OSPF Load Balancing

[Undertaken]

Hi there 

I would like to know how to configure OSPF to load balancing the routes from Router I through Router J and Router H evenly. 

Thank you 

[Julio E. Moisa]

Hi

Basically you should be receiving the prefixes behind Router J and H with the same cost, you could verify the interfaces and check the cost. The command used is: show ip ospf interface <interface>, you should verify the local outgoing interface, in few words the interface where the traffic is used to go out to the destination.

You can change the cost using under the interface the command: ip ospf cost <value>

Imagine the following scenario:

The right router could have load balance through the paths with 5 +5 + destination cost in order to reach the destination IP. The path with 5 + 10 + destination cost is dicarded for loadbalance. 

:-)

[Undertaken]

These are the configuration I have done so far on the serial links between router I and router J as well router I and router H. 

Router I: 

interface Serial0/0/0     to router H 

ip address 192.168.255.10 255.255.255.252

ip ospf cost 1000

interface Serial0/0/1    to router J 

ip address 192.168.255.14 255.255.255.252

ip ospf cost 1000

Router H:             

interface Serial0/0/0           to router I 

ip address 192.168.255.9 255.255.255.252

ip ospf cost 1000

Router J:             

interface Serial0/0/1           to router I 

ip address 192.168.255.13 255.255.255.252

ip ospf cost 1000

Is setting the cost to a specific number between the links all I have to do? I am new to ospf and I would really apperciate your feedback.

[Julio E. Moisa]

Hi,

The interface id is not showed on your picture, but if you want to see loadbalance from Router I, you should verify the cost of the interfaces into the blue circule, the right blue circles belong to Router G (I assume).

Basically you need to verify the entire path from the source to the destination, if you only check the connection between H and I and J to I you are not guaranteeing loadbalance through the path behind H and J.

Once you have verified the cost, you should see the destination with 2 path and same cost, through show ip route and also you can verify it through show ip ospf database. 

:-)

[Undertaken]

Attached different picture. As I understand all I have to do is set the cost between router I to J and H to lets say 1000 is that right? How about from router H and J to G do I also have to set the cost for ospf on the serial interfaces? 

[Deepak Kumar]

Hi, 

I shared two different pictures and we dont know about your topology. How many routes do you want to load balancing? 

Yes, you can configure but again we are waiting for above questions. It is advised to share complete topology and routes detail where you want load balancing. 

Regards,

Deepak Kumar

[Undertaken]

I need to route balance from router I to J and router I to H attached picture of the full topology. 

[Deepak Kumar]

Hi,

If you have access to the G router then you configure otherwise you must configure the OSPF Cost on routers I, H, J.

And if you want load balancing on traffic which will enter or out from this network then you need to configure the same cost on G also.

Regards,

Deepak Kumar

[Undertaken]

Hi, 

I have access to all of them. Should I configure G, I, H, J serial interfaces to ip ospf cost 1000 to make the load balancing? if yes should I do the interface from both sides like s0/0/0 on RG and s0/0/0 on RH and the rest? 

Thank you 

[Deepak Kumar]

Hi, 

I checked your topology. As all routers are on same speed and the same cost, so no need to modify any OSPF cost or bandwidth on the routers.

But 

check OSPF "sho ip ospf neighbor" on router J & G. Are they both router become neighbour?

There is authentication issue on both routers. 

Router J is having Key:

ip ospf message-digest-key 9 md5 area19 (Serial 0/0/0)

and Router G is having key:

ip ospf message-digest-key 8 md5 area19 (Serial 0/0/0)

Make it correct and your load balancing will start to work.

Regards,

Deepak Kumar

[Julio E. Moisa · Accepted solution]

Hi

If you want to have loadbalance from both ways, loadbalance from souce to destination and loadbalance from destination and source, yes you have to configure the same cost value under all the interfaces.

If you want loadbalance from Router G to I, you should have the same cost under the following interfaces:

- Router J s0/0/0/1

- Router H s0/0/0/0

- Router G s0/0/0/0 and s0/0/0/1

Loadbalance from Router I to G

- Router J s0/0/0/0

- Router H s0/0/0/1

- Router I s0/0/0/0 and s0/0/0/1

:-)

[Undertaken]

Done how would I check if the load balancing is even

[Julio E. Moisa]

You could compare the routing table from the source and destination router and you should see the prefixes with 2 paths. To test you could configure under the source and destination routers the command: ip load-sharing per-packet.

Then you can execute:

show ip cef exact-route source IP and destination IP  

But not really sure if the commands are supported on Packet Tracer.

:-)