Problem: OSPF Hellos Are Bringing Up the Link-Cause: OSPF Hellos Are Permitted as Interesting Traffic

Problem: OSPF Hellos Are Bringing Up the Link ‚ Cause: OSPF Hellos Are Permitted as Interesting Traffic

When running OSPF for dial backup purposes over DDR links, define an access list to explicitly define the interesting traffic. OSPF uses a multicast address of 224.0.0.5 to send the Hellos. This address must be denied in the access list so that OSPF doesn't bring up the link every 10 seconds.

Figure 9-89 shows a network experiencing this DDR problem.

Figure 9-89. OSPF Network Experiencing a Perpetual DDR Link Problem

Figure 9-90 shows the flowchart to follow to solve this problem.

Figure 9-90. Problem-Resolution Flowchart

Debugs and Verification

Example 9-245 shows the configuration on R1 that can produce this problem. In this configuration, only TCP traffic is denied. In other words, TCP traffic will not bring up the link, but any other IP traffic can do so.

Example 9-245 R1's Access List Denies Only TCP Traffic

 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 

The output of the show dialer command in Example 9-246 indicates that the reason for the link coming up is OSPF multicast Hellos.

Example 9-246 Determining Why the DDR Link Keeps Coming Up

 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=224.0.0.5)  Current call connected 00:00:08 Connected to 57654 (R2) 

Solution

To solve this problem, deny OSPF Hellos in the interesting traffic. Example 9-247 shows the correct configuration change in R1. In this configuration, all traffic destined to the 224.0.0.5 address is denied. This means that OSPF Hellos or updates over this point-to-point link will not bring up the link after this configuration change.

Example 9-247 Configuring R1's Access List to Deny OSPF Multicasts Hellos

 R1#   access-list 100 deny tcp any any     access-list 100 deny ip any 224.0.0.5     access-list 100 permit ip any any   dialer-list 1 protocol ip list 100 

It is not necessary to put 224.0.0.6 in the access list because only the DR uses this address. Also, because there is no DR over point-to-point links, there is no need to deny this address in the access list.