19-9 ip ospf flood-reduction

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19-9 ip ospf flood-reduction

Syntax Description:

This command has no arguments.

Purpose: An OSPF LSA, by default, is refreshed every 30 minutes even if there has been no change to the LSA. If the LSAs are not changing, then this indicates a stable network topology. For large networks, the periodic flooding of LSAs in a stable network is unnecessary. The ip ospf flood-reduction interface command causes LSAs to be sent with the DoNotAge bit set, which has the effect of disabling the periodic refresh of LSAs. If there is a change in the network topology, the LSAs will be flooded regardless of whether or not this command is used. When flood reduction is enabled on an interface, the OSPF neighbor relationship on the interface will be reset.

Initial Cisco IOS Software Release: 12.1

Configuration Example: Configuring OSPF Flood Reduction on an Interface

Configure the routers in Figure 19-6 without using flood reduction in order to investigate the changes in the OSPF database once flood reduction is enabled.

Figure 19-6. Network Used to Demonstrate the Changes in the OSPF Database when Flood Reduction Is Enabled

graphics/19fig06.gif

 Router A  interface Loopback0   ip address 1.1.1.1 255.255.255.255   !   interface Serial0/1   ip address 10.1.1.1 255.255.255.252   clockrate 64000   !   router ospf 1   network 10.1.1.0 0.0.0.3 area 0   network 1.1.1.1 0.0.0.0 area 1  _________________________________________________________________ Router B  interface Loopback0   ip address 2.2.2.2 255.255.255.255   !   interface Serial0   ip address 10.1.1.2 255.255.255.252   !   router ospf 1   network 10.1.1.0 0.0.0.3 area 0   network 2.2.2.2 0.0.0.0 area 2  

Inspect the OSPF database on Router B for network 1.1.1.1.

 rtrB#  show ip ospf database router 1.1.1.1  OSPF Router with ID (2.2.2.2) (Process ID 1)                 Router Link States (Area 0)                                     Routing Bit Set on this LSA  LS age: 113  Options: (No TOS-capability, DC)   LS Type: Router Links   Link State ID: 1.1.1.1   Advertising Router: 1.1.1.1   LS Seq Number: 80000041   Checksum: 0xDD9F   Length: 48   Area Border Router    Number of Links: 2     Link connected to: another Router (point-to-point)      (Link ID) Neighboring Router ID: 2.2.2.2      (Link Data) Router Interface address: 10.1.1.1       Number of TOS metrics: 0        TOS 0 Metrics: 1562     Link connected to: a Stub Network      (Link ID) Network/subnet number: 10.1.1.0      (Link Data) Network Mask: 255.255.255.252       Number of TOS metrics: 0        TOS 0 Metrics: 1562 

Modify the configuration on Router A to enable OSPF flood reduction on the serial interface.

 Router A  interface Serial0/1   bandwidth 64   ip address 10.1.1.1 255.255.255.252    ip ospf flood-reduction    clockrate 64000  
Verification

Verify that flood reduction is enabled on Router A by checking the OSPF database on Router B.

 rtrB#  show ip ospf database router 1.1.1.1  OSPF Router with ID (2.2.2.2) (Process ID 1)                 Router Link States (Area 0)   Routing Bit Set on this LSA  LS age: 5 (DoNotAge)  Options: (No TOS-capability, DC)   LS Type: Router Links   Link State ID: 1.1.1.1   Advertising Router: 1.1.1.1   LS Seq Number: 80000043   Checksum: 0xD9A1   Length: 48   Area Border Router    Number of Links: 2     Link connected to: another Router (point-to-point)      (Link ID) Neighboring Router ID: 2.2.2.2      (Link Data) Router Interface address: 10.1.1.1       Number of TOS metrics: 0        TOS 0 Metrics: 1562     Link connected to: a Stub Network      (Link ID) Network/subnet number: 10.1.1.0      (Link Data) Network Mask: 255.255.255.252       Number of TOS metrics: 0        TOS 0 Metrics: 1562 
Troubleshooting
Step 1. If there is an OSPF neighbor on an interface then the command ip ospf flood-reduction should work.

Step 2. The routers receiving the LSAs tagged as DoNotAge should be running Cisco IOS Software Release 11.2 or later.

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Cisco[r] OSPF Command and Configuration Handbook
Cisco OSPF Command and Configuration Handbook (paperback)
ISBN: 1587055406
EAN: 2147483647
Year: 2002
Pages: 236

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