設定

192.168.xy.0/24
Loopback: x.x.x.x/8
全I/FにてOSPFを起動

前提知識

OSPFのHello/DeadはBroadcastで10/40(s)がデフォなのは常識。

iosv-3#sh ip o int gi0/0 
GigabitEthernet0/0 is up, line protocol is up 
  Internet Address 192.168.23.3/24, Area 0, Attached via Interface Enable
  Process ID 1, Router ID 3.3.3.3, Network Type BROADCAST, Cost: 1
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           1         no          no            Base
  Enabled by interface config, excluding secondary ip addresses
  Transmit Delay is 1 sec, State BDR, Priority 1
  Designated Router (ID) 2.2.2.2, Interface address 192.168.23.2
  Backup Designated router (ID) 3.3.3.3, Interface address 192.168.23.3
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    oob-resync timeout 40
    Hello due in 00:00:02
  Supports Link-local Signaling (LLS)
  Cisco NSF helper support enabled
  IETF NSF helper support enabled
  Index 1/1/1, flood queue length 0
  Next 0x0(0)/0x0(0)/0x0(0)
  Last flood scan length is 1, maximum is 1
  Last flood scan time is 1 msec, maximum is 1 msec
  Neighbor Count is 1, Adjacent neighbor count is 1 
    Adjacent with neighbor 2.2.2.2  (Designated Router)
  Suppress hello for 0 neighbor(s)

Helloを1秒未満にしたい

DeadはHelloの1/4秒になっているので、逆もしかり、Deadを1秒にすれば0.25秒になるのではという算段。

iosv-3(config)#int gi0/0
iosv-3(config-if)#ip o dead 1

iosv-3(config-if)#do sh ip o int gi0/0
GigabitEthernet0/0 is up, line protocol is up 
  Internet Address 192.168.23.3/24, Area 0, Attached via Interface Enable
  Process ID 1, Router ID 3.3.3.3, Network Type BROADCAST, Cost: 1
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           1         no          no            Base
  Enabled by interface config, excluding secondary ip addresses
  Transmit Delay is 1 sec, State DR, Priority 1
  Designated Router (ID) 3.3.3.3, Interface address 192.168.23.3
  No backup designated router on this network
  Timer intervals configured, Hello 10, Dead 1, Wait 1, Retransmit 5
    oob-resync timeout 40
    Hello due in 00:00:09
  Supports Link-local Signaling (LLS)
  Cisco NSF helper support enabled
  IETF NSF helper support enabled
  Index 1/1/1, flood queue length 0
  Next 0x0(0)/0x0(0)/0x0(0)
  Last flood scan length is 1, maximum is 1
  Last flood scan time is 1 msec, maximum is 1 msec
  Neighbor Count is 0, Adjacent neighbor count is 0 
  Suppress hello for 0 neighbor(s

まあならないんですねこれが。Helloは変わらずDeadだけ変わる。

ip o dead minimal hello x

iosv-2(config-if)#ip o dead minimal hello 4
iosv-2(config-if)#
iosv-2(config-if)#do sh ip o int gi0/1 | i Hello
Timer intervals configured, Hello 250 msec, Dead 1, Wait 1, Retransmit 5
Hello due in 107 msec

正式には
(config-if)#ip ospf dead-interval minimal hello-multiplier 4

因みに

Helloを1秒未満にすると不一致でもネイバーを確立する。
厳密には、OSPFパケット上ではHelloが０として送信されるため、対向と０で一致してネイバーを形成している。

iosv-2(config-if)#do sh ip o nei 

Neighbor ID     Pri   State           Dead Time   Address         Interface
iosv-3            1   FULL/DR         815 msec    192.168.23.3    GigabitEthernet0/1
iosv-1            1   FULL/BDR        00:00:30    192.168.12.1    GigabitEthernet0/0
iosv-2(config-if)#
iosv-2(config-if)#do sh ip o int gi0/1 | i Hello
  Timer intervals configured, Hello 200 msec, Dead 1, Wait 1, Retransmit 5
    Hello due in 118 msec

iosv-3(config-if)#do sh ip o nei

Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           1   FULL/BDR        955 msec    192.168.23.2    GigabitEthernet0/0
iosv-3(config-if)#do sh ip o int gi0/0 | i Hello
  Timer intervals configured, Hello 250 msec, Dead 1, Wait 1, Retransmit 5
    Hello due in 147 msec

上記ではiosv-2が200msec, iosv-3が250msecだがネイバーが形成されている。

参考

CCIE Enterprise Infrastructure Foundation, 2nd Edition

OSPFのネイバーを張る為にはHello/Deadタイマーは一致する必要がある？その2（回答編）