Study Session Number 7! OSPF! #8

Hi Everyone! 

Today's subject is the biggy of the CCNA routing protocols - OSPF! I've left this to its own topic as I intend to put more time into this specific topic rather than the other two (RIP & EIGRP), by the end of this we'll both understand how to configure and verify OSPF! and if we dont...dont sue me ;p anyways lets get into it! 

OSPF shares a few of the same attributes as EIGRP - its a link state protocol with fast convergence times that uses multicast (224.0.0.5) to send out updates. Uses Dijkstra algorithm to determine what path is considered the best path.

Why use OSPF in comparison to EIGRP if there both similar? OSPF is an open standard! because of this any networking device form any vendor can use OSPF whereas in comparison the EIGRP protocol is a Cisco protocol meaning its not always supported on other vendors.

The updates sent from OSPF send out all the information it knows of the network that its connected to and the interfaces it has, this provides a birds-eye view of the network - this allows for every router on the network to know of every other router. 

Metric - route is based on cost of links, the lower cost of the links the better! the link cost itself is made up of the reference bandwidth  / Interface Bandwidth - set to 100Mbps by default an example is FastEthernet = 100 / 100 so the link cost would be 1. In ospf all interfaces with > 100mbps are treated equally, this causes routing issues, to avoid this the bandwidth can be manually changed by using the auto-cost reference-bandwidth 10000. Another way to manipulate the bandwidth is to go into the interface and use the ip ospf cost *cost* and this will manipulate the route, this done on the slower interfaces.

All routers that need to communicate with each other need to be in the same area as each other, think of ospf area's like subnets your essentially breaking the network into more manageable chunks.

Speed - Speed 10 (Ethernet), 100 (FastEthernet), 1000 Gig (GigabitEthernet).

Bandwidth - Doesn't affect transmission speed (Speed as mentioned above), bandwidth is used for policies for routes and QoS.

Clock Rate - For serial interfaces (1.544Mbps by default), clock rate command can change this. Both sides of link need to have the same clock rate.


In OSPF routing updates are referred to as link state advertisements. In OSPF there is a process to how neighbors are found and communicated with, its split into six steps - 

Discover neighbors - I like to think of this as meeting a new neighbor on the street you live in.
Form Adjacency - I imagine this as giving your neighbor a number to contact you on. 
Flood Link State Database - this is where the routes are shared, imagine giving out the number you've just received off your neighbor and giving it to your friends.  
Compute Shortest Path - You then work out what the best path is to your neighbors house.
Install Best routes into table - Remember that path for the next time you want to visit! 
Respond to network changes - Now just keep track of it all, if anything changes with your neighbor, note it down! 

ROUTER ID = HIGHEST ROUTER IP ADDRESS (SHOW IP INT BRIEF THEN SHOW IP PROTOCOLS AND COMPARED ROUTER ID TO IP), BEST PRACTICE IS TO CONFIGURE LOOPBACK OR MANUALLY SET, NEED TO RESTART FOR CONFIG TO STICK (CLEAR IP OSPF PROCESS - COMMAND).


Commands 
router ospf *process ID* - PROCESS ID IS LOCAL, DOES NOT NEED TO BE THE SAME ON OTHER ROUTERS 
network *network id* *wildcard mask* area *area number*
passive-interface *interface* - passive interfaces are advertised to other routers but wont form adjacency or give out any information.  
router id *id* - Only needed if router ID is manually configured.
auto-cost reference-bandwidth 10000 - the number needs to be the same across interfaces.

Setting Designated Router - 
int *interface*
ip ospf priority *number* 

REMEMBER - HIGHEST BRIDGE PRIORITY = DESIGNATED ROUTER (HIGHER IS BETTER).

Setting Passive Interface as default - 
router ospf *process id*
passive-interface default - sets all interfaces to passive. 
no passive-interface *interface* - sets interface to normal ospf, repeat for all non passive interfaces. 

Default route injection -
ip route 0.0.0.0 0.0.0.0 *destination ip*
router ospf *process id*
default-information originate

Verify designated router 
show ip ospf neighbor 
show ip ospf interface *interface* - tells DR/BDR in the output.

Verify 
show ip ospf neighbor 
show ip ospf database 
show ip route 
show run | sec ospf
show ip protocols 
show ip ospf interface brief 

Order for verify
show ip ospf neighbor 
show ip ospf database 
show ip route
show ip ospf int *interface*


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