Basic understanding about Border Gateway Protocol (BGP)

We really want to show you why we need BGP first but it is very difficult to explain without understanding a bit about BGP. So we will learn some basic knowledge about BGP first.

First we need to understand about the different between Interior Gateway Protocol and Exterior Gateway Protocol. The difference between them is shown below:

– Interior Gateway Protocol (IGP): A routing protocol operating within an Autonomous System (AS) like OSPF, EIGRP… Usually routers running IGP are under the same administration (of a company, corporation, individual)
– Exterior Gateway Protocol (EGP): A routing protocol operating between different AS. BGP is the only EGP used nowadays

In the topology above R1, R2 and R3 should run an IGP to communicate with each other because they are in the same AS. But to connect with other routers in another AS (like a different ISP), R1 and R3 must use an EGP.

With BGP, the term autonomous system (AS) refers to a network that operates separately from other networks and usually operates within a single administrative domain. Each AS is represented by an AS number. It is similar to EIGRP AS in this aspect. BGP is used mainly by the Internet Service Provider (ISP) all over the world. Each ISP usually has one BGP AS number (some very big ISP may have a few AS numbers). BGP AS numbers can be between 1 to 65,535.

In the topology above R1 and R3 are operating in BGP AS 1. If an AS connects to the public Internet using an EGP, then it must be assigned a unique AS number which is managed by the Internet Assigned Numbers Authority (IANA). IANA manages the AS numbers from 1 to 64,512 for public use (similar to public IP addresses) while 64,512 to 65,535 numbers are reserved for private use (similar to private IP addresses).

If we don’t want to show the routers inside each AS we can simply ignore them:

In fact, the Internet that we are going “online” everyday is a collection of interconnected autonomous systems and BGP is running to provide routing between them.

Other BGP terms that you should learn are listed below:
+ BGP speaker: a router running BGP
+ BGP peer or BGP neighbor: Any two routers that have formed a TCP connection to exchange BGP routing information (as BGP runs over TCP on port 179, not UDP)
+ Prefix: Maybe you learned the word “subnet”. In BGP world, it is usually called “prefix” because BGP usually does not advertise small subnets. It advertises blocks of large subnets so “prefix” is often used instead
+ Internal BGP (iBGP): refers to the BGP neighbor relationship within the same AS. The iBGP neighbor does not have to be directly connected
+ External BGP (eBGP): refers to the BGP neighbor relationship between two peers belongs to different AS. It is recommended that eBGP should be directly connected. Never run an IGP between eBGP peers.

In the below topology suppose all routers are running BGP then R1 is considered internal BGP to R2 and R3 (as they are running same AS 1) but is external BGP to R4. R5 is internal to R4 and R6 but external to R3.