Finished OSPF Fundamentals, onto Advanced Concepts

The last couple of videos were actually really quick, and to this point, really easy to digest after going through a condensed version of all the CCNA material.

The bandwidth command issued on the interface, is another way of adjusting in my mind basically the OSPF Cost, only it isn’t specific to OSPF so it will cause other mechanisms like QoS or other routing protocols to crash and burn if configured incorrectly.

OSPF Cost is calculated by [Reference Bandwidth/Interface Bandwidth], with the default Reference Bandwidth being 100, which works fine for calculating cost until you get above a FastEthernet interface like for example a GigEthernet interface.

The reason being for FastEthernet it would be calculated as 100/100 giving you an interface cost of 1, but being GigEthernet would be calculated as 100/1000 by default, it should give you 0.1 as an interface Cost – But OSPF does not do decimals with interface Cost!

So if you set the Reference Bandwidth in router configuration with the command “auto-cost reference-bandwidth (#)” then you will change the first value in the equation, which can help make the Cost more affective so you are not load balancing traffic over a FastEthernet and a GigEthernet interface. So if you enter “auto-cost reference-bandwidth 1000” the equations change, respectively to 1000/100 (Cost 10) for FastEthernet and 1000/1000 (Cost 1) for GigEthernet. One important note on using this method of changing cost is to be consistent with changing it on every OSPF router to match, or you may cause some bells and whistles to start alarming pretty rapidly.

Method two is to change the Interface Bandwidth, or the second value in that equation, which again is done directly on the interface with the command “bandwidth #” (in kbps), which will dynamically change the cost shown with “show ip ospf int S0/0” for example.

These are fine and good for knowledge sake, given the third option will override any changing of the cost on a specific interface I believe it is the best option, but for exam purposes of course all 3 need to be known. It is simply “ip ospf cost #” on the interface, and that is it.

One last thing to touch on before taking the night off is 4 routers on a broadcast OSPF enabled segment, and the odd behavior it causes, but only odd if you are not prepared for it! The 4 routers will go through the election process, two will become DR / BDR, the other two will become DROthers. Now when you do a “show ip eigrp nei” from either of the DROthers, you will find that their State stops at 2-way with the other DROther. This is because no two DROthers will form full adjacencies with eachother, unless the DR or BDR goes down, however they will continue to send Hello’s and their Dead timers should not drop below 30 seconds (on an Ethernet segment).

This doesn’t seem like a huge point to drive home, but there is a thing called “Stuck in 2-way” that exists with OSPF, and the above situation IS NOT STUCK IN 2-WAY <- That is very important to know. This is an expected behavior between DROther routers on any broadcast network with 4+ routers in the OSPF domain, and is not an issue.

With that I will be starting on Advanced topics such as Stub Area’s, LSA types, and all the other magic of OSPF!

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