【My Study Note】Exterior Gateway Protocols
Exterior Gateway Protocols
Exterior gateway protocols are used to communicate data between routers representing the edges of an autonomous system. Since routers sharing data using interior gateway protocols are all under control of the same organization. Routers use exterior gateway protocols when they need to share information across different organizations.
Exterior gateway protocols are really key to the Internet operating how it does today.
The Internet is an enormous mesh of autonomous systems. At the highest levels, core Internet routers need to know about autonomous systems in order to properly forward traffic.
The IANA (Internet Assigned Numbers Authority) is a non-profit organization that helps manage things like IP address allocation.
The Internet couldn’t function without a single authority for these sorts of issues. Otherwise, anyone could try and use any IP space they wanted, which would cause total chaos online.
Along with managing IP address allocation, the IANA is also responsible for ASN (Autonomous System Number) allocation. ASNs are numbers assigned to individual autonomous systems.
Just like IP addresses, ASNs are 32-bit numbers. But, unlike IP addresses, they’re normally referred to as just a single decimal number, instead of being split out into readable bits.
There are two reasons for this.
1st Reason
IP addresses need to be able to represent a network ID portion and a host ID portion for each number. This is more easily accomplished by splitting the number in four sections of eight bits, especially back in the day when address classes ruled the world.
An ASN, never needs to change in order for it to represent more networks or hosts. Its just the core Internet routing tables that need to be updated to know what the ASN represents.
2nd Reason
ASNs are looked at by humans, far less often, than IP addresses are.
So because it can be useful to be able to look at the IP 9.100.100.100 and know that 9.0.0.0/8 address space is owned by IBM, ASNs represent entire autonomous systems. Just being able to look up the fact that AS19604 belongs to IBM is enough.
Protocols
In terms of exterior gateway protocols, there is only one in use today. The entire Internet needs to agree on how to exchange this sort of information, so a single standard has emerged. This standard is known as BGP (Border Gateway Protocol).