Enjoy Networks Communication: OSPF

Showing posts with label OSPF. Show all posts

Oct 11, 2015

OSPF - Definition

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

Oct 6, 2015

OSPF - Router ID

R1(config)#router os 1
R1(config-router)#router-id 1.1.1.1

R1#sh ip protocols

Routing Protocol is "ospf 1"

Outgoing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Router ID 1.1.1.1 <== [HERE!!]

This is router ID cannot be changed, once some interface has run OSPF.

R1(config-router)#

*Mar 1 00:43:06.031: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on Serial1/0 from LOADING to FULL, Load

R1(config-router)#router-id 2.2.2.2

Reload or use "clear ip ospf process" command, for this to take effect <== [Here]

R1(config-router)#

AUTO-COST REFERENCE BANDWIDTH ON OSPF

In the cost of ospf = 10^8 / bandwidth

At here 10^8 is equal to auto-cost reference bandwidth.

[Topology]

R1 [f0/0] —- [f0/0] R2

R1(config-router)#no ?

auto-cost Calculate OSPF interface cost according to bandwidth

R1(config-router)#auto-cost reference-bandwidth ?

<1-4294967> The reference bandwidth in terms of Mbits per second

R1(config-router)#auto-cost reference-bandwidth 100 <= [Here means 10^8]

Now, let us see how it works. As we had known that the default bandwidth of fast ethernet is 100 Mbps. Therefor, the cost of fast ethernet is 10^8/ 10^8 == 1

R1(config-if)#ip ospf 12 area 0

R1(config-if)#exit

R1(config)#do sh ip ospf int fa0/0

FastEthernet0/0 is up, line protocol is up

Internet Address 12.1.1.1/24, Area 0, Attached via Interface Enable

Process ID 12, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1

R1(config-if)#bandwidth ?

<1-10000000> Bandwidth in kilobits

R1(config-if)#bandwidth 10000

R1(config-if)#do sh ip ospf int fa0/0

FastEthernet0/0 is up, line protocol is up

Internet Address 12.1.1.1/24, Area 0, Attached via Interface Enable

Process ID 12, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 10 <=> [10^8/10000,000]