Autonomous system (AS) / Routing Protocol

EIGRP (Enhanced Interior Gateway Routing Protocol) (supersedes RIP)

Cisco, being the dominant router company in the world (a crown it still wears to this day), came out with the Interior Gateway Routing Protocol (IGRP), which was quickly replaced with EIGRP. EIGRP has aspects of both distance vector and link state protocols, placing it uniquely into its own "hybrid" category. Cisco calls EIGRP an advanced distance vector protocol.

(ISIS) Intermediate System to Intermediate System (alternative to OSPF)

If you want to use a link state dynamic routing protocol and you don't want to use OSPF, your only other option is Intermediate System to Intermediate System (IS-IS). IS-IS is extremely similar to OSPF. It uses the concept of areas and send-only updates to routing tables. IS-IS was developed at roughly the same time as OSPF and had the one major advantage of working with IPv6 from the start. IS-IS is the de facto standard for ISPs. Make sure you know that IS-IS is a link state dynamic routing protocol, and if you ever see two routers using it, call me as I've never seen IS-IS in action.

OSPF (Open Shortest Path First) (IGP)

It is the most commonly used IGP in the world. Most large enterprises use OSPF on their internal networks. Even an AS, while still using BGP on its edge routers, will use OSPF internally because OSPF was designed from the ground up to work within a single AS. When you first launch OSPF-capable routers, they send out Hello packets, looking for other OSPF routers. After two adjacent routers form a neighborship through the Hello packets, they exchange information about routers and networks through link state advertisement (LSA) packets. LSAs are sourced by each router and are flooded from router to router through each OSPF area. Once all the routers communicate, they individually decide their own optimal routes, and convergence happens almost immediately. If a route goes down, OSPF routers quickly recompute a new route with stored LSAs. OSPF isn't popular by accident. It scales to large networks quite well and is supported by all but the most basic routers. By the way, did I forget to mention that OSPF also supports authentication and that the shortest-path-first method, by definition, prevents loops? OSPF Version 2 is used for IPv4 networks, and OSPF Version 3 includes updates to support IPv6.

Autonomous System (AS) (ASN vs. IP address)

One or more networks that are governed by a single protocol, which provides routing for the Internet backbone. Autonomous Systems do not deliver data between each other using IP addresses, but rather use a special globally unique Autonomous System Number (ASN) assigned by IANA. Originally a 16-bit number, the current ASNs are 32 bits, displayed as two 16-bit numbers separated by a dot. So, 1.33457 would be a typical ASN. Just as you would assign an IP address to a router, you would configure the router to use or be the ASN assigned by IANA. Autonomous Systems communicate with each other using a protocol, called generically an Exterior Gateway Protocol (EGP). The network or networks within an AS communicate with protocols as well; these are called generically Interior Gateway Protocols (IGPs). edge routers—that's what the AS-to-AS routers are called.

RIP (Routing Information Protocol)

RIP (either version) has a maximum hop count of 15, so your router will not talk to another router more than 15 routers away. This plagues RIP because a routing table request can literally loop all the way around back to the initial router. RIPv1 sent out an update every 30 seconds. This also turned into a big problem because every router on the network would send its routing table at the same time, causing huge network overloads. RIPv1 didn't know how to use variable-length subnet masking (VLSM), where networks connected through the router use different subnet masks. Plus RIPv1 routers had no authentication, leaving them open to hackers sending false routing table information. RIP needed an update. In RIPv2, VLSM has been added, and authentication is built into the protocol.

BGP (Border Gateway Protocol) 4 (EGP)

although many protocols are used within Autonomous Systems, such as RIP, the Internet has settled on one protocol for communication between each AS: the Border Gateway Protocol (BGP). BGP is the glue of the Internet, connecting all of the Autonomous Systems. BGP uses Edge Routers