OSPF Path Selection with Bandwidth

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Task 1

Remove Task 2 configuration from the previous lab.

Task 2
R2 chooses its FastEthernet interface as the outgoing path towards OSPF advertised destinations. Modify configuration on R2 so that it selects Frame-Relay interface instead. Use interface bandwidth command to accomplish this.

Questions

Try to answer the following questions:

1. What is the significance of BW in show interface command with regard to OSPF?
2. What does 'bandwidth' interface command do?

Lab Solution

Solution configuration can be accessed below (if you want to save it, click the link, then go to File-->Download):
https://drive.google.com/file/d/0BwE5C95tpjZOa1pvdlJmbUhIcXM/edit?usp=sharing

Task 1

Remove Task 2 configuration from the previous lab.

R2 Config:
!
interface FastEthernet0/0
 no ip ospf cost 65
!

Pic. 2 - R2 Current OSPF Routing Table (before configuration).

Note!

R2 calculates the best path based on the cost received from R1 by adding its own cost to reach R1 using two different interfaces:

• FastEthernet (cost = 1)
• Serial0/0 (cost = 64) 

Let's consider prefix 172.16.13.0/24 as an example of choosing the shortest path. 

There are two routers advertising 172.16.13.0/24:

• R1 with the cost of 64
• R3 also with the cost of 64

R1 cost of serial0/1 is calculated based on the formula (10^8 = 100,000,000):

cost = 10^8 / bandwidth_of_interface (bps)

Pic. 3 - R1 OSPF Cost of Serial0/1.

cost = 100,000,000 / 1,544,000 =  64 (rounded down)

Similarly, R3 is advertising 172.16.13.0/24 with the cost of 64.

Pic. 4 - R3 OSPF Cost of Serial0/1.

R2 chooses the least cost path:

• R2--->R1 (FastEthernet0/0), then R1's directly connected destination
• R2--->R1(Serial0/0), then R1's directly connected destination

It is worth noting that R1 has 172.16.13.0/24 directly connected (serial0/1) but can also reach it through R3 using its Serial0/1). It calculates the least cost path as well.

Take a closer look at the topology to understand it.

Task 2

R2 chooses its FastEthernet interface as the outgoing path towards OSPF advertised destinations. Modify configuration on R2 so that it selects Frame-Relay interface instead. Use interface bandwidth command to accomplish this.

R2 Config:

!
interface fastethernet0/0
 bandwidth 10
!

The cost of FastEthernet0/0 changes from initial 1 to 10000 (10 here is 10 kbps):

Pic. 5 - New OSPF Cost of FastEthernet0/0.

Now, R2 selects Serial0/0 as the best path:

Pic. 6 - R2's OSPF Routing Table.

Study Drill

The interface keyword 'bandwidth' followed by the number of kbps is NOT going to throttle the actual traffic. The rate stays the same. It only affects path selection routing protocols use (EIGRP and OSPF which factor in the bandwidth in the calculation).

This concept requires a bit of theory but also some hands-on verification in LSDB. Try to look at this from the practical perspective by looking in OSPF databases of R1 and R2.

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