Some Questions for MTCNA Exam

 

Bandwidth Setting and Cost Explanation in OSPF:

 

 Ethernet:

==========

10 Mb

10*1000 = Kb = 10000 Kb

10000*1000 = 10000000 b

100000000

---------

10000000

=10

=============================

Fast Ethernet:

==============

100 Mb

100 * 1000 = 1000000 Kb

100000 * 1000 = 100000000 b

100000000

---------

100000000

=1

100000000

---------

64

=1562500 b

1562500 / 1000 = 1562.5 Kb

1562.5 / 1000 = 1.56 Mb

 

OSPF Reference Bandwidth

OSPF uses a simple formula to calculate the OSPF cost for an interface with this formula:

cost = reference bandwidth / interface bandwidth

The reference bandwidth is a value in Mbps that we can set ourselves. By default this is 100Mbps on Cisco IOS routers. The interface bandwidth is something we can lookup.

 

To see your Reference Bandwidth –

Let’s take a look at an example of how this works. I’ll use this router:

The router above has two interfaces, a FastEthernet and a serial interface:

R1#show ip interface brief

Interface                  IP-Address      OK? Method Status                Protocol

FastEthernet0/0            192.168.1.1     YES manual up                    up     

Serial0/0                  192.168.2.1     YES manual up                    up

Let’s enable OSPF on these interfaces:

R1(config)#router ospf 1

R1(config-router)#network 192.168.1.0 0.0.0.255 area 0

R1(config-router)#network 192.168.2.0 0.0.0.255 area 0

After enabling OSPF we can check what the reference bandwidth is:

Router#show ip ospf | include Reference

 Reference bandwidth unit is 100 mbps

By default this is 100 Mbps. Let’s see what cost values OSPF has calculated for our two interfaces:

Router#show interfaces FastEthernet 0/0 | include BW

  MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec

Router#show ip ospf interface FastEthernet 0/0 | include Cost

  Process ID 1, Router ID 192.168.1.1, Network Type BROADCAST, Cost: 1

The FastEthernet interface has a bandwidth of 100.000 kbps (100 Mbps) and the OSPF cost is 1. The formula to calculate the cost looks like this:

100.000 kbps reference bandwidth / 100.000 interface bandwidth = 1

 

 

Changing the reference bandwidth

To change the reference bandwidth, enter a command such as the following at the OSPF configuration level of the CLI.

ITBD(config)# router ospf

device(config-ospf-router)# auto-cost reference-bandwidth 500

The reference bandwidth specified in this example results in the following costs:

• 10 Mbps port’s cost = 500/10 = 50
• 100 Mbps port’s cost = 500/100 = 5
• 1000 Mbps port’s cost = 500/1000 = 0.5, which is rounded up to 1

The costs for 10 Mbps and 100 Mbps ports change as a result of the changed reference bandwidth. Costs for higher-speed interfaces remain the same.

Syntax: [no] auto-cost { reference-bandwidth num | use-active-ports }

The num parameter specifies the reference bandwidth and can be a value from 1 - 4294967. The default is 100.

To restore the reference bandwidth to its default value and thus restore the default costs of interfaces to their default values, enter the following command.

ITBD(config-ospf-router)# no auto-cost reference-bandwidth

 

 

Router#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#interface Ethernet0
Router(config-if)#bandwidth 10000000
Router(config-if)# exit

 

The other way of affecting the cost of an interface is through the "ip ospf cost" command on the interface itself, this will override the calculated cost.

Router1(config)#int f1/0
Router1(config-if)#ip ospf cost 12345
Router(config-if)# exit

 

 

 

 

Verification commands

1) Router#sh ip ospf interface fa0/0 | in Cost
  Process ID 100, Router ID 10.1.1.2, Network Type BROADCAST, Cost: 10

 

 

2)Router#sh ip ospf interface brief

Interface    PID   Area            IP Address/Mask    Cost   State  Nbrs F/C
Fa0/0        100   0                    10.1.1.2/24            10    BDR    1/1