Cisco OSPF Command and Configuration Handbook (paperback)

19-10 ip ospf hello-interval seconds

Syntax Description:

• seconds ” The time in seconds between sending Hello packets over a link. The range of values is 1 “8192 seconds. The default value is 10 seconds (30 seconds on a nonbroadcast multiaccess [NBMA] network).

Purpose: OSPF Hello packets are used to initially establish the neighbor relationship. Once the neighbor relationship is established, the packets are used as a keepalive mechanism to determine if OSPF neighbors are active. The Hello interval should be less than the dead interval. All interfaces on a common network must have the same Hello interval or an OSPF neighbor relationship will not be established.

Initial Cisco IOS Software Release: 10.0

Configuration Example: Modifying the Interface Hello Interval

Configure the network in Figure 19-7 so you can observe the default timer values on an OSPF interface link. You will then experiment with adjusting the values of the Hello interval.

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

View the timer values for the serial interface on Routers A and B.

rtrA# show ip ospf interface s0/1 Serial0/1 is up, line protocol is up Internet Address 10.1.1.1/30, Area 0 Process ID 1, Router ID 1.1.1.1, Network Type POINT_TO_POINT, Cost: 1562 Transmit Delay is 1 sec, State POINT_TO_POINT, Timer intervals configured, Hello 10 , Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:09 Index 1/2, flood queue length 0 Next 0x0(0)/0x0(0) Last flood scan length is 1, maximum is 2 Last flood scan time is 0 msec, maximum is 4 msec Neighbor Count is 1, Adjacent neighbor count is 1 Adjacent with neighbor 2.2.2.2 Suppress hello for 0 neighbor(s) _________________________________________________________________ rtrB# show ip ospf interface serial 0 Serial0 is up, line protocol is up Internet Address 10.1.1.2/30, Area 0 Process ID 1, Router ID 2.2.2.2, Network Type POINT_TO_POINT, Cost: 64 Transmit Delay is 1 sec, State POINT_TO_POINT, Timer intervals configured, Hello 10 , Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:09 Neighbor Count is 1, Adjacent neighbor count is 1 Adjacent with neighbor 1.1.1.1 Suppress hello for 0 neighbor(s)

Notice that the default Hello interval is 10 seconds and the default dead interval is 40 seconds. Modify the configuration on Router A to change the Hello interval to 11 seconds while leaving the value for the Hello interval on Router B set to the default of 10 seconds.

Router A interface Serial0/1 bandwidth 64 ip address 10.1.1.1 255.255.255.252 ip ospf hello-interval 11 clockrate 64000

Inspect the OSPF interface timers for serial 0/1 on Router A.

rtrA# show ip ospf interface serial 0/1 Serial0/1 is up, line protocol is up Internet Address 10.1.1.1/30, Area 0 Process ID 1, Router ID 1.1.1.1, Network Type POINT_TO_POINT, Cost: 1562 Transmit Delay is 1 sec, State POINT_TO_POINT, Timer intervals configured, Hello 11, Dead 44 , Wait 44, Retransmit 5 Hello due in 00:00:05 Index 1/2, flood queue length 0 Next 0x0(0)/0x0(0) Last flood scan length is 0, maximum is 2 Last flood scan time is 0 msec, maximum is 4 msec Neighbor Count is 0, Adjacent neighbor count is 0 Suppress hello for 0 neighbor(s)

The dead timer has been automatically set to four times the Hello interval.

Verify that Routers A and B have an OSPF neighbor relationship.

rtrA# show ip ospf neighbor (no output)

Because the Hello interval time on Router A does not match the Hello interval on Router B, an OSPF neighbor relationship will not be established. This can be seen by enabling OSPF debugging on Router B.

rtrB# debug ip ospf adj OSPF adjacency events debugging is on rtrB# 17:03:20: OSPF: Rcv hello from 1.1.1.1 area 0 from Serial0 10.1.1.1 17:03:20: OSPF: Mismatched hello parameters from 10.1.1.1 17:03:20: Dead R 44 C 40, Hello R 11 C 10

Modify the Hello interval on Router B to match the Hello interval on Router A.

Router B interface Serial0 ip address 10.1.1.2 255.255.255.252 ip ospf hello-interval 11

Verification

Verify that the Hello interval on Router A matches the Hello interval on Router B.

rtrB# show ip ospf interface serial 0 Serial0 is up, line protocol is up Internet Address 10.1.1.2/30, Area 0 Process ID 1, Router ID 2.2.2.2, Network Type POINT_TO_POINT, Cost: 64 Transmit Delay is 1 sec, State POINT_TO_POINT, Timer intervals configured, Hello 11, Dead 44 , Wait 44, Retransmit 5 Hello due in 00:00:07 Neighbor Count is 1, Adjacent neighbor count is 1 Adjacent with neighbor 1.1.1.1 Suppress hello for 0 neighbor(s)

Verify that the OSPF neighbor relationship has been re-established.

rtrB# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 1.1.1.1 1 FULL/ - 00:00:42 10.1.1.1 Serial0

Troubleshooting

Step 1. Verify that all interfaces attached to a common network are using the same value for the Hello interval.