BGP4 Overview
Border Gateway Protocol version 4 (BGP4) is an exterior gateway protocol that performs inter-autonomous system (AS) or inter-domain routing. It peers to other BGP-speaking systems over TCP to exchange network reachability and routing information. BGP primarily performs two types of routing: inter-AS routing, and intra-AS routing. BGP peers belonging to different autonomous systems use the inter-AS routing, referred to as Exterior BGP (eBGP). On the other hand, within an AS BGP can be used to maintain a consistent view of network topology, to provide optimal routing, or to scale the network.
BGP4 is a path vector protocol that implements this scheme on a large scale by treating each AS as a single point on the path to any given destination. For each route (destination), BGP maintains the AS path and uses this to detect and prevent loops between autonomous systems.
Devices within an AS can use different Interior Gateway Protocols (IGPs) such as RIP and OSPF to communicate with one another. However, for devices in different autonomous systems to communicate, they need to use an EGP. BGP4 is the standard EGP used by Internet devices and therefore is the EGP implemented on RUCKUS devices.
This is a simple example of two BGP4 ASs. Each AS contains three BGP4 devices. All of the BGP4 devices within an AS communicate using iBGP. BGP4 devices communicate with other autonomous systems using eBGP. Notice that each of the devices also is running an Interior Gateway Protocol (IGP). The devices in AS1 are running OSPF and the devices in AS2 are running RIP. The device can be configured to redistribute routes among BGP4, RIP, and OSPF. They also can redistribute static routes.
