R1(config)#router os 1
R1(config-router)#router-id 1.1.1.1
R1(config-router)#network 192.168.1.1 0.0.0.0 area ?
<0-4294967295> OSPF area ID as a decimal value
A.B.C.D OSPF area ID in IP address format
a decimal value == Ax256^3 + Bx256^2 + Cx256^1 + D
For example
800 == 3x256 + 32 == 800
R1(config)#router os 1
R1(config-router)#router-id 1.1.1.1
R1(config-router)#network 1.1.1.1 0.0.0.0 a 0
R1(config-router)#net 192.168.1.1 0.0.0.0 a 0
R1(config-router)#do sh run | b r o
router ospf 1
router-id 1.1.1.1
log-adjacency-changes
network 1.1.1.1 0.0.0.0 area 0
network 192.168.1.1 0.0.0.0 area 0
R2(config-if)#router os 2
R2(config-router)#router-id 2.2.2.2
R2(config-router)#net 192.168.1.2 0.0.0.0 a 0
R2(config-router)#net 2.2.2.2 0.0.0.0 a 0
R2(config-router)#do sh ip os nei
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 1 FULL/DR 00:00:31 192.168.1.1 FastEthernet0/0
R2(config-router)#
*Mar 1 00:20:33.063: %OSPF-5-ADJCHG: Process 2, Nbr 1.1.1.1 on FastEthernet0/0 from LOADING to FULL, Loading Done
We found that the time to build neighbor between R4 and R5 is faster than R1 and R2
R5(config-router)#
*Mar 1 00:26:47.987: %OSPF-5-ADJCHG: Process 5, Nbr 4.4.4.4 on Serial1/0 from LOADING to FULL, Loading Done
R5(config-router)#
R5(config-router)#do sh ip os nei
Neighbor ID Pri State Dead Time Address Interface
4.4.4.4 0 FULL/ - 00:00:32 45.1.1.4 Serial1/0
The reason is that between R4 and R5 does not need to select DR(Design Router).
Now let us some about Hello packet and DBD packet
As you can see MTU in the DBD can be changed by manually.
What about R4 and R5 have the different MTU.
Notice, this MTU is layer-3 MTU (ip MTU ), but layer-2 MTU (MTU )
R5(config)#int s1/0
R5(config-if)#mt
R5(config-if)#mtu 1496 <--- layer-2="" mtu="" p="">
R5(config-if)#shut
R5(config-if)#no shut
R4(config-router)#int s1/0
R4(config-if)#mtu
R4(config-if)#mtu 1500 <--- div="" layer-3="" mtu="">
R4(config-if)#shut
R4(config-if)#no shut
R5(config-if)#
*Mar 1 01:02:09.479: %OSPF-5-ADJCHG: Process 5, Nbr 4.4.4.4 on Serial1/0 from INIT to DOWN, Neighbor Down: Interface down or detached
R5(config-if)#do sh ip os nei
Neighbor ID Pri State Dead Time Address Interface
4.4.4.4 0 EXSTART/ - 00:00:35 45.1.1.4 Serial1/0
R5(config)#int s1/0
R5(config-if)#ip mtu 1490 <--- layer-3="" mtu="" p="">
R5(config-if)#shut
R5(config-if)#no shut
R4(config)#int s1/0
R4(config-if)#ip mtu 1490 <--- layer-3="" mtu="" p="">
R4(config-if)#shut
R4(config-if)#no shut
R5(config-if)#
*Mar 1 01:10:35.615: %OSPF-5-ADJCHG: Process 5, Nbr 4.4.4.4 on Serial1/0 from LOADING to FULL, Loading Done
R5(config-if)#
R5(config-if)#do sh ip os nei
Neighbor ID Pri State Dead Time Address Interface
4.4.4.4 0 FULL/ - 00:00:35 45.1.1.4 Serial1/0
This experiment can explain that OSPF only care layer-3 MTU.
For some reason, such as VPN, we need to set different layer-3 MTU / layer-2 MTU.
What can we do on OSPF. The answer is to use
R5(config-if)#ip ospf mtu-ignore ?
Notice this command must be set on the interface which has less layer-3 MTU.
R4(config-if)#do sh run int s1/0
interface Serial1/0
ip address 45.1.1.4 255.255.255.0
ip mtu 1480
R5(config-if)#do sh run int s1/0
interface Serial1/0
ip address 45.1.1.5 255.255.255.0
ip mtu 1490
R4(config-if)#ip ospf mtu-ignore <-- b="" nbsp=""> the interface which has less layer-3 MTU.
R4#sh ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
5.5.5.5 0 FULL/ - 00:00:32 45.1.1.5 Serial1/0
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