# 10/11 (CEF/dijkstra algorithm/OSPF

###### tags: `Networking` `CCNA`

[TOC]

## CEF(cisco express forwarding)

CEF is a CISCO default Rule, the Rule is True.
IF(CEF is True ){
it's not going to look up a routing table, it will only go one of these ways.
}


### CEF == TRUE(default)


### CEF == False

R1(config)#no ip cef
R2(config)#no ip cef

### Result

0/0

0/1

RIP-ECMP(equal cost multipathpaht routing) only consider the hop of numbers
OSPF-ECMP, only consider the bandwidth
EIGRP
It consider a bandwidth, delay,reliability,cost and MTU.
>
>- Advantage: Load balance
>- Disadvantage: It's going to increase the occurred rate of the out-of-order problem

## dijkstra algorithm

dijkstra is to find the shortest path between any two vertex in the graph

## Find the shortest path from vertex A to every other vertex.

### example

**TO get from A to C

## Generate the table

we use two list, one to keep track of the vertices that we visited and another to keep track of the vertices that we haven't visited yet.


Algorithm

let distance of start vertex from start vertex =0 
let distance of all other vertices form start = infinity

while 1 
	Visit the unvisited vertex with the smallest known distance from the start vertex
	For the current vertex examine its unvisited neighbourt from start vertex
	if(the calculated distance of a vertex is less than known distance){
		update the shortest path distance 
	}
	Update the previous vertex for each of the updated distances 
	add the current vertex to the list of visited vertices 
	
	Until all vertices visited 

## OSPF(Open Shortest Path First)

cost -> dijkstra algorithm



Configure OSFP

R1

r1(config)#router ospf 100   #A Process ID is a local remark, so each device can use a different Process ID
r1(config-router)#network 12.0.0.0 0.0.0.255 area 0

R2

r2(config)#router ospf 100 
r2(config-router)#network 12.0.0.0 0.0.0.255 area 0
r2(config-router)#network 23.1.0.0 0.0.0.255 area 0

R3

r3(config)#router ospf 100
r3(config-router)#network 23.1.0.0 0.0.0.255 area 0

Wildcard mask: specify segment of IP address that needs to be checked, using deny or allow the IP whether it could go through or not.

Result

ad = 110

cost = 10**8 bps / bandwidth
Ethernet = 10 mps

10**8 / 10 * 10\**6 = 10
one hop = 10 (cost)
So, we could know there have two hops.

### Packet types

1.hello
2.Database Description
3.LSR(link state request)
4.LSU(link state update)
5.LSA(link state acknowlege)

### OSPF table

neighbor table

r1(config)#do show ip ospf neighbor

r2(config)#do show ip ospf database


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