1. Introduction to Networks
Routing Protocols Comparisons

2. Routing Scope
Routing protocols are the methods through which a router will learn about and share connected routes to map out their networks
There are many different routing protocols, each with their own strengths so we need a way to compare the protocols to decide which ones to use
The first classification we’ll look at is routing scope
A routing scope is how much information is shared and remembered by the routers

3. IGP vs EGP
An IGP is an Internal Gateway Protocol. This is the protocol used inside autonomous systems
An autonomous system is a private network connected to the internet. These networks are fairly independent, only sharing a link to the internet
Each autonomous system is assigned a unique ASN, autonomous system number, used to identify that system
The routing protocol defines how far and what information is shared
IGP will share information within the autonomous system about the internal network, but won’t share any information outside of it
An EGP is an External Gateway Protocol. This is the protocol used to share information between autonomous systems over the internet
Network admins typically deal with IGP, except if they work for exceptionally large networks

4. Metrics - Hop Count
Another way to classify routing protocols is by comparing metrics
A metric is a value set to a route between networks that determine which route to take when there are multiple ways to send data
The route with the lowest value indicates the best route
The first metric a protocol could use is hop count
Hop Count simply measures how many routers we would need to pass through to get to our destination through a certain path
The path chosen will always be the route that takes us through the fewest routers to get to the network

5. Metrics - Bandwidth
A more advanced metric we could use is bandwidth or latency (delay)
This attempts to measure how quickly a message would move through a route
Each connection between routers will be given a value based on the rated bandwidth of the connection. The slower the connection, the higher the value
With a protocol using this metric data may choose to move through several faster routers than a few slow ones

6. Metrics - Relative Value
Relative value is a modified version of hop count
Every router starts at a default value, and the protocol will choose the path with the lowest value
If there is no change, then this works exactly like hop count, however a network admin can assign each router a value to customize how likely the router will be used as part of a path
These values are often called link values, and can be based on any reasoning a network admin may have

7. Distance Vector
Another property we can use to compare routing protocols is the method used to share routing information
The first method is the distance vector method
The distance vector method is when routing tables are propagated by routers sharing with their neighbors
Each router knows about its directly connected networks. Periodically each router will send its routing table to its neighbors. Any new networks get added to the routing table of the router, so that when the new router sends its routing table it is included
When the routing tables of every router know of every sub-net, then we say this network has achieved convergence

8. Link State
Another way to propagate information about routing tables is called the link state method
With the link state method, routers only send out the information about the networks that they are directly connected to using a special message called a link state advertisement (LSA) and link state packets (LSP)
Once a router takes in this information and updates its routing table, it forwards the information to the next router
Once convergence is achieved, routers will only send out LSAs if something with their local network changes
This process of sending out advertisements is called flooding because a single message is flooded through the network
This method achieves convergence faster and uses less network traffic, but the distance vector method requires less router processing and is simpler

9. VLSM
The final method we have of classifying routing protocols is their support for variable length subnet masks (VLSM)
Early routing protocols relied on networks using the default subnet mask, which created problems between confusing subnets
Newer protocols send the network ID with a subnet mask to accurately identify the network
Almost all protocols used today supports VLSM. These are called classless protocols, whereas the earlier protocols that relied on an address’s class to know its subnet mask were called classful protocols