Understanding Border Gateway Protocol (BGP)
BGP is a complex, advanced distance Exterior Gateway Protocol (EGP), BGP exchange routing information between Autonomous Systems (ASs).
Unlike Interior routing protocols such as RIP, EIGRP, and OSPF that run inside a company’s network, BGP uses a Best Path Selection Algorithm (BPSA) to choose and install the best routes into the router’s routing table. This is a different basic algorithm for building a loop-free topology than any of the above mentioned protocols.
BGP is especially used for exchanging routing information between all of the major Internet Service Providers (ISPs), as well between larger client sites and their respective ISPs. And, in some large enterprise networks, BGP is used to interconnect different geographical or administrative regions.
BGP is Primarily used to support the complexity of the public Internet, Cisco has added several clever and useful features to its BGP implementation (BGP 4). Some of the primary attributes of BGP is the use of pieces of information about a known route, where it came from, and how to reach it, A BGP router will also generate an error message if it receives a route that is missing these are mandatory attributes.
Types of BGP
There are different terms used when describing BGP. these including:
1.Internal BGP (iBGP) operates inside an autonomous System (AS)
2. External BGP (eBGP), which is also known as an inter-domain routing protocol, operates outside an AS and connects one AS to another. These terms are just used to describe the same protocol just the area of operation is what differs.
Autonomous Systems (AS)
An autonomous system can be a company, ISP or an entire corporate network comprised of multiple locations connecting to the network.
Each autonomous System (AS) uses BGP to advertise routes in its network that need to be visible outside of the network; it also uses BGP to learn about the reachability and routes by listening to advertisement announcements from other autonomous systems.
Each of these enterprise network, commercial enterprise or ISP must be identified by an autonomous system number (ASN). This number allows a hierarchy to be maintained when sharing route information.
There are 65,535 (from 0 to 65,535) available autonomous system numbers that can be assigned. BGP assigns 64,512 – 65,534 ASNs to be private. Being private means this ASN connect to only one other ASN (sometimes multiple ASN) and these ASNs can’t cause loop by themselves
Characteristics of BGP-4
The key features of BGP-4 include and not limited to these:
• It is an advanced distance-vector protocol.
• BGP sends full routing updates at the start of the session, trigger updates are sent afterward.
• BGP maintains connection by sending periodic keepalives.
• It creates and maintains connections between peers, using TCP port 179.
• BGP sends a triggered update when a keepalive, an update, or a notification is not received
• It has its own routing table, although it is capable of both sharing and inquiring of the interior IP routing table.
• BGP uses a very complex metric, and is the source of its strength. The metric, referred to as attributes, allows great flexibility in path selection.