Concept and network topology of BGP protocol

The BGP protocol in the routing protocol is a type of protocol we will analyze for you below. As this part of the content is very important, we will mainly introduce it in two parts: first, the introduction of the concept above, and then a brief analysis of its topology structure.

1. Introduction

BGP is a routing protocol between autonomous systems. The network accessibility information exchanged by BGP provides sufficient information to detect routing loops and make decisions based on performance precedence and policy constraints, BGP exchange includes all the network accessibility information of the AS path and implements routing policies according to the configuration information. With the progress and growth of the Internet in recent years, it has to face some serious scale problems.

Class B network address space depletion: one of the main causes of this problem is the lack of medium-size networks suitable for medium-sized organizations; Class C network, with a maximum of 254 host addresses, there are too few, and B-type networks allow a maximum of 65534 addresses, but they are too large to be fully used. The increasing number of Route tables in Internet Routers makes the current software (and people) unable to effectively manage ｡

32-bit IP address space depletion: Obviously, the first two and last problems may become urgent in the next one or two years and three years, respectively. classless Inter-Domain Routing (CIDR) trying to solve these problems, designing a mechanism to reduce the growth rate of Route tables and new IP network allocation requirements, does not solve the third problem that is more long-term, but to put the recent problem delayed so that the Internet can still operate effectively, while starting a long-term solution. BGP-4 has expanded the BGP-3, this memorandum describes the application of BGP-4 in Internet ｡

All the discussions are based on the following assumptions: the Internet is a collection of autonomous systems that are randomly connected. The classic definition of autonomous systems is that a group of routers are under unified management, data packets are routed using the internal gateway protocol and unified measurement in AS, and data packets are routed to other AS through the external gateway protocol, some AS uses multiple internal gateway protocols and measurements. Here, we emphasize the meaning of autonomous systems in this document. Even if it uses multiple IGP and measurements, its management is different from other, its internal routes are consistent. When a route passes through it, it is regarded AS a node. Each AS is managed by a management organization, at least externally it represents the routing information of the system ｡

2. BGP protocol topology model

When we talk about a connection between two AS, it means two things: physical connection: there is a shared data link subnet between the two AS, and on this subnet, each AS has at least one VBR. Therefore, each as vbr can forward data packets to other AS vbrs, there is no need to use the routing between AS and AS. BGP connection: There is a BGP session process between the BGP spokespersons of each AS, through the session communication routing, after the declared AS to reach a target network ｡

We impose additional restrictions on the BGP spokespersons that form BGP connections: they must share their own data link subnets directly. Therefore, BGP sessions between adjacent AS instances do not require routes between AS instances or AS instances. cases beyond the scope of this article may be inconsistent with this restriction. Therefore, in each connection, each AS has more than one BGP speaker and Border Gateway Router. These BGP speakers and Border Gateway routers are distributed on the Internet of shared data links ｡

Note that the BGP protocol spokesman is not necessarily a Border Gateway Router, and vice versa. The path stated by the BGP spokesman connecting to the previous AS can be used by the Border Gateway Router of other AS on the same shared subnet, that is, the non-direct neighbor is allowed. The traffic in an AS is either from the AS or finally the AS (that is, the source IP address or destination IP address of the IP packet is in the AS). Traffic that meets the preceding description is called "local traffic ", otherwise, it is called "transitional traffic". The main purpose of BGP is to control the transitional traffic ｡

AS can be divided into the following categories based on how an AS handles transitional traffic:

End AS: connect to only one other AS. Naturally, end AS only ships local traffic ｡

Multi-host AS: connects to more than one other AS, but does not transport the transitional traffic ｡

Transition AS: connects to more than one other AS to transport local and transitional traffic ｡

A complete AS path provides an effective and simple way to avoid routing loops and eliminate the "count to infinity" problem of the companion Distance Vector Algorithm. Therefore, the BGP protocol does not limit the connection topology between ｡