The Border Gateway Protocol is used online as a path vector and routing protocol. It forms the basis for ex­chan­ging data about the reach­ab­il­ity of available routers and managing data packets. BGP can connect in­tern­ally and ex­tern­ally autonom­ous systems and provides network stability in the event of router failures.

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What is a BGP?

The Border Gateway Protocol is used to stabilise the internet network of the global routing system. Ab­bre­vi­ated to BGP or BGP protocol, it’s a path vector protocol that provides and exchanges in­form­a­tion about reachable or failed routing paths. It provides better networks stability by acting as an exterior gateway protocol and an interior gateway protocol.

With BGP, autonom­ous systems can be connected in­tern­ally and across systems in order to exchange data packets ef­fi­ciently. The BGP basics, including the current version BGPv4, are defined in detail in RFC 1163. BGP routing uses TCP port 179 to exchange data and in­form­a­tion. The basis for net­work­ing using BGP is the stand­ard­ised internet protocol under the ab­bre­vi­ation TCP/IP.

When is BGP used?

The Border Gateway Protocol is used as external BGP (eBGP) and as internal BGP (iBGP). Being the only external gateway protocol on the internet, it’s used for routing and managing data exchange between and within autonom­ous systems. By making decisions based on policies and rules defined by network ad­min­is­trat­ors, it guar­an­tees better network stability. This is possible, for example, by allowing routers to flexibly adapt in case of failures and to choose other logical routing paths available via BGP for ex­chan­ging packets. In addition, BGP updates enable routers to add new ones to the default routing table in use.

Note

The IPv6 ad­dress­ing type anycast is closely related to the BGP protocol. BGP routing is used to enable com­mu­nic­a­tion between computers in anycast networks.

How do BGP networks work?

BGP networks use a routing table to control and manage data packets for net­work­ing and data exchange between routers. Routers generate the table’s BGP in­form­a­tion with incoming router in­form­a­tion and the RIB (Routing In­form­a­tion Database) stored on the router. The RIB not only includes in­form­a­tion about external and internal peers, but also updates the routing table based on new in­form­a­tion about the current BGP peer group and available or no longer available paths, routers and peers. BGP routers use TCP con­nec­tions and TCP port 179 to exchange messages and data, for example:

  • OPEN: starts the exchange of in­form­a­tion of a BGP session
  • UPDATE: transmits in­form­a­tion about modified or new paths
  • KEEPALIVE: regular KEEPALIVE messages ac­know­ledge OPEN messages and inform connected routers that a session should be main­tained
  • NO­TI­FIC­A­TION: used to delete routing paths or to abort a session if the KEEPALIVE message fails to appear

BGP de­term­ines the best path for data exchange based on the routing table and cor­res­pond­ing path at­trib­utes such as:

  • Reachable autonom­ous systems (AS_PATH)
  • Required hops or in­ter­me­di­ate nodes (Next Hop)
  • Cost (IGP metric)
  • Pri­or­it­ised parallel con­nec­tions (multi-exit dis­crim­in­at­or)

BGP routing can be applied with IPv4 and IPv6 and other internet protocols or MPLS labels. In addition, BGP works according to the OSI model on the OSI transport layer to control the network layer

Providing a constant exchange to available or new paths via BGP, BGP routers can create graphs to map networked paths within or between autonom­ous systems. This ensures reliable data and in­form­a­tion exchange in networks, improves network stability and prevents loop formation.

Image: Overview of how BGP works
BGP strengthens network stability and prevents looping.

What’s the dif­fer­ence between internal and external BGP?

The dis­tinc­tion between external BGP (eBGP) and internal BGP (iBGP) depends on whether networks share data between different autonom­ous systems and BGP peer groups or within an autonom­ous system (AS) with internal peer groups.

Where routers are networked within an AS, either the OSPF protocol (Open Shortest Path First), based on the mesh principle, or iBGP as the Interior Gateway Protocol (IGP) are used for the shortest path between all internal routers meshed via BGP. Route re­flect­ors can prevent scaling problems in large networks. Con­nec­tion to the route reflector is suf­fi­cient for BGP routers. In turn, these forward learned route in­form­a­tion to internal AS routers via the external BGP. In order to avoid a single point of failure due to route re­flect­ors, these are usually used as clusters.

Issues of security and problems caused by the Border Gateway Protocol

When it comes to network security, BGP routers are quite vul­ner­able to attacks such as Denial of Service (DoS). This is where routers are flooded with packets. Another BGP routing vul­ner­ab­il­ity is BGP hijacking, where hackers im­per­son­ate the AS and routing source to intercept or redirect traffic.

BGP service issues and error or subcode for possible timeout or pro­cessing issues may result if:

  • in­form­a­tion exchange fails due to in­cor­rectly formatted or in­com­plete/false router in­form­a­tion.
  • memory or working memory is in­suf­fi­cient.
  • updates are too slow.
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