Sunday, 25 November 2018

Introduction of BGP

Introduction of BGP

Border Gateway Protocol (BGP) advertises, learns, and chooses the best paths inside the wide internet. When two ISPs connect, they typically use BGP to exchange routing information. The ISPs of the worldwide exchange routing information with one or more than one ISPs.

Autonomous system

An autonomous system is a collection of networks under a single technical administration domain. IGPs like OSPF EIGRP operates within an Autonomous system. We use BGP between the autonomous systems to exchange  loop-free routing information.

Internal and External BGP

BGP defines in two classes for neighbors.
  1. Internal BGP ( iBGP ) operates within the same autonomous system.
  2. External BGP ( E BGP) operates in between the multiple autonomous systems  

BGP  features 

  • BGP is an open standard protocol
  • exterior gateway protocol designed for inter-AS domain routing to scale huge neter-network like internet 
  • It supports classless, VLSM, CIDR, auto, and manual summary 
  • updates are incremental and trigger BGP send updates to manually defined neighbor as unicast.
  • BGP is an application layer protocol that uses TCP for reliability, TCP  port 179
  • Metric is attributes
  • Administrative distance  is 20 for external updates ( EBGP) and 200 for internal updates (i BGP)

Types of ISP connections 

  • Single Homed
  • Dual-Homed sites
  • Multi-Homing
  • Dual Multi-Homing

Single homed 

Single homed a site with a single ISP connection is single-homed. Single homed is good for a site that does not depend heavily on Internet or WAN connectivity. you can advertise the site route or static routes and receives a default route from the internet service provider.


 A Dual-homed site has two connections to the same ISP either from one router or two routers. one link might be primary and the other as backup, or the site might be load balancing over both the connections. we can use static or dynamic routing.


 Multi-homing means having connecting links to one or more ISPs at the same time. it is done for redundancy and backup if one ISP fails and for better performance if one ISP provides a better path to frequently used networks. multi-homed also gives you an ISP-independent solution. BGP is typically used with multihomed connections.

Dual multi-homed

Dual multi-homed means having two connections with multiple ISPs. Dual multihomed gives you the most redundancy. BGP  is used with ISPs and can be implemented internally also.







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