OSPF - Common OSPF Optimizations

4 important questions on OSPF - Common OSPF Optimizations

In OSPF, Link Cost is Reference Bandwidth divided by the Interface Bandwidth.
What is the default reference bandwidth and why can this be a problem?

Default reference bandwidth is 100Mbit.
Because of this value the Cost for a 100Mbit or 1Gbit interface is the same.

Which 2 ways can be used to change the OSPF cost

1. Change the reference bandwidth using OSPF process command auto-cost reference-bandwidth <Bandwidth in Mbits>. Should be done on all routers in the area.
2. Change it on a specific interface using the interface command ip ospf cost <cost>

Hello packets are used between OSPF neighbors to check if they are still healthy and available. These are send on an interval based on the Hello timer.
What is another timer related to the Hello Timer?

The Dead interval timer. This timer decrements until an Hello packet is received, after it is reset again and starts to decrement again.
The Dead Interval timer defaults to 4 times the Hello timer.

What are the 5 OSPF network types and their information/usage?

1. Broadcast (Enabled by default on OSPF Ethernet links. DR/BDR enabled. Hello Time 10 seconds
2. Non-Broadcast (Enabled by default on OSPF Frame Relay (Multipoint)interfaces. DR/BDR enabled. Hello Time 30 seconds.
3. Point-to-Point (Enabled by default on Frame Relay p2p sub interfaces. DR/BDR not enabled. Hello timer 10 seconds.
4. Point-to-Multipoint (Not enabled by default. No DR/BDR. Hello Timer 30 seconds.
5. Loopback (Default on OSPF enabled Loopback interfaces. DR/BDR and Timers not available.

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