Border Gateway Protocol

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The Border Gateway Protocol (BGP) is the core routing protocol of the Internet. It works by maintaining a table of IP networks or 'prefixes' which designate network reachability between autonomous systems (AS). It is described as a path vector protocol. BGP does not use technical metrics, but makes routing decisions based on network policies or rules. As of January 2006, the current version of BGP, version 4, is codified in (request for comment) RFC 4271 (which obsoletes RFC 1771).

BGP supports classless interdomain routing and uses route aggregation to decrease the size of routing tables. Since 1994, version four of the protocol has been in use on the Internet; all previous versions are considered obsolete.

BGP was created to replace the EGP routing protocol to allow fully decentralized routing in order to allow the removal of the NSFNET Internet backbone network. This allowed the Internet to become a truly decentralized system.

Very large private IP networks can also make use of BGP; an example would be the joining of a number of large Open Shortest Path First (OSPF) networks where OSPF by itself would not scale to size. Another reason to use BGP would be multihoming a network for better redundancy.

Most Internet users do not use BGP directly. However, since most Internet service providers must use BGP to establish routing between one another, it is one of the most important protocols of the Internet. Compare and contrast this with Signalling System 7, which is the inter-provider core call setup protocol on the PSTN.

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BGP operation

BGP neighbours, or peers, are established by manual configuration between routers to create a TCP session on port 179. A BGP speaker will periodically send 19-byte keepalive messages to maintain the connection (every 60 seconds by default). Among routing protocols, BGP is unique in using TCP as its transport protocol.

When BGP is running inside an autonomous system (AS), it is referred to as Internal BGP (IBGP Interior Border Gateway Protocol). iBGP routes have an administrative distance of 200. When BGP runs between AS, it is called External BGP (EBGP Exterior Border Gateway Protocol), and it has an administrative distance of 20. If the role of a BGP router is to route IBGP traffic, it is called a transit router. Routers that sit on the boundary of an AS and that use EBGP to exchange information with the ISP are called border or edge routers.

All routers within a single AS and participating in BGP routing must be configured in a full mesh: each router must be configured as peer to every other router. This causes obvious scaling problems, since the number of required connections grows quadratically with the number of routers involved. To get around this, two solutions are built into BGP: route reflectors (RFC 2796) and confederations (RFC 3065).

Route reflectors reduce the number of connections required in an AS. A single router (or two for redundancy) can be made a route reflector: other routers in the AS need only be configured as peer to them.

Confederations are used in very large networks where a large AS can be configured to encompass smaller more manageable internal ASs. Confederations can be used in conjunction with route reflectors.


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Finite State Machine

A BGP peer uses a simple Finite State Machine (FSM) to make decisions in its operations with other BGP peers. The FSM consists of six states - Idle, Connect, Active, OpenSent, OpenConfirm, and Established. A BGP peer will transition the TCP connection to another peer between these states as it attempts to establish and maintain a session with that peer.


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Path Selection

BGP uses the following criteria to determine the path to use (from top to bottom):

  • An explicit route (i.e. not a default route) for the next-hop router must exist in the routing table
  • Prefer the path with the highest weight (Only on Cisco routers)
  • Prefer the path with the highest local preference
  • Prefer any BGP originated on this router
  • Prefer the route with the shortest AS path
  • prefer the route with the lowest origin (IGP < EGP < ?)
  • Prefer the path with the lowest MED (Multi exit discriminator)
  • Prefer external paths to internal paths
  • Prefer the path with the lowest IGP metric to the next hop
  • If all remaining paths are external choose the oldest one
  • Prefer the next hop router with the lowest BGP ID
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BGP problems and mitigation

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Route flapping

A feature known as "damping" is built into BGP to mitigate the effects of route flapping. Flapping of routes can be caused by WAN / WLAN links or physical interfaces mending and breaking or by misconfigured or mismanaged routers. Without damping, routes can be injected and withdrawn rapidly from routing tables, possibly causing a heavy processing load on routers thus affecting overall routing stability.

With damping, a route's flapping is exponentially decayed. At first instance when a route becomes unavailable but quickly reappears for whatever reason, then the damping does not take effect, so as to maintain the normal fail-over times of BGP. At the second occurrence, BGP shuns that prefix for a certain length of time; subsequent occurrences are timed out exponentially. After the abnormalities have ceased and a suitable length of time has passed for the offending route, prefixes can be reinstated and its slate wiped clean. Damping can also mitigate malicious denial of service attacks; damping timings are highly customizable.

As backbone links and router processors have become faster, some network architects have suggested that flap damping may not be as important as it used to be, since changes to the routing table can be absorbed much faster by routers. Some have even suggested that damping may make things worse, not better, in such an environment. This topic is controversial and is the subject of much research.

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Routing table growth

One of the largest problems faced by BGP, and indeed the Internet infrastructure as a whole, comes from the growth of the Internet routing table. If the global routing table grows to the point where some older, less capable, routers cannot cope with the memory requirements or the CPU load of maintaining the table, these routers will cease to be effective gateways between the parts of the Internet they connect. In addition, and perhaps even more importantly, larger routing tables take longer to stabilize (see above) after a major connectivity change, leaving network service unreliable, or even unavailable, in the interim.

Until late 2001, the global routing table was growing exponentially, threatening an eventual widespread breakdown of connectivity. In an attempt to prevent this from happening, there was a cooperative effort by ISPs to keep the global routing table as small as possible, by using CIDR and route aggregation. While this slowed the growth of the routing table to a linear process for several years, with the expanded demand for multi-homing by end user networks the growth was once again exponential by mid 2004.

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External links

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