The protocol I chose to write about is the Border Gateway Protocol because it is the main protocol used to operate the internet and recommended protocol for Cisco design certifications. The BGP protocol is an inter-autonomous protocol; which means it is for connecting separate large networks, such as a college network to a corporate network or the internet. Due to the large number of routes and connection this protocol must manage; it uses very many features, or attributes, to achieve this. Cisco defines these attributes as weight, local preference, multi-exit discriminator, origin, AS_path, next hop, and community. These attributes are used to discover networks and determine the best path to use for sending packets to those networks.
BGP was designed in accordance with RFC 1771, which was started in October of 1991 and published in March of 1995. In January of 2006 RFC 1771 was made obsolete by RFC 4271 which contains the updated information about BGP. These RFCs, along with a quite a few more, describe the design and operation of BGP. Cisco uses a much smaller paper to give a basic rundown of BGP, plus they carry an extensive library of web pages to go more in depth. To get a basic understanding of BGP we need to look at the attributes that make it work.
The first attribute we will look at is called the weight attribute. This is a Cisco proprietary attribute, so it is not found with other vendors. This attribute simply assigns a weight value to any know routes on an individual router. This value is not passed on to any other routers. Each router can have its own independent weight values for any known routes. If two separate routes to the same network are known, the one that has the highest assigned weight value will be the preferred route.
The local preference attribute is used to determine the best exit point for an autonomous system (AS). This value is shared by all the routers in the AS to select the best interfaces and routers for reaching known networks. Each known network may exit on any router in the AS, or all on one router in the AS. The route is determined by which...