Introduction to BGP

 

Introduction to BGP

Border Gateway Protocol (BGP) advertises, learns, and chooses the best paths inside the global Internet. When two ISPs connect, they typically use BGP to exchange routing information. Collectively, the ISPs of the world exchange the Internet’s routing table using BGP. And enterprises sometimes use BGP to exchange routing information with one or more ISPs, allowing the enterprise routers to learn Internet routes.

Interior Gateway Protocols (IGPs) are used for exchanging routing information within an autonomous system (AS). Ex: RIP, IGRP, EIGRP, OSPF, and IS-IS.

Exterior Gateway Protocols (EGPs) are used for communication between autonomous systems. The main goal of EGPs is to provide an inter-domain routing system that exchanges guaranteed loop-free routing information between autonomous systems throughout the Internet and within multinational organizations. EGPs are not meant to locate a specific network, but rather provide the information to find the autonomous system in which the network resides. Ex: BGP.

One key difference when comparing BGP to the usual IGP routing protocols is BGP’s robust best-path algorithm. BGP uses this algorithm to choose the best BGP path (route) using rules that extend far beyond just choosing the route with the lowest metric. This more complex best-path algorithm gives BGP the power to let engineers configure many different settings that influence BGP best-path selection, allowing great flexibility in how routers choose the best BGP routes.

BGP Basics

BGP, specifically BGP version 4 (BGPv4), is the one routing protocol in popular use today that was designed as an exterior gateway protocol (EGP) instead of as an interior gateway protocol (IGP). As such, some of the goals of BGP differ from those of an IGP, such as Open Shortest Path First (OSPF) or Enhanced Interior Gateway Routing Protocol (EIGRP), but some of the goals remain the same.

Path Attributes: AS_PATH

Path attributes (PAs) are factors that allow BGP to select a route over another. By default, no BGP PAs have been set, and BGP use AS_PATH (autonomous system path) PA when choosing the best route among many routes.

When a router uses BGP to advertise a route with AS_PATH, it will tell which list of ASN the path will go through. AS_PATH can:

- Choose the best route by using the route with shortest AS_PATH

- Prevent routing loops.

IBGP Vs EBGP

1. EBGP is peering between two different AS, whereas IBGP is between same AS (Autonomous System).

2. Routes learned from eBGP peer will be advertised to other peers (BGP or IBGP); however, routes learned from IBGP peer will not be advertised to other IBGP peers.

3. EBGP routes have administrative distance of 20, whereas IBGP has 200.

iBGP

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connections are those within your company or Autonomous System (AS). Two or more routers internally connected together share iBGP links. You would typically see this inside of the network routers of an Internet Service Provider (ISP).

eBGP

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connections are those between two companies or Autonomous Systems (AS). Traffic is able to be passed between companies using these types of links, which in essence is the backbone of the Internet.