP traffic will not bring up and keep up the link. IGRP broadcasts are IP packets; because the permit ip any any command allows any IP traffic to go through besides TCP, IGRP broadcast traffic will be considered interesting traffic.

Example 5-87 R1 Configuration in Which IGRP Broadcasts Are Not Denied

 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 broadcast 57654   dialer-group 1   isdn switch-type basic-net3   ppp authentication chap    access-list 100 deny tcp any any    access-list 100 permit ip any any   dialer-list 1 protocol ip list 100  

Example 5-88 shows the output of show dialer, which indicates that the reason for the link coming up is IGRP broadcast.

Example 5-88 show dialer Output Indicates the Last Time the Link Was Up Was Because of IGRP Broadcast

 R1#  show dialer  BRI1/1:1 - dialer type = ISDN Idle timer (120 secs), Fast idle timer (20 secs) Wait for carrier (30 secs), Re-enable (2 secs) Dialer state is data link layer up  Dial reason: ip (s=192.168.254.13, d=255.255.255.255)  Current call connected 00:00:08 Connected to 57654 (R2) 

Solution

When running IGRP and DDR, define the access list to define the interesting traffic. In Example 5-87, the access list is denying only the TCP traffic and is permitting all the IP traffic. IGRP uses an IP broadcast address of 255.255.255.255 to send the routing updates. This address must be denied in the access list so that IGRP does not bring up the link every 90 seconds.

Example 5-89 shows the correct configuration change in Router R1. In this configuration, all traffic destined to the 255.255.255.255 address is denied. This covers IGRP broadcast, so IGRP will not bring up the link after this configuration change.

Example 5-89 Configuring R1's Access List to Deny IGRP Broadcast Traffic

 R1#  access-list 100 deny tcp any any    access-list 100 deny ip any 255.255.255.255    access-list 100 permit ip any any   dialer-list 1 protocol ip list 100