Troubleshooting a Flapping ISDN Link in OSPF

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ISDN BRI

R4# show dialer

Idle timer (120 sees). Fast idle timer (20 sees) Wait for earrier (30 sees). Re-enable (2 sees) Dialer state is data link layer up Dial reason: ip (s=172.16.14.4, d=224.0.0.5)

Numerous changes to the network topology can cause DDR links to be frequently connected

© 2002, Cisco Systems, Inc. All rights res o CCIE Prep v1.0—Module 7-80

When an Integrated Services Digital Network (ISDN) link is configured as an OSPF demand circuit, OSPF hellos are suppressed and periodic LSA refreshes are not flooded over the link. These packets bring up the link only when they are exchanged for the first time, or when a change occurs in the network. The OSPF demand circuit feature allows the underlying DataLink Layer to be closed when the OSPF network topology is stable. This is critical in a Dial-on-Demand Routing (DDR) environment.

In the diagram above, R4 and R1 are running an OSPF demand circuit across the ISDN link. The link between R4 and R1 keeps coming up, which defeats the purpose of the OSPF demand circuit feature. The output of the show dialer command shows that the link came up because of an OSPF hello packet.

R4# show dialer

Idle timer (120 sees), Fast idle timer (20 sees) Wait for earrier (30 sees), Re-enable (2 sees) Dialer state is data link layer up Dial reason: ip (s=172.16.14.4, d=224.0.0.5)

OSPF could cause the ISDN link to activate for several reasons. These reasons will be discussed on the following pages.

Reason 1: Change in the Network Topology

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OSPF: Schedule SPF in area 0.0.0.0

Change in LS ID 5.5.5.5, LSA type R, OSPF: schedule SPF: spf time 1620348064ms wait interval 10s

• Network changes can be monitored using the "debug ip ospf monitor" command

© 2002, Cis

o Systems, Inc. All lights reserved. Cisco CCIE Prep v1.

—Module 7-81

Reason 1: Change in the Network Topology

Whenever there is a change in an OSPF network topology, OSPF routers must be notified. In this situation, the OSPF demand circuit must be brought up, so that the neighbors can exchange the new LSA information. Once the new databases have been exchanged and synchronized, the link can go down again, and the adjacency remains in the FULL state.

Solution

To determine if the link is being brought up due to a change in network topology, use the debug ip ospf monitor command. It shows which LSA is changing, as shown below:

R4# debug ip ospf monitor

OSPF: Schedule SPF in area 0.0.0.0

OSPF: schedule SPF: spf_time 1620348064ms wait_interval 10s

The output above shows there was a change in the LSA with the router ID of 5.5.5.5, which causes the database to be resynchronized. If the network is stable, then this debug output will not display anything when the ISDN link comes up.

To reduce the chance of link flaps on the demand circuit, configure the area that contains the demand circuit as a stub or totally stubby area, if possible.

R4# show dialer

Idle timer (120 sees). Fast idle timer (20 sees)

Wait for earrier (30 sees). Re-enable (2 sees)

Dialer state is data link layer up

Interfaee bound to profile Di1

Current eall eonneeted 00:00:08

Conneeted to 57654 (R6)

R4# show dialer

Idle timer (120 sees). Fast idle timer (20 sees)

Wait for earrier (30 sees). Re-enable (2 sees)

Dialer state is data link layer up

Interfaee bound to profile Di1

Current eall eonneeted 00:00:08

Conneeted to 57654 (R6)

OSPF network type should be set to point-to-point or point-to-multipoint on DDR links

Note: By default, ISDN is considered a point-to-point network by OSPF

o CCIE Prep v1.0—Module 7-82

Reason 2: Network Type Defined as Broadcast

When you configure the OSPF demand circuit on a link, the OSPF network type must be defined as point-to-point or point-to-multipoint. Any other link type will cause the link to come up unnecessarily. This is due to the fact that OSPF hellos are not suppressed if the network type is anything other than point-to-point or point-to-multipoint. For example, with the network type defined as broadcast, OSPF hellos will bring the link up at every hello interval. The show dialer output here shows that the ISDN link was brought up because of an OSPF hello packet.

R4# show dialer

Idle timer (120 sees), Fast idle timer (20 sees) Wait for earrier (30 sees), Re-enable (2 sees) Dialer state is data link layer up Dial reason: ip (s=172.16.14.4, d=224.O.O.5)

Interfaee bound to profile Di1 Current eall eonneeted 00:00:08 Conneeted to 57654 (R6)

Solution

To solve this problem, change the OSPF network type to either point-to-point or point-to-multipoint with the ip ospf network command. By changing the OSPF network type to point-to-point or point-to-multipoint, the OSPF hellos will be suppressed on the link, and the ISDN link will stop flapping.

Note By default, ISDN is considered a point-to-point network by OSPF.

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