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The Border Gateway Protocol (BGP) is the routing protocol of the Internet, used to route traffic across the Internet. For that reason, it's a pretty important protocol, and it can also be the hardest one to understand.
From our overview of Internet routing, you should realize that routing in the Internet is comprised of two parts: the internal fine-grained portions managed by an IGP such as OSPF, and the interconnections of those autonomous systems (AS) via BGP.
When BGP is configured incorrectly, it can cause massive availability and security problems, as Google discovered in 2008 when its YouTube service became unreachable to large portions of the Internet. What happened was that, in an effort to ban YouTube in its home country, Pakistan Telecom used BGP to route YouTube's address block into a black hole. But, in what is believed to have been an accident, this routing information somehow got transmitted to Pakistan Telecom's Hong Kong ISP and from there got propagated to the rest of the world. The end result was that most of YouTube's traffic ended up in a black hole in Pakistan.
More sinisterly, 2003 saw a number of BGP hijack attacks, where modified BGP route information allowed unknown attackers to redirect large blocks of traffic so that it travelled via routers in Belarus or Iceland before it was transmitted on to its intended destination.
Clearly, BGP is significant. Here we'll provide a short overview of how BGP works, along with the problems it solves and causes.
Once BGP is enabled, your router will pull a list of Internet routes from your BGP neighbors, who in this case will be your two ISPS. It will then scrutinize them to find the routes with the shortest AS paths. These will be put into the router's routing table. (If you only connect to a single ISP then you don't need BGP. That's because there's only one path to the Internet, so there's no need for a routing protocol to select the best path.)
BGP (Border Gateway Protocol) is protocol that manages how packets are routed across the internet through the exchange of routing and reachability information between edge routers. BGP directs packets between autonomous systems (AS) -- networks managed by a single enterprise or service provider. Traffic that is routed within a single network AS is referred to as internal BGP, or iBGP. More often, BGP is used to connect one AS to other autonomous systems, and it is then referred to as an external BGP, or eBGP.
Border Gateway Protocol (BGP) is an Internet Engineering Task Force (IETF) standard, and the most scalable of all routing protocols. BGP is the routing protocol of the global Internet, as well as for Service Provider private networks. BGP has expanded upon its original purpose of carrying Internet reachability information, and can now carry routes for Multicast, IPv6, VPNs, and a variety of other data.
Border Gateway Protocol (BGP) is a standardized exterior gateway protocol designed to exchange routing and reachability information among autonomous systems (AS) on the Internet.[1] The protocol is often classified as a path vector protocol but is sometimes also classed as a distance-vector routing protocol. The Border Gateway Protocol makes routing decisions based on paths, network policies, or rule-sets configured by a network administrator and is involved in making core routing decisions.
BGP may be used for routing within an autonomous system. In this application it is referred to as Interior Border Gateway Protocol, Internal BGP, or iBGP. In contrast, the Internet application of the protocol may be referred to as Exterior Border Gateway Protocol, External BGP, or eBGP.
Border Gateway Protocol (BGP) is a standardized exterior gateway protocol designed to exchange routing and reachability information among autonomous systems (AS) on the Internet. The protocol is often classified as a path vector protocol but is sometimes also classed as a distance-vector routing protocol.
BGP neighbors, called peers, are established by manual configuration between routers to create a TCP session on port 179. A BGP speaker sends 19-byte keep-alive messages every 60 seconds to maintain the connection. Among routing protocols, BGP is unique in using TCP as its transport protocol.When BGP runs between two peers in the same autonomous system (AS), it is referred to as Internal BGP (iBGP or Interior Border Gateway Protocol). When it runs between different autonomous systems, it is called External BGP (eBGP or Exterior Border Gateway Protocol). Routers on the boundary of one AS exchanging information with another AS are called border or edge routers or simply eBGP peers and are typically connected directly, while iBGP peers can be interconnected through other intermediate routers. Other deployment topologies are also possible, such as running eBGP peering inside a VPN tunnel, allowing two remote sites to exchange routing information in a secure and isolated manner. The main difference between iBGP and eBGP peering is in the way routes that were received from one peer are propagated to other peers. For instance, new routes learned from an eBGP peer are typically redistributed to all other iBGP peers as well as all eBGP peers (if transit mode is enabled on the router). However, if new routes were learned on an iBGP peering, then they are re-advertised only to all other eBGP peers. These route-propagation rules effectively require that all iBGP peers inside an AS are interconnected in a full mesh.Filtering routes learned from peers, their transformation before redistribution to peers or before plumbing them into the routing table is typically controlled via route-maps mechanism. These are basically rules which allow the application of certain actions to routes matching certain criteria on either ingress or egress path. These rules can specify that the route is to be dropped or, alternatively, its attributes are to be modified. It is usually the responsibility of the AS administrator to provide the desired route-map configuration on a router supporting BGP.
BGP is relevant to network administrators of large organizations which connect to two or more ISPs, as well as to Internet Service Providers (ISPs) who connect to other network providers. If you are the administrator of a small corporate network, or an end user, then you probably don't need to know about BGP.BGP basicsThe current version of BGP is BGP version 4, based on RFC4271.BGP is the path-vector protocol that provides routing information for autonomous systems on the Internet via its AS-Path attribute.BGP is a Layer 4 protocol that sits on top of TCP. It is much simpler than OSPF, because it doesn’t have to worry about the things TCP will handle.Peers that have been manually configured to exchange routing information will form a TCP connection and begin speaking BGP. There is no discovery in BGP.Medium-sized businesses usually get into BGP for the purpose of true multi-homing for their entire network.An important aspect of BGP is that the AS-Path itself is an anti-loop mechanism. Routers will not import any routes that contain themselves in the AS-Path. Why do you need to understand BGP?When BGP is configured incorrectly, it can cause massive availability and security problems, as Google discovered in 2008 when its YouTube service became unreachable to large portions of the Internet. What happened was that, in an effort to ban YouTube in its home country, Pakistan Telecom used BGP to route YouTube's address block into a black hole. But, in what is believed to have been an accident, this routing information somehow got transmitted to Pakistan Telecom's Hong Kong ISP and from there got propagated to the rest of the world. The end result was that most of YouTube's traffic ended up in a black hole in Pakistan.More sinisterly, 2003 saw a number of BGP hijack attacks, where modified BGP route information allowed unknown attackers to redirect large blocks of traffic so that it travelled via routers in Belarus or Iceland before it was transmitted on to its intended destination.Clearly, BGP is significant. Here we'll provide a short overview of how BGP works, along with the problems it solves and causes.Autonomous systemsFirst a little terminology. In the world of BGP, each routing domain is known as an autonomous system, or AS. What BGP does is help choose a path through the Internet, usually by selecting a route that traverses the least number of autonomous systems: the shortest AS path.You might need BGP, for example, if your corporate network is connected to two large ISPs. To use BGP you would need an AS number, which you can get from the American Registry of Internet Numbers (ARIN).Once BGP is enabled, your router will pull a list of Internet routes from your BGP neighbors, who in this case will be your two ISPS. It will then scrutinize them to find the routes with the shortest AS paths. These will be put into the router's routing table. (If you only connect to a single ISP then you don't need BGP. That's because there's only one path to the Internet, so there's no need for a routing protocol to select the best path.)Generally, but not always, routers will choose the shortest path to an AS. BGP only knows about these paths based on updates it receives.Route updatesUnlike Routing Information Protocol (RIP), a distance-vector routing protocol which employs the hop count as a routing metric, BGP does not broadcast its entire routing table. At boot, your peer will hand over its entire table. After that, everything relies on updates received.Route updates are stored in a Routing Information Base (RIB). A routing table will only store one route per destination, but the RIB usually contains multiple paths to a destination. It is up to the router to decide which routes will make it into the routing table, and therefore which paths will actually be used. In the event that a route is withdrawn, another route to the same place can be taken from the RIB.The RIB is only used to keep track of routes that could possibly be used. If a route withdrawal is received and it only existed in the RIB, it is silently deleted from the RIB. No update is sent to peers. RIB entries never time out. They continue to exist until it is assumed that the route is no longer valid.BGP path attributesIn many cases, there will be multiple routes to the same destination. BGP therefore uses path attributes to decide how to route traffic to specific networks.The easiest of these to understand is Shortest AS_Path. What this means is the path which traverses the least number of AS "wins."Another important attribute is Multi_Exit_Disc (Multi-exit discriminator, or MED). This makes it possible to tell a remote AS that if there are multiple exit points on to your network, a specific exit point is preferred.The Origin attribute specifies the origin of a routing update. If BGP has multiple routes, then origin is one of the factors in determining the preferred route.BGP issuesTo get a true sense of how BGP works, it's important to spend some time talking about the issues that plague the Internet.First, we have a very big problem with routing table growth. If someone decides to deaggregate a network that used to be a single /16 network, they could potentially start advertising hundreds of new routes. Every router on the Internet will get every new route when this happens. People are constantly pressured to aggregate, or combine multiple routes into a single advertisement. Aggregation isn't always possible, especially if you want to break up a /19 into two geographically separate /20s. Routing tables are approaching 200,000 routes now, and for a time they were appearing to grow exponentially.Second, there is always a concern that someone will "advertise the Internet." If some large ISP's customer suddenly decides to advertise everything, and the ISP accepts the routes, all of the Internet's traffic will be sent to the small customer's AS. There's a simple solution to this. It's called route filtering. It's quite simple to set up filters so that your routers won't accept routes from customers that you aren't expecting, but many large ISPs will still accept the equivalent of "default" from peers that have no likelihood of being able to provide transit.Finally, we come to flapping. BGP has a mechanism to "hold down" routes that appear to be flaky. Routes that flap, or come and go, usually aren't reliable enough to send traffic to. If routes flap frequently, the load on all Internet routes will increase due to the processing of updates every time someone disappears and reappears. Dampening will prevent BGP peers from listening to all routing updates from flapping peers. The amount of time one is in hold-down increases exponentially with every flap. It's annoying when you have a faulty link, since it can be more than an hour before you can get to many Internet sites, but it is very necessary.This quick discussion of BGP should be enough to get you thinking the right way about the protocol but is by no means comprehensive. Spend some time reading the RFCs if you're tasked with operating a BGP router. Your peers will appreciate it.
Border Gateway Protocol, an exterior gateway routing protocol that enables groups of routers (called autonomous systems) to share routing information so that efficient, loop-free routes can be established. BGP is commonly used within and between Internet Service Providers (ISPs).
